Natural Dyes and Pigments: Sustainable Applications and Future Scope

Abstract

Natural dyes and pigments are gaining importance as a sustainable alternative to synthetic dyes. Sourced from renewable materials, they are known for their biodegradable and non-toxic properties, offering a diverse range of color profiles and applications across industries such as textiles, cosmetics, food, and pharmaceuticals. This manuscript discusses various aspects of natural dyes and pigments (derived from plants and microbes), including anthocyanins, flavonoids, carotenoids, lactones, and chlorophyll. Furthermore, it highlights the polyphenolic nature of these compounds, which is responsible for their antioxidant activity and contributes to their anticancer, antibacterial, antifungal, antiprotozoal, and immunomodulatory effects. However, natural dyes are often categorized as pigments rather than dyes due to their limited solubility, a consequence of their molecular characteristics. Consequently, this manuscript provides a detailed discussion of key structural challenges associated with natural dyes and pigments, including thermal decomposition, photodegradation, photoisomerization, cross-reactivity, and pH sensitivity. Due to these limitations, natural dyes are currently used in relatively limited applications, primarily in the food industry, and, to lesser extent, in textiles and coatings. Nevertheless, with ongoing research and technological innovations, natural dyes present a viable alternative to synthetic dyes, promoting a more sustainable and environmentally conscious future.

1. Introduction

Synthetic dyes are integral to numerous industries due to their vibrant hues and cost-effectiveness [1,2,3,4]. However, despite their industrial success, synthetic dyes pose significant challenges related to occupational hazards, environmental toxicity, and escalating global use [5,6,7,8]. For example, workers involved in the production and application of synthetic dyes face numerous health risks. Many dyes contain toxic substances that can cause skin irritations and respiratory issues, and even prolonged exposure leads to hormonal disruptions. Notably, certain azo dyes have been scrutinized for their potential carcinogenic effects; when they decompose, they can release aromatic amines, some of which are known carcinogens and endocrine disruptors [8]. Although synthetic dyes, in particular azo dyes, are known to be useful as therapeutic tools [9,10,11,12,13,14,15,16], their large-scale applications are often associated with significant chemical waste and a high environmental footprint.

Additionally, the lack of technical expertise and transparency in chemical usage exacerbates these risks, especially in large-scale industries. For example, a significant portion of chemicals used in textile manufacturing and dyeing is often kept confidential, suggesting that potentially harmful substances might be utilized without adequate disclosure [17,18,19]. In textile dyeing, chemical quantities are often expressed as a percentage of the weight of fabric (abbreviated as o.w.f.), indicating the proportion of dye relative to the fabric’s weight. In recent years, awareness of the usage and environmental impact of synthetic dyes has increased significantly.

The environmental ramifications of synthetic dyes are profound. The textile industry, which consumes approximately 80% of the produced synthetic dyes, generates substantial amounts of dye-containing wastewater annually [5,20,21]. This wastewater often contains toxic chemicals and heavy metals, which, when discharged untreated into water bodies, can destroy the aquatic ecosystems. The coloration or high dye content in waters reduces light penetration, hindering photosynthesis in aquatic plants and therefore leading to the disruption of marine ecosystems. Microalgae, which form the foundation of aquatic ecosystems, are particularly sensitive to synthetic dyes, experiencing growth inhibition and cell malfunction upon exposure. However, most reports on this issue are based on laboratory-scale studies, which may differ from real-world scenarios where these chemicals are usually present in aquatic environments at ppm levels. Additionally, dyes can accumulate in fish, leading to their deposition in certain body parts. When consumed by humans, this could pose health risks to human societies. On land, these dyes can accumulate in soil, potentially disrupting soil microbial communities, highlighting their significant environmental impact. Furthermore, the environmental impact of synthetic dyes and their associated materials (used in the textile and plastic industries), such as dye-fiber materials could pose environmental hazards. These materials represent a significant threat to marine life and soil microbial consortia. Moreover, trace amounts of these materials have also been found in birds, though the exact pathways through which they reach avian species remain unknown [22,23,24,25].

The rapid increase in the human population has extensively driven overuse in recent years, leading to a significant rise in global consumption and market growth. The global synthetic dyes market has been experiencing substantial expansion. In 2024, the market was valued at USD 7.1 billion and is projected to reach USD 9.1 billion by 2029, growing at a compound annual growth rate (CAGR) of 5.0% during this period. However, growing awareness of the health risks associated with certain synthetic dyes has significantly restrained further market growth. Studies indicate that some synthetic dyes may be linked to allergic reactions and other health issues, leading to increased scrutiny from consumers and regulatory bodies [26,27,28,29]. For instance, certain azo dyes have been banned in the European Union due to their potential carcinogenic effects. This heightened awareness has prompted consumers to seek safer, natural alternatives, thereby impacting the demand for synthetic dyes in the finished products. The rising popularity of natural dyes presents a formidable challenge to the synthetic dye market, as consumers increasingly prefer products made with natural ingredients, especially in cosmetic products [30]. However, the issue extends beyond environmental toxicity awareness.

The textile industry remains the predominant consumer, accounting for approximately 60% of the total dye market [31,32]. This escalating demand is driven by the industry’s pursuit of vibrant, long-lasting colors and the constant introduction of new seasonal shades. However, this growth comes with significant environmental consequences. The textile industry is ranked as the second-largest contributor to water pollution, following the leather industry. A majority of textile manufacturers are located in Southeast Asian countries, where substantial amounts of textile wastewater are discharged each year. This exacerbates water pollution issues due to inadequate wastewater treatment regulations and facilities. Due to such concerns, synthetic dyes have come under scrutiny from various regulatory bodies, including academic institutions, governmental agencies, and non-governmental organizations. Stringent environmental regulations pose a significant challenge for the synthetic dyes market, as governments worldwide are increasingly implementing restrictions on hazardous chemicals used in dye production. For instance, the European Union’s REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals) regulation requires manufacturers to demonstrate the safety of their products, leading to higher compliance costs and potential market entry barriers. Similarly, the U.S. Environmental Protection Agency (EPA) has established guidelines that encourage the use of environmentally safe synthetic dyes. These regulatory frameworks not only aim to ensure consumer safety but also stimulate market growth by fostering innovation in dye formulations.

2. Natural Dyes and Pigments

Natural dyes or pigments have historically been used in textiles, foods, cosmetics, and pharmaceuticals. They are derived from a variety of sources, including terrestrial and marine plants, animals, microbes, and even minerals. However, most natural dyes and pigments are sourced from plants and can be extracted from different parts such as leaves, flowers, roots, and fruits. These colorants exhibit a diverse range of chemical structures, allowing them to produce a wide variety of color properties.

As modern societies and organizations increasingly focus on achieving sustainability goals, interest in utilizing natural resources, particularly plant-based colorants, has grown. Compared to synthetic dyes, many plant-based colorants are preferred for their sustainability, safety, and lower environmental impact. In contrast, animal-based natural dyes and colorants raise ethical concerns. Additionally, extracting mineral-based colorants can be harmful to native ecosystems, while marine-derived colorants are often limited by lower yield or expression levels.

The structural specificity of plant-based colorants is crucial to their function as dyes and pigments. These colorants often contain conjugated systems of double bonds that absorb light in the visible spectrum, resulting in various colors. For example, anthocyanins, which are water-soluble pigments found in red, purple, and blue fruits and flowers such as grapes, blueberries, and red cabbage, have a flavonoid backbone with conjugated carbon-carbon double bonds. This structure enables them to absorb light and produce characteristic colors, while changes in pH can modify their structure, leading to variations in color intensity and hue. The structure of chlorophyll, the green pigment commonly found in plants and algae, plays a vital role in photosynthesis. Chlorophyll contains porphyrin rings with a centrally coordinated magnesium ion, which enables it to absorb light efficiently in the red and blue regions of the spectrum while reflecting green light. This structural arrangement is essential for its role in photosynthesis. In contrast, carotenoids, another class of colorants, are commonly found in edible vegetables and fruits. One prominent carotenoid, known as beta-carotene, is found in carrots, sweet potatoes, and pumpkins. Carotenoids contain long conjugated chains of carbon-carbon double bonds, absorbing light in the blue-green region of the spectrum and reflecting yellow, orange, and red colors. While primarily found in plants, carotenoids are also present in algae and some bacteria. These pigments serve multiple functions in plants, including light harvesting, photoprotection, and regulating photosystem activity. They act as accessory pigments, expanding the light absorption range beyond chlorophyll and providing protection against damage caused by excess light exposure by quenching reactive oxygen species (ROS). However, their radical-scavenging properties highly depend on the conjugated bond system in their structure. Therefore, carotenoids can be used as food and cosmetic additives, acting as antioxidants or UV-protecting excipients.

2.1. Color Properties of Natural Dyes and Pigments

The color properties of plant-based colorants rely heavily on their molecular structure, particularly the functional groups within the molecules, which determine the wavelengths of light they absorb and reflect. This color profile can be influenced by factors such as the concentration of the colorant, the solution or state of the colorant, pH, and the presence of other compounds in the same solution. For instance, anthocyanins exhibit a broad color spectrum from red to blue, but their color can vary significantly with pH changes. In acidic conditions, anthocyanins appear red, while in neutral or alkaline conditions, they shift to blue or purple hues. Curcumin (a diarylheptanoid class of compounds), derived from turmeric, offers a bright yellow color pigment. Curcumin’s color remains stable at neutral pH but can degrade in alkaline conditions, leading to a loss of vibrancy. For example, it produces a reddish-brown color in alkaline conditions, while its limited aqueous solubility often requires mixing with alcohol (e.g., ethanol). It remains yellow in acidic and neutral solutions, which allows it to serve as a pH indicator. However, due to its diarylheptanoid structure (an α,β-unsaturated β-diketone moiety and an aromatic O-methoxy-phenolic group), curcumin easily loses its color properties (photofading) upon exposure to light, heat, or oxygen. Because of this photofading issue, it is not used for textile applications and is limited to food (as a natural coloring agent, E100) and cosmetics.

On the other hand, betalains (which can be classified as cationic natural pigments), found in beets, also display a range of colors from red to yellow. Betacyanins, responsible for red and purple hues, and betaxanthins, which produce yellow-orange colors, are sensitive to pH changes, with red betacyanins turning blue in alkaline environments. There are several noteworthy natural dyes or pigments, such as Indigo, Lac Dye, and Carminic acid. Indigo, being one of the most common dyes used in textile industrial applications, is derived from the plant species Indigofera tinctoria and is a deep blue pigment traditionally used in textiles. The indigo molecule absorbs light in the red and yellow regions of the spectrum and reflects blue, making it highly valued for fabric dyeing.

2.2. UV-Vis Absorption Spectrum of Natural Dyes and Pigments

The UV-Vis absorption of plant-based colorants is crucial in determining their utility and effectiveness as a color additive. UV-Vis absorption refers to the interaction of light in the ultraviolet and visible regions of the electromagnetic spectrum with the colorant molecules, leading to the absorption of specific wavelengths of light. This absorption is somewhat highly dependent on the molecular features and three-dimensional structure, particularly the presence of conjugated double bonds, and to an extent, functional groups. Anthocyanins, for example, typically exhibit strong absorption bands in the 500–550 nm range, corresponding to their red-to-blue color. They also absorb significantly in the UV region (200–400 nm), providing natural protection to plants from harmful UV radiations. This property has made anthocyanins appealing for use in cosmetics and sunscreens, where they help protect the skin.

Chlorophyll, which plays a central role in photosynthesis, exhibits absorption maxima around 430 nm (blue) and 662 nm (red) in the visible spectrum. These absorption properties are essential for light capture and conversion during photosynthesis and help protect plants from harmful UV radiation. Carotenoids, with their typical absorption in the 400–500 nm range, contribute to the yellow, orange, and red pigmentation observed in many fruits and vegetables. In addition to their role as colorants, carotenoids also protect plant tissues from UV-induced oxidative damage, making them valuable in dietary and cosmetic applications. Flavonoids, another group of plant-based colorants, show a broad range of UV-Vis absorption patterns based on their specific structure. For instance, quercetin, a dietary flavonol found in apples, onions, and tea, exhibits characteristic UV-Vis absorption with a Band I maximum around 370 nm (band I, linked to the B-ring cinnamoyl system, π→π transitions), and band II around 255 nm, with weak visible absorption extending into the yellow region (430–480 nm), which gives it a pale yellow color. This UV-Vis absorption also aids in protecting plants from UV radiation and oxidative stress. Annatto, derived from the Bixa orellana plant, is used in food products and contains the carotenoid bixin. Bixin absorbs light in the blue-green region (450–470 nm) and imparts a yellow to orange color to food. It also absorbs UV light, which contributes to its antioxidant properties. Tannin-based colorants, derived from various plants’ bark and leaves, have aromatic structures containing hydroxyl groups that interact with UV light, making them useful in leather production and textiles. These colorants absorb strongly in the UV region (280–320 nm), enhancing the longevity and durability of products exposed to sunlight.

2.3. Examples of Commercial Success of Natural Dyes and Pigments

As consumers increasingly seek natural and sustainable alternatives to synthetic dyes, several natural dyes and pigments have found commercial success, being sold under brand names in industries such as textiles, cosmetics, food, and beverages. Examples of natural dyes and pigments include Indigo, as ‘Dharma Trading Co. Indigo Dye’ for textile dyeing, particularly in traditional methods like tie-dyeing and Shibori. Turmeric, known for its bright yellow color, is sold as ‘ColorGarden Organic Turmeric Powder’ and is used in fabric dyeing and food products. Cochineal, an insect-derived dye, is marketed by various brands, including Madder Root, producing reds and purples used in textiles, cosmetics, and food. Annatto, commonly used as a food coloring, provides yellow to orange shades and is sold under the brand name ‘Saffron Road Annatto Dye’ for products like cheeses and snacks. Hibiscus flowers, a source of red and purple dyes, are sold by The Herbal Garden as hibiscus powder for use in cosmetics, food, and fabrics. Spirulina, a green pigment from Arthrospira platensis, is marketed by Earth Circle Organics as ‘Spirulina Powder’, used both for its nutritional value and as a colorant in foods and cosmetics. Woad, a natural blue dye historically used in Europe as an alternative to indigo, is sold by Earthues as ‘Woad Natural Indigo’. Madder Root, providing red, orange, and brown dyes, is sold by The Madder Company as ‘Madder Root’ for textile applications. Pomegranate, which yields shades of yellow, brown, and red, is used in eco-friendly fabric dyeing, with Eco-Color offering ‘Pomegranate Dye’. Beetroot, known for its vibrant red and pink pigments, is marketed by Organic Herb Trading as ‘Beetroot Powder’ for food and cosmetics. Black Walnut hulls, used to create rich brown dyes, are sold by Wild Harvest as ‘Black Walnut Hulls’ for fabric and wood dyeing. Safflower, producing yellow and orange dyes, is marketed by Wild Colors as ‘Safflower Natural Dye’ for fabric dyeing. Curry Leaf, traditionally used for its green color, is available from Spicely Organic as ‘Curry Leaf Powder,’ although its applications are less widespread. Logwood, which produces colors ranging from purple to black, is sold by Earthues as ‘Logwood Natural Dye’ for textile dyeing. Finally, Marigold flowers, used to create yellow to orange dyes, are sold by Wild Colors as ‘Marigold Dye’ for eco-friendly fabric dyeing. These natural dyes and pigments offer a sustainable and natural alternative to synthetic options, catering to the growing demand for eco-friendly products across various industries.

3. Classification of Natural Dyes and Pigments

There are different ways of classifying natural colorants based on their origin, chemical structure, and solubility. One of the common ways is based on their source, but since similar natural colorant chemotypes are available in different types of sources in nature, such classification may not be appropriate. For example, based on their sources, dyes can be classified as plant-based or animal-based. Plant-based dyes are extracted from various parts of plants, including roots, leaves, flowers, bark, seeds, or even the entire plant. Examples include indigo from Indigofera tinctoria, turmeric from Curcuma longa, and madder from Rubia tinctorum. In contrast, animal-based dyes are derived from insects or mollusks, obtained either from secreted colorants produced during their metabolic processes or as waste products. Notable examples of natural dyes include carmine red, derived from cochineal insects (Dactylopius coccus), and Tyrian purple, obtained from sea snails such as Murex brandaris. On the other hand, microbial dyes and pigments are produced by bacteria, fungi, and algae, with examples such as prodigiosin (from Serratia marcescens) and fungal indigo (from Saccharothrix azulens). Mineral-based pigments are those obtained from naturally occurring minerals such as ochre, malachite, and ultramarine, which have historically been used in painting and textile dyeing. However, classification based on the source or origin of natural dyes and pigments does not provide significant value in understanding their properties, applications, and limitations in various industries. Another way of classifying natural dyes and pigments is based on solubility, as some colorants exhibit solubility in water while others do not, and some are specifically lipid soluble. Water-soluble dyes dissolve in water and mainly contain highly polar functional groups in their molecular structure, such as phenolics, anthocyanins, betalains, and flavonoids, as well as those natural colorants containing carboxylic acids. Lipid-soluble dyes dissolve in oils, fats, and non-polar organic solvents, making them suitable for cosmetic applications. Examples include carotenoids like β-carotene and lycopene. Pigments insoluble in both water and non-polar solvents (such as oils) are classified as “insoluble pigments.” These pigments remain suspended and are suitable for applications like coatings, paints, and textiles. Examples include mineral-based pigments like ochre and ultramarine. However, classifications based solely on solubility or origin provide limited insight into the chemical nature of these colorants. Such methods do not fully inform researchers about their chemical properties, which are crucial for improving extraction processes, developing derivatives with enhanced value, or identifying targeted industrial applications. Therefore, chemotaxonomic-based classification, which considers the chemical structure and biosynthetic origin of pigments, is often more informative and preferable. In this approach, if the same natural dye or pigment is available in more than one species or source, it can easily be categorized. Based on chemical structure, these can be classified into different categories: anthocyanins, carotenoids, flavonoids, betalains, and chlorophylls, as shown in Figure 1.

3.1. Anthocyanin-Based Natural Dyes and Pigments

Anthocyanins are a class of water-soluble pigments found in a variety of fruits, vegetables, and flowers, responsible for the red, purple, and blue colors seen in many plants [33,34]. Despite their widespread use in the food, cosmetics, and textile industries, anthocyanins face several structural issues that can affect their stability, color, and utility as natural colorants. There are numerous examples of anthocyanins being utilized in material applications [35,36,37,38,39], such as the development of plasma-treated corn-starch-based films incorporated with acerola and grape pomace extracts [40]. These films have pH-sensing capabilities, making them useful as smart packaging materials or pH sensor films. Both acerola (Malpighia emarginata) and grape (Vitis labrusca) pomace extracts contain various types of anthocyanins, which are water-soluble pigments. For example, prominent anthocyanins found in acerola include cyanidin-3-O-glucoside, cyanidin, and derivatives of peonidin and malvidin [41,42,43]. Meanwhile, grape pomace extract is a rich source of anthocyanins, including cyanidin-3-glucoside, peonidin-3-glucoside, delphinidin-3-glucoside, malvidin-3-glucoside, and petunidin-3-glucoside [44,45,46,47,48]. However, the phytochemical composition and specificity can vary based on regional differences and species. Importantly, acerola (Malpighia emarginata) is rich in vitamin C and also contains carotenoids.

Table 1. Anthocyanins as natural dyes and pigments [33,34,35,36,37,38,39,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58].

Anthocyanin	Source	Structural Specificity	Applications	Biological Activities
6-Hydroxycyanidin	corn, berries	C6-Hydroxylated anthocyanin	Nutraceuticals, food	Antioxidant, anti-aging
6-Hydroxypeonidin	corn, grapes	C6-Hydroxylated anthocyanin	Beverages, textiles, cosmetics	Antioxidant, anticancer
Acylated cyanidin	Red cabbage, radishes	Cyanidin with acyl groups	Food, textiles, cosmetics	Antioxidant, UV protection
Acylated delphinidin	Eggplants, violets	Delphinidin with acyl groups	Cosmetics, food	Antioxidant, neuroprotective
Acylated pelargonidin	Strawberries, red currants	Pelargonidin with acyl groups	Food, pharmaceuticals, textiles	Antioxidant, anti-inflammatory
Apigeninidin	Sorghum, millet	Flavylium cation, flavone derivative	Food, cosmetics, beverages	Antioxidant, cardiovascular health
Caffeoylated cyanidin	Red cabbage, black rice	Cyanidin with caffeoyl group	Food, cosmetics	Antioxidant, cardiovascular health
Caffeoylated delphinidin	Grapes, eggplants	Delphinidin with caffeoyl group	Food, beverages	Antioxidant, anticancer
Coumaroylated cyanidin	Raspberries, grapes	Cyanidin with coumaroyl group	Beverages, cosmetics, textiles	Antioxidant, UV protection
Coumaroylated delphinidin	Plums, berries	Delphinidin with coumaroyl group	Food, pharmaceuticals	Antioxidant, antimicrobial
Cyanidin	Blackberries, cherries, apples	Flavylium cation, hydroxyl groups	Food, cosmetics, textiles	Antioxidant, anti-inflammatory
Cyanidin-3-rutinoside	Black rice, red cabbage	Cyanidin with rutin sugar (O-linkage)	Nutraceuticals, cosmetics	Antioxidant, supports circulation
Delphinidin	Blueberries, eggplants	Flavylium cation, multiple hydroxyl groups	Food, nutraceuticals, textiles	Antioxidant, cardiovascular health
Delphinidin-3-rutinoside	Blueberries, violets	Delphinidin with rutin sugar	Cosmetics, food	Antioxidant, anti-aging
Europinidin	Flowers (Ajuga species)	Flavylium cation, hydroxylated	Food, cosmetics, textiles	Antioxidant, anti-inflammatory
Europinidin-3-glucoside	Ajuga flowers, grapes	Europinidin with glucose (O-linkage)	Cosmetics, textiles	Antioxidant, neuroprotection
Feruloylated malvidin	Wine grapes, bilberries	Malvidin with feruloyl group	Wine, textiles	Antioxidant, neuroprotection
Feruloylated peonidin	Red grapes, plums	Peonidin with feruloyl group	Food, supplements	Antioxidant, anticancer
Glucoside cyanidin	Blackcurrants, cherries	Cyanidin with glucose moiety	Food, cosmetics	Antioxidant, neuroprotective
Glucoside delphinidin	Blue corn, blackberries	Delphinidin with glucose moiety	Textiles, cosmetics	Antioxidant, antimicrobial
Glucoside pelargonidin	Strawberries, roses	Pelargonidin with glucose moiety	Food, beverages	Antioxidant, anti-inflammatory
Glycosylated cyanidin	Blackberries, cherries	Cyanidin with sugar moieties	Beverages, textiles, cosmetics	Antioxidant, improves gut health
Glycosylated delphinidin	Blueberries, plums	Delphinidin with sugar moieties	Food, textiles	Antioxidant, UV protection
Glycosylated pelargonidin	Strawberries, raspberries	Pelargonidin with sugar moieties	Food, cosmetics	Antioxidant, anti-inflammatory
Hirsutidin	Cranberries, grapes	Flavylium cation, methoxy groups	Cosmetics, food colorant	Antioxidant, neuroprotective
Luteolinidin	Sorghum, millet	Flavylium cation, hydroxyflavone	Food, beverages, textiles	Antioxidant, anti-diabetic
Malonyl cyanidin	Black rice, black currants	Cyanidin with malonyl group	Food, textiles, pharmaceuticals	Antioxidant, anti-diabetic
Malonyl pelargonidin	Cherries, strawberries	Pelargonidin with malonyl group	Food, nutraceuticals	Antioxidant, anti-inflammatory
Malvidin	Grapes, red wine	Flavylium cation, methoxy groups	Wine, textiles, food	Antioxidant, neuroprotective
Malvidin-3-glucoside	Grapes, red wine	Malvidin with glucose moiety	Wine, food, cosmetics	Antioxidant, cardiovascular health
Methylated cyanidin	Black carrots, berries	Methylated anthocyanin	Food, beverages, nutraceuticals	Antioxidant, cardiovascular support
Methylated delphinidin	Petunias, blue flowers	Delphinidin with methyl groups	Textiles, food, pharmaceuticals	Antioxidant, UV protection
Pelargonidin	Strawberries, raspberries	Flavylium cation, fewer hydroxyl groups	Food, cosmetics, beverages	Antioxidant, anticancer
Pelargonidin-3-rutinoside	Strawberries, pomegranates	Pelargonidin with rutin sugar	Food, beverages	Antioxidant, UV protection
Peonidin	Cranberries, plums	Flavylium cation, methylated hydroxyl groups	Food, supplements, cosmetics	Anti-inflammatory, neuroprotective
Peonidin-3-glucoside	Cranberries, plums	Peonidin with glucose moiety	Food, beverages, pharmaceuticals	Antioxidant, anti-inflammatory
Petunidin	Grapes, bilberries	Flavylium cation, additional methoxy groups	Food, beverages, textiles	Antioxidant, anticancer properties
Rosinidin	Catharanthus roseus	Flavylium cation, methyl groups	Food, cosmetics	Antioxidant, antimicrobial
Rosinidin-3-glucoside	Carnations, Catharanthus roseus	Rosinidin with glucose	Food, textiles, research	Antioxidant, antimicrobial
Rutinose cyanidin	Red cabbage, radishes	Cyanidin with rutinose sugar	Food, supplements	Antioxidant, vascular support
Rutinose delphinidin	Blue grapes, plums	Delphinidin with rutinose sugar	Food, textiles	Antioxidant, UV protection
Sophoroside cyanidin	Black rice, grapes	Cyanidin with sophoroside sugar	Food, cosmetics	Antioxidant, anti-diabetic
Sophoroside delphinidin	Blue grapes, eggplants	Delphinidin with sophoroside sugar	Textiles, food	Antioxidant, cardiovascular support

presents a compilation of common anthocyanins recognized as natural dyes and colorants.

Certain issues are associated with anthocyanin-based natural dyes and pigments, primarily due to their molecular chemical structures. They are susceptible to changes in environmental conditions such as pH, light, temperature, and oxygen exposure [59,60,61,62,63,64]. Some of the primary structural issues associated with anthocyanins include:

(a) pH-sensitive: The color properties of anthocyanins are highly sensitive to any change in the pH levels, and their structure is vulnerable to any change in pH (acidic or alkaline environment) [65,66,67,68]. At lower pH (acidic conditions), anthocyanins exhibit red, purple, or blue shades. However, in alkaline conditions, anthocyanins can undergo chemical changes, resulting in their color loss. Specifically, under alkaline conditions, anthocyanins undergo a transformation where the flavylium ion (the chromophore responsible for their color) is deprotonated (as shown in Figure 2), resulting in a shift to a colorless or brownish form. For example, the purple pigment from red cabbage, which is rich in anthocyanins, can change to green or blue when exposed to alkaline substances such as baking soda or certain salts. This structural sensitivity to pH limits the use of anthocyanins in products where pH stability is crucial. For example, delphinidin (found in blueberries and grapes), are highly sensitive to pH. For example, cyanidin-3-glucoside (found in strawberries and raspberries) changes color depending on the pH, becoming more purple in neutral conditions and red in acidic conditions. This pH sensitivity makes it difficult to maintain consistent color in food products that undergo pH changes during processing or storage.

(b) Chemical structure degradation: The anthocyanin molecule consists of a flavonoid structure as a molecular core with various sugar residues attached (either as O-glycosyl units or C-glycosyl units). The common sugars (such as glucose, rhamnose, or galactose) that are found in their structure are critical for polar solubility and, to an extent, their structural stability. These glycosyl units or sugar moieties have a polyhydroxy functional group, which impacts aqueous solubility and stability of the glycosylated anthocyanin structure through H-bonding. However, these glycosidic linkages (O-linkage) can be hydrolyzed, especially under acidic conditions, leading to the breakdown of the anthocyanin molecule from its sugar moiety. This hydrolysis can influence their color properties and result in the anthocyanidin aglycone moiety to exhibit lesser stability. For example, cyanidin, a common anthocyanidin, is prone to hydrolysis into a colorless compound under acidic conditions, reducing its effectiveness as a colorant in food and cosmetics. The structural breakdown of anthocyanins through hydrolysis is one of the primary reasons why their shelf life is relatively short, unless stabilized.

(c) Oxidative degradation: Anthocyanins are highly susceptible to oxidation, which is another structural issue that affects their stability. The oxidative degradation of anthocyanins typically occurs when they are exposed to light, oxygen, or heat. The oxidized forms of anthocyanins often lose their intense color and degrade into colorless or brownish compounds. The presence of oxygen can lead to the oxidation of the phenolic groups on the anthocyanin molecule, breaking down the conjugated system of double bonds that are responsible for light absorption and vibrant color. For instance, delphinidin, a type of anthocyanin found in blueberries, is particularly prone to oxidative degradation, leading to a loss of its blue color when exposed to air or sunlight. This oxidative instability can be a significant challenge for industries that use anthocyanins as colorants, as it limits their ability to maintain long-term stability, especially in light-sensitive products like beverages, cosmetics, and processed foods.

(d) Temperature sensitivity: Temperature is another factor that influences the structural integrity of anthocyanins [69]. High temperatures can accelerate their degradation through hydrolysis, oxidation, or a combination of both. Heat can cause the breakdown of the anthocyanin molecule, especially in the presence of light and oxygen. As a result, when products containing anthocyanins are stored or processed at elevated temperatures, the color can fade or change, making anthocyanins less reliable for use in products that require long shelf life or high processing temperatures. For example, anthocyanins found as red, purple, and blue natural colorants in many fruits like berries, grapes, and red cabbage are particularly sensitive to heat. For example, cyanidin-3-glucoside (a common anthocyanin found in cherries and red cabbage) loses its red color when subjected to heat. This is due to the breakdown of the anthocyanin molecule under thermal conditions, which results in color degradation. Extended exposure to elevated temperatures results in the degradation of anthocyanins, leading to color loss, structural breakdown, and reduced antioxidant activity. For example, high temperatures can accelerate the hydrolysis of anthocyanin glycosides (i.e., breaking the glycosidic bonds between the anthocyanin and sugar units), oxidation of the flavylium ring structure, resulting in discoloration, or polymerization to form brown pigments in the presence of oxygen. One example is blueberry aqueous extract, where heating above 70 °C leads to rapid anthocyanin degradation, causing visible color fading and browning. Various studies have reported nearly 50% loss of anthocyanin content after pasteurization (e.g., 85 °C for 2 min or 90 °C for 1 min) [59,69,70,71,72,73,74,75,76]. Similarly, anthocyanins in purple corn are often stabilized by co-pigments. However, thermal processing or extraction procedures at 80–100 °C for prolonged durations reduce their stability, leading to significant degradation of anthocyanins. Another example is red wine. During fermentation or aging at elevated temperatures, anthocyanin degradation can alter wine color, affect shelf life, and potentially influence taste. Hence, storage facilities for red wine often require air conditioning and regulated temperatures [67]. Storage at 25 °C or higher is known to accelerate anthocyanin breakdown compared to cooler environments. In the case of strawberries, continuous heating during aqueous extraction at 60–90 °C significantly reduces anthocyanin content. This degradation is further accelerated in the presence of light and oxygen. This limitation often necessitates the use of stabilizers or refrigeration to preserve the color.

(e) Intermolecular interactions: The stability and color of anthocyanins can also be influenced by their interactions with other compounds, both natural and synthetic, within a system. These intermolecular interactions can alter the structure of anthocyanins, leading to changes in their color and stability. Several factors can influence the color properties and aqueous solubility of anthocyanins when used as natural pigments or dyes:

(i)

Presence of metal ions: Anthocyanins contain electronegative groups, such as polyphenolic hydroxyls, which can coordinate with metal ions like iron or copper to form metal-anthocyanin complexes. While some organic–metal complexes are known for their bright color properties, in the case of anthocyanins, these interactions often result in undesirable color changes, such as browning or dulling of the original hue.

(ii)

Presence of other natural compounds: Anthocyanins may interact with tannins, proteins, and other polyphenols present in their natural sources or extracts [77]. These interactions enhance intermolecular bonding, leading to the formation of insoluble complexes that can precipitate out of solution. As a result, the color intensity and solubility of anthocyanins are reduced.

(iii)

Presence of glycosyl units: Glycosylation generally stabilizes anthocyanins by increasing their water solubility and structural integrity. However, the presence of multiple glycosyl (sugar) units can increase the overall polarity of the molecule and, in some cases, cause positional isomerization. This equilibrium is sensitive to changes in environmental conditions (such as pH and temperature), leading to inconsistent color intensity and sometimes visible color shifts.

(f) Light sensitivity: Anthocyanins are susceptible to photodegradation when exposed to ultraviolet (UV) or visible light [78]. Prolonged or high-intensity light exposure can break down chemical bonds in the structure, particularly in the conjugated double-bond systems responsible for the pigment’s color. This degradation can result in color fading or a shift to less vibrant hues. The issue is especially problematic in food products, beverages, and cosmetics that are exposed to light for extended periods. For example, malvidin, an anthocyanin found in grapes, can lose its vibrant color under prolonged sunlight exposure, turning dull brown or gray [79,80,81,82,83]. Light sensitivity is a major concern for product shelf life and esthetic quality.

(g) Molecular diversity and variability: Anthocyanins come in various forms, with slight structural differences across different plant species. These structural differences, including variations in the type of sugar or the hydroxylation pattern on the aromatic ring, can affect the color and stability of anthocyanins. Some anthocyanins are more stable than others due to their molecular structure, and this variability can create challenges for standardizing the use of anthocyanins in industrial applications [84]. For example, pelargonidin and cyanidin structures (as shown in Figure 2) are two common anthocyanins that produce red and purple colors, respectively. However, pelargonidin is generally less stable than cyanidin and more prone to degradation when exposed to light, oxygen, slightly alkaline pH, and heat [59]. The degradation of anthocyanins like cyanidin and pelargonidin is influenced by the number and position of hydroxyl groups on their B-ring, not the C-ring. Cyanidin has two hydroxyl groups on the B-ring (at positions 3′ and 4′), while pelargonidin has only one (at position 4′). Although ortho-dihydroxyl groups (adjacent hydroxyls) can stabilize the flavylium ion through intramolecular hydrogen bonding, neither cyanidin nor pelargonidin possesses an ortho-dihydroxy arrangement on the B-ring, so intramolecular hydrogen bonding is unlikely in these anthocyanins. However, reports indicate the presence of additional hydroxyl groups can facilitate intermolecular hydrogen bonding with solvent molecules or other compounds, indirectly helping to stabilize the flavylium ion’s positive charge. Moreover, compounds with multiple hydroxyl groups can exhibit better electron delocalization, contributing to greater resistance to degradation, especially under acidic conditions [84,85,86,87,88].

This variability in stability and color performance adds complexity to the commercial use of anthocyanins, as it may require careful selection and formulation to ensure consistent results across different plant sources.

(h) Solubility issues: The aqueous solubility of anthocyanins is one of the major challenges that sustainably affect their commercial use. Although anthocyanins are known to dissolve in water, their solubility can vary significantly. This variation primarily depends on the type and position of glycosyl moieties attached to their structure and, to some extent, on the presence of polyphenolic groups. Additionally, the pH of the solution plays a critical role, as shown in Figure 3. At higher pH levels, anthocyanins tend to form insoluble aggregates or complexes, which reduces their effectiveness as colorants in aqueous solutions. In some cases, the use of solvents or stabilizers is necessary to enhance their solubility, which can complicate their application in product formulations.

3.2. Betalain-Based Natural Dyes and Pigments

Betalains are a class of plant pigments that are responsible for the red, purple, and yellow colors in certain plants, such as beets, chard, and prickly pears. These pigments are structurally distinct from anthocyanins, as they belong to a different family of nitrogen-containing pigments, but exhibit color change properties similar to anthocyanins, therefore can be used in developing smart packaging, colorimetric methods, and other materialistic applications [89]. Also, they exhibit, different structural features, physicochemical properties, and therefore have distinctive therapeutic benefits [90,91], as shown in Figure 4. Betalains are primarily divided into two categories: betacyanins, which provide red to violet colors, and betaxanthins, which provide yellow to orange colors.

While betalains have many potential applications in food coloring, cosmetics, and pharmaceuticals, they also face several structural challenges that impact their stability, color retention, and overall applicability. There are some key structural issues associated with betalains, therefore, pretreatment methods plays important role in their extraction [92,93].

(a) Susceptibility to pH changes: One of the major structural issues with betalains is their pH sensitivity. Similarly to anthocyanins, the color properties of betalain structure can be influenced by the acidity or alkalinity of the environment, although their response is different due to their unique chemical structure. For example, betacyanins are more stable in acidic conditions and can lose their bright color in alkaline (basic) environments. At higher pH, the betalain molecule undergoes structural changes, leading to a loss of the red to violet color and often resulting in a shift toward a brownish or colorless form. This occurs because the betalain molecule is chemically unstable in alkaline conditions, causing a degradation of the chromophore (the part of the molecule responsible for its color). Betaxanthins can also experience color shifts under extreme pH levels, although they are generally more stable in neutral to slightly acidic conditions. In general, this pH sensitivity limits the use of betalains in formulations that require stable color properties across a wide range of pH values (e.g., in beverages or processed foods with varying acidity).

(b) Thermal degradation: Betalains are highly sensitive to heat, and their stability is compromised when exposed to high temperatures. Thermal degradation can cause the betalain molecules to degrade, resulting in color shades and the formation of colorless or degraded products. This is one of the significant structural limitations of the use of betalains in food processing or cosmetics, where heat is often applied during cooking, pasteurization, or sterilization. For example, the red color of beet juice, which is rich in betacyanins, can significantly fade or change to a dull brown when exposed to heat. This degradation can be accelerated if the heat is combined with light exposure or oxygen. The breakdown of the betalain molecule involves the breakdown of the conjugated system that is responsible for the bright color shades, rendering the pigment less effective for use in colorant applications.

(c) Sensitivity towards oxidation: Oxidation is another major structural issue with betalains. Like other natural dyes and pigments, betalains are highly susceptible to oxidation, which occurs when the pigment molecules are exposed to oxygen, especially under light or heat. Oxidative degradation involves the breakdown of the betalain structure, particularly the nitrogen-containing ring system, which is essential for the pigment’s color properties. For example, betacyanins, such as betanin (the primary betalain found in beets), can undergo oxidation, leading to a loss of their characteristic vibrant red color. The oxidation process typically involves the break down of the betalain molecule’s aromatic ring, causing a shift to a more brownish or colorless compound. Betaxanthins, although more stable than betacyanins, are still prone to oxidative degradation, especially in the presence of light and oxygen. The process of oxidation reduces the intensity of the yellow color and can lead to fading or browning of products containing betalains. The oxidative instability of betalains poses a significant challenge for their use in long-term applications, such as food colorants and cosmetics, where consistent color retention is a prerequisite requirement. In my opinion, due to their structural substitution (trisubstituted pyridine), synthetic compounds with similar substitutions could potentially replace betalains. Although the synthetic versions may produce different colors, they tend to be more resistant to oxidation and light, which is advantageous for achieving stable color properties [94,95,96,97].

(d) Light sensitivity: Betalains are also sensitive to light, particularly ultraviolet (UV) light, which can induce structural changes in the betalain molecules. The absorption of UV light can break the chemical bonds in betalain structure, leading to a loss of color intensity or the degradation of the pigment. For example, betacyanins, such as betanin, are especially vulnerable to photodegradation. When exposed to light, especially in the presence of oxygen, the betalain molecules undergo structural changes that result in a fading or brownish discoloration. This light sensitivity is a significant issue for commercial products such as beverages, cosmetics, and packaged foods, which may be exposed to light during storage or retail. Betaxanthins are somewhat more stable to light than betacyanins, but they still degrade under prolonged exposure to light, leading to a loss of their yellow or orange color. There are several factors that contribute to the observed difference in light stability between betaxanthins and betacyanins. One key distinction lies in their molecular complexity. Betaxanthins possess a nitrogen-containing structure linked to amino acids or amines, resulting in a simpler molecular framework. In contrast, betacyanins feature a more complex structure, including an additional cyclic amine ring fused to the betalain core, e.g., cyclo-DOPA moiety, which increases both their size and degree of conjugation [98,99,100,101]. This structural difference influences how these pigments interact with light. Betacyanins absorb light at longer wavelengths (approximately 535–540 nm), meaning they are more reactive to visible and UV light. This absorption can promote molecular excitation into reactive states, making them more susceptible to photodegradation. In contrast, betaxanthins absorb at shorter wavelengths (around 480–500 nm) with generally lower molar absorptivity, which contributes to their relatively higher photostability and reduced photochemical reactivity under similar conditions [90,102,103,104,105,106]. Consequently, the higher conjugation in betacyanins leads to more easily excited electronic states when exposed to light, increasing the likelihood of photooxidative reactions that degrade their structure [106]. In comparison, betaxanthins, with their simpler conjugation, are less prone to such reactions, which contributes to their slightly greater light stability. In summary, light sensitivity is one of the most important structural issues that limit the use of betalains in certain products, especially in packaging and environments where light exposure is unavoidable.

(e) Interaction with metals: The coexistence of metal ions with betalains can lead to the formation of coordination complexes, which impact both the color properties and molecular stability of the pigment. This occurs due to the presence of nitrogen atoms in the betalain structure, particularly in the betacyanin class, which can coordinate with metal ions such as iron, copper, and magnesium. These intermolecular interactions and coordination bonds can cause the pigment to lose its vibrant color or form undesirable precipitates, thereby reducing its effectiveness in applications. Betacyanins are particularly prone to forming complexes with iron ions, which may cause the formation of a dark or brownish color instead of the desired vibrant red or violet hue. These interactions can also cause the betalain to precipitate out of the solution, reducing its effectiveness as a colorant. Betaxanthins, while less prone to these interactions, can still undergo some binding with metal ions that may lead to instability, especially in aqueous solutions. This sensitivity to metal ions can create challenges in formulations where metal contamination is present, such as in the processing of food or cosmetics.

(f) Solubility issues: Betalains are water-soluble, which makes them suitable for use in aqueous systems such as beverages, juices, and food products. However, their solubility can be affected by environmental factors such as temperature, pH, and the presence of other solutes. For example, betacyanins can exhibit reduced solubility in neutral or slightly alkaline conditions, leading to the formation of insoluble aggregates or precipitates. This solubility issue can affect the uniformity of color distribution in products, leading to uneven color intensity or sedimentation. Betaxanthins are more stable in neutral to slightly acidic environments and generally exhibit good solubility in water, but their solubility can still be influenced by pH and other solutes present in the system. The solubility issues of betalains can limit their use in certain applications, especially those requiring stability across a range of pH values or in more complex formulations.

(g) Molecular instability due to nitrogen content: Betalains contain a nitrogen atom within their molecular structure, which is a key feature that differentiates them from anthocyanins. However, this nitrogen atom can contribute to the instability of the betalain molecule. Nitrogen-containing compounds are often more prone to nucleophilic attack or deamination reactions, resulting in their chemical degradation and loss of color properties. For example, betacyanins in particular, susceptible to reactions involving their nitrogen atoms (due to its high electronegativity character), which can result in the breakdown of the betalain structure and the loss of their characteristic color. Deamination (the removal of an amino group) can lead to the formation of colorless or brownish byproducts. This structural issue contributes to the relatively short shelf life of betalains as colorants, especially in products known to be continuously exposed to fluctuating ambient conditions.

(h) Biosynthetic pathways and variability: The biosynthesis of betalains involves complex metabolic pathways, and the concentration of betalains in different plant species can vary significantly depending on environmental factors, growth conditions, and genetic factors. This variability can lead to inconsistency in the structure and quality of betalains from different sources. For example, the variability in betalain content and structure may make it difficult to standardize betalain-derived colorants for commercial use. For instance, betalains from different beetroot varieties can exhibit slight differences in their molecular structures, affecting their color intensity and stability. This natural variability poses challenges for large-scale production and standardization of betalain-based colorants, particularly when consistency is required for industrial applications.

3.3. Carotenoids-Based Natural Dyes and Pigments

Carotenoids are a diverse class of naturally occurring pigments that are responsible for the yellow, orange, and red colors in a variety of fruits, vegetables, and other plants. They play crucial roles in photosynthesis and also serve as precursors to vitamin A in animals. Carotenoids are widely used in the food, cosmetic, and pharmaceutical industries due to their color and health benefits [107,108], as compiled in

Table 2. Carotenoids-based natural dyes and pigments [109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133].

Carotenoid	Source	Structural Specificity	Applications	Biological Activities
Antheraxanthin	Green plants, algae	Xanthophyll, epoxide groups	Food, cosmetics	Antioxidant, photoprotection
Astaxanthin	Microalgae, salmon, shrimp	Xanthophyll, keto groups	Aquaculture, cosmetics	Antioxidant, skin protection
Bixin	Annatto seeds	Carotenoid, apocarotenoid	Dairy, snacks, cosmetics	Antioxidant, anti-inflammatory
Canthaxanthin	Crustaceans, fungi, flamingos	Xanthophyll, keto groups	Poultry feed, aquaculture	Antioxidant, skin pigment enhancer
Capsanthin	Red peppers, paprika	Xanthophyll, keto groups	Food colorant, cosmetics	Anti-inflammatory, antioxidant
Capsorubin	Red peppers, paprika	Xanthophyll, keto groups	Food, cosmetics	Anti-inflammatory, UV protection
Deoxyxanthin	Marine bacteria	Xanthophyll, deoxygenated	Biotechnology, food	Antioxidant, anti-aging
Diadinoxanthin	Diatoms, algae	Xanthophyll, epoxide group	Nutraceuticals, aquaculture	Antioxidant, photoprotection
Diatoxanthin	Diatoms, algae	Xanthophyll, epoxide group	Aquaculture, research	Photoprotection, antioxidant
Echinenone	Cyanobacteria, microalgae	Xanthophyll, keto group	Food, supplements	Antioxidant, anti-inflammatory
Echinenone	Cyanobacteria, algae	Xanthophyll, ketone group	Aquaculture, research	Antioxidant, supports photosynthesis
Flexixanthin	Marine bacteria	Xanthophyll, ketone group	Research, pharmaceuticals	Antioxidant, antimicrobial
Fucoxanthin	Brown seaweed, diatoms	Xanthophyll, epoxide groups	Nutraceuticals, food	Anti-obesity, anti-inflammatory
Lutein	Marigold, spinach, kale	Xanthophyll, hydroxyl groups	Eye health supplements, food	Antioxidant, eye health
Luteoxanthin	Carrots, plants	Xanthophyll, hydroxyl groups	Food, dietary supplements	Antioxidant, UV protection
Lycopene	Tomatoes, watermelon	Acyclic carotenoid	Food, cosmetics, supplements	Antioxidant, anti-inflammatory
Methylhexacosahexaenoate	Algae, bacteria	Carotenoid, esterified	Nutraceuticals, pharma	Anti-inflammatory, neuroprotective
Mutatochrome	Mushrooms, bacteria	Carotene derivative	Research, food colorant	Antioxidant
Myxoxanthophyll	Cyanobacteria	Xanthophyll, glycosylated	Biotechnology	Antioxidant, photoprotection
Neoxanthin	Green leafy vegetables	Xanthophyll, epoxide groups	Food, cosmetics	Antioxidant, neuroprotective
Norbixin	Annatto seeds	Carotenoid, apocarotenoid	Food, dairy products	Antioxidant, neuroprotective
Oscillaxanthin	Cyanobacteria	Xanthophyll, oxygenated	Research, biotechnology	Antioxidant, anti-inflammatory
Peridinin	Dinoflagellates (algae)	Xanthophyll, lactone group	Fluorescent dyes, research	Antioxidant, photoprotection
Phytoene	Tomatoes, carrots, algae	Precursor carotenoid, colorless	Food, cosmetics	Antioxidant, anti-aging
Phytofluene	Tomatoes, carrots, oranges	Precursor carotenoid, colorless	Food, skincare	Anti-inflammatory, UV protection
Rhodoxanthin	Conifers, flamingos	Xanthophyll, oxygenated	Food, bird pigmentation	Antioxidant, pigmentation
Rubixanthin	Rose hips, red berries	Xanthophyll, hydroxyl groups	Food, cosmetics	Antioxidant, UV protection
Salinixanthin	Halophilic bacteria	Xanthophyll, glycosylated	Biotechnology	Antioxidant, membrane stabilizer
Saproxanthin	Bacteria (Sphingomonas)	Xanthophyll, hydroxyl groups	Biotechnology	Antioxidant, anti-UV effects
Spheroidene	Purple bacteria, plants	Carotene, cyclized chain	Research, food	Antioxidant, photoprotection
Spheroidenone	Photosynthetic bacteria	Carotene, oxygenated chain	Biotechnology	Antioxidant, photoprotective
Spirilloxanthin	Photosynthetic bacteria	Xanthophyll, conjugated chain	Biotechnology, research	Antioxidant, UV protection
Synechoxanthin	Cyanobacteria	Xanthophyll, keto group	Research, biotechnology	Antioxidant, photoprotection
Tetradehydrolycopene	Algae, bacteria	Carotenoid, acyclic	Research, nutraceuticals	Antioxidant
Torularhodin	Yeast (Rhodotorula)	Xanthophyll, hydroxyl groups	Pharmaceuticals, food	Antioxidant, immune-boosting
Torulene	Yeast (Rhodotorula)	Carotenoid, hydrocarbon	Food, cosmetics	Antioxidant, antimicrobial
Trollichrome	Fungi, lichens	Carotenoid-like structure	Food, pharmaceuticals	Antioxidant, antibacterial
Violaxanthin	Yellow flowers, paprika	Xanthophyll, epoxide groups	Food, nutraceuticals	Antioxidant, anti-inflammatory
Zeaxanthin	Corn, paprika, egg yolk	Xanthophyll, hydroxyl groups	Vision supplements, food	Antioxidant, prevents macular degeneration
Zeaxanthin-diglucoside	Maize, bacteria	Xanthophyll, glycosylated	Nutraceuticals, food	Antioxidant, eye health
α-Carotene	Pumpkins, carrots	Tetraterpene, hydrocarbon	Food, nutraceuticals	Pro-vitamin A, antioxidant
β-Carotene	Carrots, sweet potatoes, spinach	Tetraterpene, hydrocarbon	Food, cosmetics, supplements	Pro-vitamin A, antioxidant
β-Cryptoxanthin	Oranges, papaya, red peppers	Xanthophyll, hydroxyl groups	Pro-vitamin A, food	Antioxidant, supports immune function
β-Isorenieratene	Bacteria, fungi	Xanthophyll, methoxy group	Food, pharmaceuticals	Antioxidant, anticancer properties
γ-Carotene	Oranges, pumpkins	Carotene, single cyclization	Food, supplements	Antioxidant, vitamin A precursor

.

However, despite their utility, carotenoids face several challenges that limit their effectiveness and stability in various applications. These challenges arise from their chemical composition, especially the polyene chain, which is sensitive to environmental changes or factors such as light, oxygen, and temperature [134,135,136,137]. These issues are more relevant considering the molecular structure of carotenoids, illustrated with specific examples.

(a) Susceptibility towards oxidation: One of the most prominent structural issues with carotenoids is their susceptibility to oxidation. Carotenoids are composed of long conjugated polyene chains (alternating single and double bonds), which are responsible for their contrasting colors. However, these conjugated systems are highly reactive to oxygen. When carotenoids are exposed to oxygen, especially under heat or light, the polyene chain can break, leading to the formation of oxidized byproducts that often have a lesser or no color. For example, beta-carotene (the orange pigment found in carrots, sweet potatoes, and pumpkins) is a well-known example of a carotenoid that is prone to oxidation. Exposure to oxygen, especially under high temperatures or in the presence of light, causes beta-carotene to degrade into colorless or brownish compounds. This makes beta-carotene less stable in processed foods, such as juices and oils, unless proper packaging or stabilizing agents are used. Lycopene, the carotenoid responsible for the red color of tomatoes, watermelon, and red peppers, is also highly sensitive to oxidation. Oxidative degradation of lycopene leads to a loss of red color and a decrease in nutritional value, particularly its antioxidant properties. This instability limits the use of lycopene as a stable colorant in food products.

(b) Thermal degradation: Carotenoids are sensitive to high temperatures, which can cause degradation of their structure and color [59,138,139,140]. Heat can break the conjugated polyene chain, leading to the loss of the pigment’s characteristic color. This thermal degradation is particularly problematic in food processing, where high temperatures are often applied during cooking, pasteurization, or sterilization. Alpha-carotene, another carotenoid found in carrots and pumpkins, is highly susceptible to thermal degradation. When exposed to high temperatures, alpha-carotene loses its color and is often converted into colorless or brownish products. This instability limits the use of carotenoids in processed foods that require heat treatment. Lutein, a carotenoid found in leafy greens such as spinach and kale, is also prone to thermal degradation. When heated, lutein can degrade into less stable compounds, reducing its color intensity and its nutritional value as an antioxidant. This degradation process can reduce the overall quality of the product, particularly in processed food and supplements.

(c) Light sensitivity: Carotenoids are highly light-sensitive, particularly ultraviolet (UV) and visible light [135,141]. Light exposure can lead to the photodegradation of carotenoids, breaking the conjugated polyene chain and causing a loss of color. This structural sensitivity to light is a significant limitation for carotenoids used in commercial products that are exposed to light, such as beverages, cosmetics, and certain food products. For example, beta-carotene undergoes photodegradation when exposed to light, particularly UV light, which can cause it to lose its vibrant orange color. This is a common issue in products such as fruit juices, smoothies, and packaged foods, which are often stored in transparent containers that allow light to penetrate. Astaxanthin, a red carotenoid found in marine organisms like shrimp, salmon, and krill, is used in various supplements and food products. However, astaxanthin is highly sensitive to light, and exposure to UV light can degrade the pigment, reducing its color and antioxidant properties. This light sensitivity limits its use in clear packaging or products exposed to sunlight.

(d) Solubility: Carotenoids are generally lipophilic (fat-soluble), which means they tend to be soluble in fats and oils but show average aqueous solubility. This can create challenges in formulating carotenoids in aqueous systems, such as beverages, dairy products, and certain cosmetics. The poor aqueous solubility of carotenoids limits their bioavailability and their ability to be used effectively in water-based formulations. For example, lycopene is a lipophilic carotenoid, which means it has aqueous insolubility or limited solubility in hydroalcoholic solutions. This poor solubility can make it difficult to incorporate into water-based products like fruit juices or beverages without the use of emulsifiers or solvents. Additionally, its lipophilic nature can affect its absorption in the body when consumed in water-based foods or supplements. Lutein, which is used in eye health supplements, is also fat-soluble. Its low solubility in water presents challenges in the development of oral supplements or the fortification of water-based products. Specialized formulations like liposomal lutein or carotenoid emulsions are often used to improve their solubility and bioavailability.

(e) Interference with other ingredients: Carotenoids can sometimes interact with other ingredients in a formulation, leading to changes in their structure or color properties. These interactions can result in the loss of color intensity or the formation of undesirable compounds that reduce the effectiveness of carotenoids as colorants or nutraceutical ingredients. For example, beta-carotene can react with certain metals, such as iron or copper, forming complexes that reduce its stability and color intensity. These metal-carotenoid interactions can result in a dulling or darkening of the orange color, which is undesirable in food products where bright color is essential. The presence of high concentrations of these metals can aggravate this issue. Lycopene and other carotenoids can interact with certain proteins, oils, or fatty acids in food systems. These interactions may reduce their color intensity or cause them to precipitate out of the solution, leading to uneven color distribution in processed products.

(f) Isomerization and structural changes: Carotenoids can undergo isomerization, particularly under exposure to light (photoisomerization), or oxygen (oxidative isomerization). Isomerization involves the rearrangement of the double bonds within the polyene chain of carotenoids, resulting in the formation of different geometric isomers that may have different color properties. The isomerization of carotenoids can result in a loss of the desired color and reduced effectiveness as a pigment. For example, beta-carotene is known to undergo isomerization when exposed to heat or light. The most common isomerization occurs between the all-trans-beta-carotene and its cis-isomers, which can have different color properties and bioavailability. While cis-isomers are less intense in color than their all-trans counterparts, they may be more easily absorbed by the human body [142,143,144,145]. Color intensity is generally greater in trans-isomers, particularly all-trans forms, because their more linear and extended conjugated structures enable stronger and more specific light absorption, resulting in more vivid coloration. In contrast, cis-isomers introduce a bend in the molecular structure that disrupts conjugation, leading to a blue shift (absorption at shorter wavelengths) and reduced molar absorptivity, which produces a fainter color. This structural difference also influences their bioavailability. For example, in carotenoids such as lycopene and β-carotene, cis-isomers tend to be more soluble in mixed micelles during digestion, which enhances their intestinal absorption. Additionally, cis-isomers are less prone to crystallization within the food matrix, making them more accessible for uptake by the body [108,142,146]. However, the color loss associated with isomerization can affect the esthetic appeal of carotenoid-based products. Another example is lycopene also undergoes isomerization, and the trans-isomer (the most stable form) is typically the most intensely colored. However, exposure to heat or light can convert lycopene into various cis-isomers, which may have reduced color intensity and stability. The presence of these isomers in carotenoid formulations can affect the consistency and quality of the product.

(g) Biosynthesis, variability, and genetic factors: The content and composition of carotenoids can vary significantly across different plant species, cultivars, and ambient conditions. This variability can make it challenging to obtain consistent quality and color from natural carotenoid sources. The structure of carotenoids can also be influenced by genetic factors, leading to differences in their stability and color properties. For example, tomatoes are a common source of lycopene, but the concentration of lycopene can vary between different tomato varieties, while environmental conditions (such as nutrient richness in soil, pH of the soil, moisture content, temperature, and light exposure) can affect lycopene biosynthesis. This variability in the carotenoid composition may lead to inconsistencies in the color and stability of lycopene-derived colorants used in food or cosmetic products. Carrots are a source of beta-carotene, but the amount of beta-carotene in carrots can vary depending on the cultivar and growing conditions. This variability can affect the color intensity of beta-carotene, making it difficult to ensure consistency in carotenoid-based colorants.

3.4. Flavonoids-Based Natural Dyes and Pigments

Flavonoids are a large class of plant secondary metabolites that are widely distributed in fruits, vegetables, grains, and herbs, and are primarily responsible for the vibrant colors in many plants, including the reds, blues, and purples [147,148]. They also exhibit antioxidant, anti-inflammatory, and other potential health benefits [149,150,151,152,153,154,155,156,157,158,159]. Despite their wide array of beneficial properties and applications in food, medicine, and cosmetics, flavonoids face several structural challenges that limit their stability, bioavailability, and overall effectiveness in various formulations [160,161,162,163,164,165,166,167,168,169], as shown in

Table 3. Flavonoid-based natural dyes and pigments [147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,181,182,183,184,185,186,187,188,189,190].

Name	Source	Structural Specificity	Applications	Biological Activities
Amentoflavone	Selaginella species (Spikemoss)	Biflavonoid	Potential pharmaceuticals	Antioxidant; antiviral; neuroprotective
Apigenin	Matricaria chamomilla (Chamomile)	Flavone	Textile dye; food colorant; cosmetic applications	Antioxidant; anti-inflammatory; anticancer
Baicalein	Scutellaria baicalensis (Baikal skullcap)	Flavone	Traditional medicine; potential cosmetic applications	Antioxidant; anti-inflammatory; neuroprotective
Baohuoside I	Epimedium species (Horny Goat Weed)	Flavonol glycoside	Traditional Chinese medicine	Antioxidant; vasodilatory; neuroprotective
Bilobetin	Ginkgo biloba	Biflavonoid	Herbal medicine; supplements	Antioxidant; neuroprotective
Biochanin A	Red clover (Trifolium pratense)	Isoflavone	Dietary supplement; potential cosmetic applications	Phytoestrogenic activity; antioxidant; anticancer
Butin	Dalbergia sissoo (Indian Rosewood)	Flavanone	Traditional medicine	Antioxidant; anti-inflammatory
Carlinoside	Phlomis umbrosa	Flavone glycoside	Traditional medicine	Antioxidant; hepatoprotective
Chrysin	Passiflora caerulea (Blue passionflower)	Flavone	Dietary supplement; potential cosmetic applications	Antioxidant; anti-inflammatory; anxiolytic
Corylifolinin	Psoralea corylifolia (Babchi)	Flavone	Traditional medicine	Antioxidant; antifungal
Cryptostrobin	Cryptocarya species	Flavonoid	Traditional medicine	Antioxidant; antimicrobial
Daidzein	Soybeans (Glycine max)	Isoflavone	Food colorant; dietary supplement; cosmetic applications	Phytoestrogenic activity; antioxidant; anticancer
Derrone	Derris scandens (Climbing Derris)	Flavone	Traditional medicine	Antioxidant; anti-inflammatory
Echinatin	Glycyrrhiza glabra (Licorice)	Chalcone	Traditional medicine; cosmetics	Antioxidant; antimicrobial
Engeletin	Engelhardtia roxburghiana	Flavanone glycoside	Traditional medicine	Antioxidant; anti-inflammatory
Eriodictyol	Citrus fruits, Yerba Santa	Flavanone	Food additive; bitter blocker in flavors	Antioxidant; anti-inflammatory; neuroprotective
Eriodictyol-7-O-glucoside	Eriodictyon californicum (Yerba Santa)	Flavanone	Herbal medicine	Antioxidant; expectorant
Eupatilin	Artemisia asiatica (Mugwort)	Flavone	Traditional medicine; pharmaceuticals	Antioxidant; anti-inflammatory; gastroprotective
Eupatorin	Eupatorium perfoliatum (Boneset)	Flavone	Herbal medicine	Antioxidant; anticancer
Farrerol	Rhododendron spp.	Flavanone	Traditional Chinese medicine	Antioxidant; anti-inflammatory; antibacterial
Fisetin	Rhus cotinus (Smoke tree)	Flavonol	Textile dye; food colorant; cosmetic applications	Antioxidant; neuroprotective; anticancer
Formononetin	Red clover (Trifolium pratense)	Isoflavone	Dietary supplement; potential cosmetic applications	Phytoestrogenic activity; antioxidant; anticancer
Genistein	Soybeans (Glycine max)	Isoflavone	Food colorant; dietary supplement; cosmetic applications	Phytoestrogenic activity; antioxidant; anticancer
Gossypetin	Hibiscus flowers (Hibiscus sabdariffa)	Flavonol	Natural textile dye; food colorant	Antioxidant; antimicrobial; anti-inflammatory
Helichrysetin	Helichrysum species	Chalcone	Traditional medicine	Antioxidant; antimicrobial
Hesperidin	Citrus fruits (e.g., oranges, lemons)	Flavanone glycoside	Food colorant; dietary supplement; cosmetic applications	Antioxidant; anti-inflammatory; cardioprotective
Hinokiflavone	Hinokitiol (Japanese Cypress)	Biflavonoid	Traditional medicine	Antioxidant; antibacterial
Hispidulin	Salvia species (Sage)	Flavone	Traditional medicine; potential dietary supplement	Antioxidant; neuroprotective; anticonvulsant
Hispidulin	Salvia miltiorrhiza (Red Sage)	Flavone	Traditional medicine; supplements	Antioxidant; neuroprotective; anticonvulsant
Hispidulin	Marrubium vulgare (White Horehound)	Flavone	Herbal medicine; potential pharmaceutical use	Antioxidant; neuroprotective
Isobavachalcone	Psoralea corylifolia (Babchi)	Chalcone	Traditional medicine	Antioxidant; antimicrobial
Isoliquiritigenin	Glycyrrhiza uralensis (Chinese Licorice)	Chalcone	Traditional medicine; potential food coloring	Antioxidant; hepatoprotective
Isookanin	Achillea millefolium (Yarrow)	Flavonol	Herbal medicine	Antioxidant; antimicrobial
Isorhamnetin	Hippophae rhamnoides (Sea buckthorn)	Flavonol	Food colorant; dietary supplement; cosmetic applications	Antioxidant; anti-inflammatory; anticancer
Kaempferol	Ginkgo biloba leaves	Flavonol	Textile dye; food colorant; cosmetic applications	Antioxidant; anti-inflammatory; cardioprotective
Karanjin	Pongamia pinnata (Indian Beech)	Flavonol	Pesticide; medicine	Antioxidant; antimicrobial
Licarin A	Myristica fragrans (Nutmeg)	Flavonoid	Herbal medicine	Antioxidant; neuroprotective
Licoflavone A	Glycyrrhiza glabra (Licorice)	Flavone	Herbal medicine; cosmetics	Antioxidant; anti-inflammatory
Lippiflavone	Lippia species	Flavone	Herbal medicine	Antioxidant; antimicrobial
Lonchocarpol A	Lonchocarpus species	Flavonoid	Traditional medicine	Antioxidant; anti-inflammatory
Luteolin	Reseda luteola (Weld)	Flavone	Textile dye for silk and wool; painting pigment; manuscript illumination	Antioxidant; anti-inflammatory; antimicrobial
Methylnaringenin	Citrus species	Flavanone	Food additive; supplements	Antioxidant; anti-inflammatory
Morin	Maclura pomifera (Osage orange)	Flavonol	Textile dye; food colorant; wood staining	Antioxidant; anti-inflammatory; anticancer
Myricetin	Myrica rubra (Bayberry)	Flavonol	Food colorant; dietary supplement; cosmetic applications	Antioxidant; anti-inflammatory; neuroprotective
Naringenin	Grapefruit, tomatoes	Flavanone	Food colorant; dietary supplement	Antioxidant; cardioprotective
Naringin	Citrus paradisi (Grapefruit)	Flavanone glycoside	Food colorant; dietary supplement; cosmetic applications	Antioxidant; cholesterol-lowering; anticancer
Neobavaisoflavone	Psoralea corylifolia (Babchi)	Isoflavone	Traditional medicine	Antioxidant; anti-inflammatory
Nobiletin	Citrus peel (Citrus reticulata)	Flavone	Functional food ingredient; dietary supplement	Antioxidant; anti-inflammatory; neuroprotective
Pectolinarigenin	Cirsium japonicum (Japanese Thistle)	Flavone	Traditional medicine	Antioxidant; anti-inflammatory
Pinocembrin	Eucalyptus spp., propolis	Flavanone	Cosmetic applications; pharmaceutical potential	Antioxidant; antibacterial; neuroprotective
Pinostrobin	Boesenbergia rotunda (Fingerroot)	Flavanone	Traditional medicine	Antioxidant; anti-inflammatory
Quercetin	Tagetes spp. (Marigold petals)	Flavonol	Textile dye; food colorant; dietary supplement	Antioxidant; antiviral; anticancer
Retusin	Alnus glutinosa (Black Alder)	Flavonol	Natural dye; herbal medicine	Antioxidant; antimicrobial
Robinetin	Robinia pseudoacacia (Black Locust)	Flavonol	Natural dye; cosmetics	Antioxidant; antimicrobial
Rutin	Sophora japonica (Japanese pagoda tree)	Flavonol glycoside	Textile dye; food colorant; dietary supplement	Antioxidant; vascular protection; anti-inflammatory
Sakuranetin	Prunus species (Cherry blossoms)	Flavanone	Potential pharmaceutical and cosmetic applications	Antioxidant; anti-inflammatory; antifungal
Sappanchalcone	Caesalpinia sappan (Sappanwood)	Chalcone	Natural dye; herbal medicine	Antioxidant; antimicrobial
Scutellarein	Scutellaria lateriflora (Skullcap)	Flavone	Herbal medicine; supplements	Antioxidant; neuroprotective
Sibiricaxanthone B	Hypericum perforatum (St. John’s Wort)	Flavone	Traditional medicine	Antioxidant; antidepressant
Silymarin	Silybum marianum (Milk thistle)	Flavonolignan complex	Liver health supplement; potential cosmetic applications	Hepatoprotective; antioxidant; anti-inflammatory
Sophoranone	Sophora flavescens (Shrubby Sophora)	Flavonoid	Herbal medicine	Antioxidant; anti-inflammatory
Sophoricoside	Sophora japonica (Japanese Pagoda Tree)	Flavonol glycoside	Traditional medicine; dietary supplements	Antioxidant; anti-inflammatory; anticancer
Sotetsuflavone	Cycas revoluta (Sago Palm)	Biflavonoid	Traditional medicine	Antioxidant; anticancer
Tangeretin	Tangerine peel (Citrus tangerina)	Flavone	Food colorant; dietary supplement	Antioxidant; anticancer; neuroprotective
Taxifolin	Siberian larch (Larix sibirica)	Flavanonol	Food colorant; dietary supplement; cosmetic applications	Antioxidant; anti-inflammatory; cardioprotective
Tectochrysin	Tectona grandis (Teak)	Flavone	Traditional medicine	Antioxidant; anti-inflammatory
Tricetin	Eucalyptus globulus (Eucalyptus leaves)	Flavone	Textile dye; potential cosmetic applications	Antioxidant; anti-inflammatory; antiviral
Wogonin	Scutellaria baicalensis (Baikal skullcap)	Flavone	Traditional medicine; potential cosmetic applications	Antioxidant; anti-inflammatory; anticancer
Xanthohumol	Humulus lupulus (Hops)	Chalcone	Beer additive; dietary supplement	Antioxidant; anticancer

. Being polyphenolic in nature, these compounds are among the most extensively studied natural colorants for dyeing cellulose-based fibers [170,171,172,173,174,175]. However, due to the inherently low dyeability of cellulose, the fibers are typically modified using chemical processes to activate the surface and enhance dye absorption [176,177,178,179,180]. In this context, the most practical approach for utilizing such polyphenols may involve the use of blended cellulose-based fibers, such as polycotton.

There are certain issues associated with flavonoids, most of which arise from their chemical structures. These structures include aromatic rings and conjugated systems that can be reactive to environmental factors such as light, oxygen, and pH. Key structural issues associated with flavonoids, along with examples.

(a) Sensitivity towards oxidation: Flavonoids are highly sensitive to oxidative conditions due to their aromatic structures with hydroxyl groups (-OH) attached to the benzene rings. These hydroxyl groups are prone to react with oxygen, leading to the formation of oxidized products that often lack color or bioactivity. The oxidation process can result in the degradation of the flavonoid and the loss of both color and beneficial properties (such as antioxidant or other therapeutic efficacies). For example, quercetin (3,3′,4′,5,7-pentahydroxyflavone), a flavonoid found in apples, onions, and kale, is highly susceptible to oxidation. When exposed to oxygen, quercetin can form oxidized products, leading to the loss of its yellow color and a reduction in its antioxidant capabilities. This oxidation also limits its use in food products where stable color retention is essential. Catechins (such as epicatechin and epigallocatechin gallate), which are abundant in green tea, are also prone to oxidation. When catechins are oxidized, they form degradation products, such as theaflavins and thearubigins, which alter the color and flavor of the tea. The oxidation of catechins also reduces their antioxidant capacity, making them less effective in promoting health.

(b) Thermal decomposition and degradation: At high temperature or prolong heating, flavonoids tend to degrade chemically, breaking down the polyphenolic structure of flavonoids, leading to the loss of color and bioactivity [191]. This structural instability is a concern for the use of flavonoids in food processing, where heat treatment (e.g., cooking, pasteurization, or sterilization) is often involved. Apigenin (a flavonoid found in parsley, celery, and chamomile), a yellow-flavored flavonoid, undergoes thermal degradation when exposed to high temperatures, especially during cooking or food processing. The prolonged heating breaks down the flavonoid’s aromatic ring system, causing it to lose its color and potentially reduce its health benefits. However, in certain food products, the application of thermal energy through microwave treatment has been shown to enhance antioxidant activity. For example, Polish researchers investigated how microwave processing affects the physical properties and bioactive compound content of acorn flour, as well as its subsequent impact on muffin quality [192]. Both dry and hydrated forms of acorn flour (using a 1:1 flour-to-water ratio, w/v) were subjected to microwave heating at various power levels (925, 1295, and 1850 W) for different durations (0.5, 1.0, 1.5, 2.0, and 3.0 min).

The treated flours were then incorporated into composite wheat–acorn muffins, which were evaluated for physical characteristics such as volume, porosity, crumb firmness, and color. The muffins’ chemical composition was also analyzed, with particular attention to antioxidant activity and the content of bioactive compounds, including total free phenolics, flavonoids, and tannins. The study revealed that microwave processing significantly influenced both the antioxidant potential and technological properties of the muffins. Notably, treatment at 925 W for 3 min or 1295 W for 1.5 min under moist conditions resulted in muffins with enhanced antioxidant capacity—demonstrating increases in total free phenolics (up to 61.4%) and flavonoids (up to 135.9%), as well as a reduction in tannins (up to 38.2%)—alongside improvements in volume and sensory appeal. In contrast, exposure to the highest power level (1850 W) adversely affected muffin volume, texture, and porosity. These findings underscore the potential of controlled microwave treatment to improve both the nutritional and technological quality of bakery products enriched with acorn flour [192].

Another Polish researcher from the University of Life Sciences in Lublin, Poland, studied the fruits of the common quince (Cydonia oblonga), known for their extensive health benefits due to their rich composition [193]. However, because of the fruit’s hard texture and astringency, consumers rarely eat it raw. Therefore, selecting an appropriate processing method, including heat treatment, is crucial to maintaining the quince’s quality. The study investigated the impact of freeze drying and convection drying at two temperatures (40 °C and 60 °C) on the physicochemical, bioactive, and antioxidant characteristics of Cydonia oblonga fruits. Results showed that freeze drying best preserved the fruit’s qualities, producing samples most similar to fresh fruit. These freeze-dried samples exhibited the highest rehydration rate (3.53 ± 0.04), the lowest shrinkage (9.87 ± 0.29%), and the lowest bulk density (0.41 ± 0.01 g/cm³). Additionally, freeze drying maintained the brightest fruit color (L* = 75.70 ± 1.71) and the highest total acidity (1.34 ± 0.01 g/100 g DM). While drying reduced tannin content overall, no significant differences were observed between freeze-dried and convection-dried samples at 40 °C and 60 °C. Notably, freeze-dried quince retained high levels of polyphenols (233.56 ± 5.96 mg GEA/100 g DM), flavonoids (36.79 ± 0.74 mg EPI/100 g DM), and antioxidant activities measured by ABTS (364.51 ± 9.12 µM Trolox/100 g DM) and DPPH (258.78 ± 5.16 µM Trolox/100 g DM). The greatest losses in these bioactive compounds and antioxidant capacity were found in fruits dried by convection at 60 °C. Together, these studies emphasize that flavonoids and other bioactive compounds can be effectively preserved using modern processing strategies—if researchers carefully select and optimize the techniques employed [193].

(c) Sensitivity to pH changes: Due to the presence of phenolic groups, flavonoids exhibit different colors depending on the pH of their environment [194], as they can form both protonated and deprotonated species. These polyphenolic –OH groups have different pKa values, meaning they become protonated or deprotonated at different pH levels. As a result, flavonoids are pH-sensitive, which affects their color stability and, to some extent, limits their applications in food products where pH changes commonly occur during processing or storage. At extreme pH values, the flavonoid molecules may undergo chemical breakdown and structural changes, leading to a loss of color or bioactivity (if present). Chalcones, a specific subclass of flavonoids, are considered precursors in the biosynthesis of various flavonoid compounds in nature. Chemically, they are characterized by two aromatic rings connected by a three-carbon α,β-unsaturated carbonyl system. Chalcones serve as intermediates in the biosynthesis of flavonoids such as flavones, flavonols, and isoflavones, with the specific conversion depending on the enzyme class and species involved. Several food sources naturally contain chalcones, including apples, citrus fruits (such as grapefruit and oranges), berries (like strawberries, blueberries, and raspberries), carrots, red bell peppers, tomatoes, spinach, onions, and other common fruits and vegetables. However, because chalcones are structural precursors and lack the typical heterocyclic oxygen ring found in other flavonoids, they tend to be more reactive—particularly toward nucleophilic attack. This reactivity makes them less stable and more sensitive to pH changes. For example, liquiritigenin (found in licorice) is pH-sensitive and prone to structural changes that can affect its color properties and stability. In alkaline solutions or at higher pH values, chalcones can undergo isomerization and structural rearrangement, resulting in color loss or the formation of unwanted compounds.

(d) Light sensitivity (photodegradation): Flavonoids are also prone to light-induced degradation, or chemical changes, especially ultraviolet (UV) light exposure [195,196,197,198,199,200]. The energy from UV light can cause the aromatic rings in flavonoids to undergo photochemical reactions [201], leading to structural changes and a loss of color or bioactivity. The photodegradation sensitivity of flavonoids poses a significant challenge for their use in light-exposed products such as beverages, cosmetics, and natural dyes. For example, Quercetin is particularly sensitive to UV light, and when exposed to light, it can undergo photodegradation, resulting in the breakdown of its structure and a loss of color and antioxidant activity. This is one of the reasons why quercetin is often used in products with opaque packaging to prevent light exposure. Luteolin (found in celery, parsley, and thyme) also undergoes photodegradation when exposed to light, especially in the presence of oxygen. This leads to a breakdown of its structure, resulting in a loss of its yellow color and potential reduction in its beneficial properties. Light exposure during storage or in product formulations can limit luteolin’s stability and effectiveness.

(e) Isomerization and structural changes: Flavonoids, particularly those with conjugated systems, can undergo isomerization, where the double bonds in the molecule are rearranged, leading to different isomers with distinctive physicochemical properties. The change in structure or isomerization can bring changes in color properties, stability, and bioactivity of that flavonoid. Depending on the conditions (e.g., heat, light, pH), flavonoids can change from one form to another, leading to a loss of color and bioactivity. For example, Isoflavones, such as genistein (found in soybeans), can undergo isomerization, which alters their biological activity and solubility. The trans and cis isomers of genistein exhibit different levels of estrogenic activity, and this can influence their effectiveness in health applications, especially in the context of hormonal therapies or dietary supplements [202,203,204,205,206]. Kaempferol (found in kale, spinach, and tea) can also undergo isomerization when exposed to heat, light, or acidic conditions. The isomerization of kaempferol leads to the formation of compounds that are less effective as antioxidants and may exhibit reduced color intensity.

(f) Instability in aqueous environments: Flavonoids are typically water-soluble, but their solubility can vary depending on the chemical structure. Certain flavonoids can undergo precipitation or aggregation in aqueous environments, especially at high concentrations, which can affect their stability, color, and bioavailability. Additionally, some flavonoids may degrade when exposed to water, leading to a loss of color and functionality. For example, Rutin (a flavonoid glycoside found in citrus fruits and buckwheat) is a water-soluble flavonoid, but it can undergo degradation when exposed to water, leading to a loss of its antioxidant properties. The stability of rutin in aqueous environments can be compromised, especially in the presence of light or heat. Hesperidin (found in citrus fruits like oranges) is another flavonoid glycoside that is water-soluble but can suffer from instability in aqueous solutions. The molecule may undergo hydrolysis (where the glycosidic bond is broken), which leads to the loss of its color and may reduce its health benefits, particularly its antioxidant properties.

(g) Interaction with metal ions: Flavonoids, especially those with hydroxyl groups, can interact with metal ions such as iron, copper, and magnesium [207,208,209,210,211]. These interactions can affect the stability and color of flavonoids, leading to the formation of metal-flavonoid complexes that may precipitate out of solution or result in color changes. For example, Quercetin has been shown to form complexes with iron and copper, which can affect its stability and color. These metal-flavonoid complexes can also reduce quercetin’s antioxidant activity, limiting its effectiveness in food products or supplements. Catechins (such as epicatechin and epigallocatechin gallate, found in green tea) can also interact with metal ions, leading to a loss of color and bioactivity. These interactions can reduce their solubility and their ability to scavenge free radicals effectively.

3.5. Chlorophyll-Based Natural Dyes and Pigments

Chlorophyll-based colorants, derived from the chlorophyll pigment found in plants, are often used in various industries, including food, cosmetics, and textiles, due to their natural, vibrant green color [212,213,214,215,216,217,218,219]. Various chlorophyll pigments and their therapeutic benefits are summarized in

Table 4. Chlorophyll-based natural dyes and pigments [212,213,214,215,216,217,218,219,220,221,222,223,224,225,226].

Name	Source	Structural Specificity	Applications	Biological Activities
Bacteriochlorophyll a	Purple bacteria	Similar to chlorophyll a but modified	Bacterial photosynthesis	Helps anoxygenic photosynthesis
Bacteriochlorophyll b	Green sulfur bacteria	Similar to chlorophyll b	Bacterial photosynthesis	Photosynthetic function
Bacteriochlorophyll c	Green bacteria	Modified porphyrin ring	Bacterial photosynthesis	Light-harvesting role
Bacteriochlorophyll d	Green bacteria	Structural variant	Bacterial photosynthesis	Energy conversion
Bacteriochlorophyll e	Green bacteria	Highly modified porphyrin structure	Light-harvesting	Alternative to chlorophyll in bacteria
Bacteriochlorophyll f	Green bacteria	Modified bacteriochlorophyll c	Bacterial photosynthesis	Supports light absorption
Chlorin e6	Semi-synthetic from chlorophyll	Partially hydrogenated porphyrin ring	Photodynamic therapy	Antimicrobial, anticancer
Chlorophyll a	Green plants, algae	Methyl group at C7, phytyl ester at C17, Mg center	Photosynthesis	Antioxidant properties
Chlorophyll b	Green plants, algae	Formyl group at C7, phytyl ester at C17, Mg center	Photosynthesis	Antioxidant properties
Chlorophyll c	Diatoms, brown algae	Lacks phytyl chain, porphyrin ring	Light absorption in algae	Supports photosynthesis
Chlorophyll d	Red algae, cyanobacteria	Formyl group at C3	Far-red light absorption	Photosynthetic function
Chlorophyll f	Cyanobacteria	Formyl group at C2	Absorbs infrared light	Alternative light-harvesting pigment
Chlorophyllide a	Enzymatic breakdown of chlorophyll a	Lacks phytyl chain	Intermediate in metabolism	Participates in biosynthesis
Chlorophyllide b	Enzymatic breakdown of chlorophyll b	Lacks phytyl chain	Chlorophyll metabolism studies	Involved in plant biochemistry
Chlorophyllin	Alkaline hydrolysis of chlorophyll	Water-soluble derivative	Food colorant, health supplements	Detoxifying, anti-inflammatory
Co-Chlorophyllin	Cobalt-substituted chlorophyllin	Co replaces Mg	Industrial colorants	Used in synthetic chemistry
Copper Chlorophyllin	Chlorophyllin with Cu substitution	Copper replaces Mg	Natural food colorant	Antioxidant, potential anticancer
Dihydro-Chlorophyll	Reduced chlorophyll derivative	Hydrogenation at C7–C8	Biomedical research	Potential therapeutic applications
Ethyl Chlorophyllide	Oxidized chlorophyll derivative	Ethyl ester modification	Biochemical research	Intermediate in metabolism
Fe-Chlorophyllin	Iron-substituted chlorophyllin	Fe replaces Mg	Biomedical research	Possible enzyme mimic
Hydroxy-Chlorophyll	Hydroxylated chlorophyll derivative	Hydroxylation at various positions	Research on chlorophyll modifications	Antioxidant potential
Methyl Chlorophyllide	Oxidized chlorophyll derivative	Methyl ester modification	Plant biochemistry studies	Intermediate in biosynthesis
Mg-Chlorin	Modified chlorophyll structure	Mg at center, hydrogenated porphyrin	Phototherapy	Photosensitizer
Ni-Chlorophyllin	Nickel-substituted chlorophyllin	Ni replaces Mg	Synthetic pigment research	Potential catalytic applications
Pheophorbide a	Chlorophyll a breakdown product	Lacks phytyl chain, Mg replaced by hydrogen	Photodynamic therapy	Photosensitizer
Pheophorbide b	Chlorophyll b breakdown product	Lacks phytyl chain, Mg replaced by hydrogen	Cancer treatment research	Photosensitizer
Pheophytin a	Chlorophyll a degradation	Mg replaced by hydrogen	Food colorant	Antioxidant properties
Pheophytin b	Chlorophyll b degradation	Mg replaced by hydrogen	Food colorant	Antioxidant properties
Phytyl Chlorophyllide	Partial hydrolysis product	Retains phytyl chain	Chlorophyll metabolism studies	Biosynthesis intermediate
Protochlorophyllide	Chlorophyll precursor	Lacks phytol chain	Essential in chlorophyll biosynthesis	Involved in plant development
Pyropheophorbide a	Thermal degradation of pheophorbide a	Decarboxylation at C132	Medical applications	Photosensitizing properties
Pyropheophorbide b	Thermal degradation of pheophorbide b	Decarboxylation at C132	Cancer research	Photosensitizer
Pyropheophytin a	Heat-treated chlorophyll a	Decarboxylation at C132	Indicator of food processing	Antioxidant potential
Pyropheophytin b	Heat-treated chlorophyll b	Decarboxylation at C132	Food quality assessment	Antioxidant properties
Zn-Chlorophyllin	Zinc-replaced chlorophyllin	Zn replaces Mg	Research in metal-substituted pigments	Potential antimicrobial

.

However, several structural issues can affect the stability, appearance, and functionality of chlorophyll-based colorants. These issues often arise from the pigment’s inherent chemical instability, degradation processes, or improper extraction methods. There are some structural issues of chlorophyll-based colorants.

(a) Chlorophyll degradation: Chlorophyll-based colorants are highly susceptible to degradation when exposed to light, heat, and oxygen. This degradation can lead to fading (photofading) or color change, which is a common issue in the use of chlorophyll-based colorants. For example, degradation in food products, where chlorophyll pigments are present in food items, such as spinach or parsley powder, start degrading with prolonged exposure of heat (or warm conditions), leading to them starting to lose the contrast of their green color. For example, chlorophyll is often unstable in high-temperature cooking processes, causing a brownish color to develop. Meanwhile, chlorophyll-based colorants in cosmetic formulations, such as green shampoos or lotions, can lose their vibrancy when exposed to light and air. For instance, chlorophyllin, a water-soluble derivative of chlorophyll, can darken and lose its green color in skincare products over time due to oxidation. However, chlorophyll-based dyes used for coloring textiles can fade when exposed to sunlight and washing, especially if the fabric is not treated properly. For example, the use of chlorophyll as a natural dye in wool or cotton may result in fading after repeated exposure to UV light.

(b) Photobleaching: Chlorophyll-based colorants are prone to photobleaching, a process in which the color fades or disappears due to prolonged exposure to light, particularly ultraviolet (UV) light. For example, Chlorophyll-based colorants in food packaging materials (e.g., in green-colored wrappers) can undergo photobleaching when exposed to sunlight, causing the color to fade. For example, chlorophyll colorants in herb-flavored snacks may lose their green color if exposed to UV rays. In cosmetics, products containing chlorophyll-based colorants, such as green toothpaste or face masks, can undergo photobleaching when exposed to light, causing the color to deteriorate quickly. Chlorophyll-based pigments used in green paints or coatings may experience photobleaching when exposed to sunlight, leading to a duller appearance of the painted surface over time.

(c) pH sensitivity affecting the structure and color properties: Chlorophyll-based colorants are highly sensitive to pH changes [134,227,228,229]. Variations in pH can alter the structure of the chlorophyll molecule, leading to a change in its color or complete loss of colorant properties. In products like green juices or herbal teas, chlorophyll-based colorants can change from green to brown or yellow when exposed to acidic conditions, such as when the product is mixed with lemon juice or citric acid. This color change is due to the protonation of chlorophyll molecules under acidic pH conditions. Chlorophyll derivatives in green creams or lotions may change color when the formulation’s pH is adjusted. In highly acidic environments, the green hue may shift toward a brownish color, resulting in an unattractive appearance in the product. Chlorophyll-based dyes used in textile industries may show a color shift when exposed to high pH conditions, especially during washing. This can cause an unintended change in the final color of dyed fabrics, resulting in less vibrant greens.

(d) Oxidation and rancidity: Oxidation is a common problem for chlorophyll-based colorants, especially in food and cosmetic products, where exposure to oxygen can lead to rancidity and loss of the colorant’s effectiveness. This occurs because chlorophyll molecules are sensitive to oxidative degradation. For example, chlorophyll-based colorants like chlorophyllin used in processed food products (e.g., candies, green sauces, or ice creams) can oxidize, leading to a brownish or dull appearance. This is particularly problematic in products that are stored for long periods. Chlorophyll-based colorants used in products such as green facial masks or moisturizers may lose their color over time if exposed to air, particularly in products with poor packaging that does not prevent oxidation. This can lead to an undesirable shift in the product’s appearance. Chlorophyll-based dyes in fabrics can oxidize over time, causing the color to fade or shift, especially in environments with high humidity or poor storage conditions.

(e) Structural Instability at high temperatures: Chlorophyll-based colorants are sensitive to high temperatures, as when subjected to heating, chlorophyll molecules can undergo structural changes, losing in Mg²⁺, resulting in the formation of degraded compounds (pheophytins) known for loss of their original colorant properties or a shift in color. For example, chlorophyll-based colorants used in food, such as those derived from spinach or kale, or broccoli, often lose their vibrant green color when exposed to high temperatures during cooking [230]. Another example is the stability of the green color in 1–60% ethanolic solutions of chlorophyll a, which exhibits color loss following first-order reaction kinetics, similar to that in aqueous systems [231]. Additionally, color loss increases with temperature and varies with ethanol concentration. For example, blanching spinach leaves to extract chlorophyll may result in the loss of bright green hues and a dull, olive color instead. In lipsticks, eyeshadows, or hair dyes that incorporate chlorophyll-based colorants, exposure to high temperatures during manufacturing or storage can result in the fading or alteration of the intended color. When using chlorophyll-based colorants to dye fabrics, exposure to high temperatures during the dye-setting process may cause the color to fade or change. This is especially problematic for materials that require high-temperature treatment.

(f) Inconsistent extraction and purification: The extraction and purification process of chlorophyll-based colorants is often challenging. Variability in the extraction method can lead to inconsistent pigment quality, resulting in color variations in the final product. For example, variations in the extraction process of chlorophyll from different plants (e.g., alfalfa or spinach) can lead to slight differences in the shade of green in food products. For example, chlorophyll extracted from spirulina might have a different hue compared to chlorophyll from parsley. Chlorophyll-based colorants used in cosmetics may have varying purity levels depending on the extraction method. Inconsistent extraction could lead to a product that is either too pale or too dark in color, leading to product inconsistency in the marketplace. The dyeing process with chlorophyll extracts may yield fabrics with varying shades of green due to differences in chlorophyll content and extraction methods. This inconsistency can be problematic for mass production, especially when uniform color is essential.

(g) Competition with other natural colorants: Chlorophyll-based colorants may face competition from other natural colorants, which could offer better stability or performance under specific conditions. This can reduce the demand for chlorophyll-based alternatives. For example, chlorophyll-based colorants may be replaced by spirulina, matcha, or green tea extracts, which offer similar green hues but greater stability in certain food products. These alternatives might be preferred due to better resistance to heat or oxidation. In cosmetics, green tea extract or algae-derived colorants might replace chlorophyll due to their superior stability in formulations, making chlorophyll less popular in certain cosmetic applications. Chlorophyll-based dyes can be substituted with indigo or chlorophyll derivatives that offer better performance in terms of lightfastness or washability.

Microbial Dyes and Pigments: The rising demand for natural colorants across industries is largely fueled by growing consumer concerns over the health risks of synthetic dyes. As regulatory agencies increase scrutiny of synthetic colorants, natural alternatives are gaining market traction. Microbial sources—bacteria, fungi, and algae—are becoming prominent due to their rapid growth, ease of cultivation, and adaptability to diverse environments. Microbial pigments offer a promising alternative to synthetic and plant-based dyes. Unlike many synthetic dyes, which may pose toxicity, allergenic, or carcinogenic risks, microbial pigments are generally safer and more environmentally friendly. Besides imparting color, many microbial pigments provide health benefits such as antioxidant, antimicrobial, and anticancer effects, making them attractive for pharmaceuticals and functional foods.

Compared to plant pigments, microbial production is more predictable, efficient, and scalable. Microorganisms produce higher, more consistent pigment yields year-round in controlled fermenters, independent of seasonal or geographic constraints. Plant cultivation requires more land and resources and is vulnerable to environmental variations. Additionally, pigment extraction from microbes tends to be simpler and less labor-intensive, improving cost efficiency.

Microbial pigments have diverse applications. In cosmetics, pigments like lycopene (from Anabaena vaginicola), β-carotene, astaxanthin (from Haematococcus pluvialis and marine yeast), lutein, and zeaxanthin are valued for their vibrant colors and antioxidant, photoprotective properties in sunscreens and anti-aging products. Melanin from Halomonas venusta and Streptomyces bellus is used for natural pigmentation and UV protection in products like lip balms. Phycobiliproteins (phycocyanin from Spirulina and phycoerythrin from Spirulina and Porphyridium) produce blue and red hues in lipsticks and eyeshadows. Emerging pigments such as ankaflavin (Monascus spp.), chlorophyll (from microalgae), and indigoidine (Streptomyces chromofuscus) are under investigation. In food packaging, microbial pigments enhance both appearance and function of biodegradable materials. For example, pigments from Talaromyces amestolkiae impart yellow to red hues and antioxidant activity to cassava starch films, potentially extending shelf life in products like butter. These uses are being optimized for pigment concentration and efficacy. The food industry employs several microbial pigments: riboflavin (from Ashbya gossypii) imparts yellow and is common in dairy, sauces, and baby food; β-carotene (from Dunaliella salina and Blakeslea trispora) offers orange-red tones and vitamin A supplementation; canthaxanthin (from Bradyrhizobium spp.) colors food and feed orange-pink; prodigiosin (from Serratia marcescens) colors yogurt and beverages; and phycocyanin (from Spirulina) adds blue hues to sweets and frozen desserts. Other notable pigments include melanin, Monascus pigments, astaxanthin (Xanthophyllomyces dendrorhous), and lycopene (Blakeslea trispora). Usage levels vary; for example, β-carotene is often used as a 30% micronized suspension, while astaxanthin is critical in aquaculture feeds. Pharmaceutical applications explore microbial pigments for their dual roles as colorants and bioactives. Beta-carotene, astaxanthin, canthaxanthin, and riboflavin are prized antioxidants with nutritional benefits. Pigments like prodigiosin, violacein, and pyocyanin exhibit anticancer and antimicrobial activities. Others—such as scytonemin, rubrolone, undecylprodigiosin, and naphthoquinones from Cordyceps unilateralis—show diverse pharmacological effects. While in vitro efficacy data exist, safe human dosage ranges require further study.

Safety and regulation are paramount. Food-grade pigments must be free from mycotoxins (e.g., citrinin in some Monascus strains) and microbial contaminants. Allergenicity must be assessed. Cosmetics require stringent hygiene and preservation to prevent irritation or infection. Pharmaceuticals enforce strict GMP standards to maintain sterility and prevent cross-contamination. Regulatory bodies like the FDA and EFSA mandate safety data and concentration limits for food colorants. Cosmetic regulations under the FD&C Act, MoCRA, and the EU Cosmetics Regulation require proper labeling and safety. Pharmaceuticals require comprehensive toxicological evaluation before approval.

Natural Bacterial dyes and pigments: Among microbial sources, bacteria are notable for their rapid growth and ability to thrive on inexpensive media, making them well-suited for industrial-scale pigment production. Productivity depends on strain and fermentation conditions, requiring careful optimization. Using low-cost substrates such as agro-industrial waste can enhance cost-effectiveness, though some bacterial pigments may still be pricier than synthetic dyes. Bacteria produce a wide spectrum of pigments across various chemical classes. Examples include the red prodigiosin (Serratia marcescens), yellow carotenoids and staphyloxanthin (Staphylococcus aureus), blue phycocyanin (cyanobacteria), and purple violacein (Chromobacterium violaceum). Bacterial pigments fall mainly into carotenoids, prodiginines, phycobiliproteins, xanthomonadins, and violacein. Carotenoids are isoprenoids with conjugated double bonds responsible for yellow to red hues and antioxidant activity. Prodiginines feature a tripyrrole core, producing red pigments with anticancer, antimicrobial, and immunosuppressive effects. Phycobiliproteins (e.g., phycocyanin) are water-soluble blue proteins used in food and supplements. Xanthomonadins are yellow aryl-polyene pigments protecting against UV radiation. Violacein is a purple indolocarbazole with antibacterial, antiviral, and anticancer properties. Minor structural variations—such as double bond number, functional groups, and stereochemistry—influence pigment color and biological function. β-carotene is common in foods and pharmaceuticals; prodigiosin in food and cosmetics; phycocyanin in food and beauty products; and violacein is under investigation for multiple industries. A number of bacterial strains have been evaluated for the production of pigments, especially those targeted for the food industry. Furthermore, numerous applications of bacterial pigments have been discussed in recent articles, which can offer a deeper understanding of specific bacterial pigments; therefore, readers are encouraged to explore them further for additional insights [108,232,233,234,235,236,237,238,239].

Natural Fungal dyes and pigments: Fungi produce diverse pigments including carotenoids (yellow to orange), melanins (black and brown), azaphilones (yellow to purple–red), quinones (orange to purple), and flavins (yellow riboflavin). Melanins provide UV and oxidative protection, azaphilones have a pyranoquinone bicyclic core responsible for their vivid hues and reactivity, and quinones have antimicrobial and anticancer properties. Riboflavin serves as a widely used nutritional supplement and food additive. Fungal pigment structural diversity arises from polyketide variations such as chain length, cyclization, and functional groups. Chromophores and conjugated double bonds determine light absorption and color. Applications include Monascus pigments in food, β-carotene in food and pharma, riboflavin supplements, and melanin in cosmetics and pharma. Filamentous fungi are favored for ease of cultivation, rapid growth, and high pigment yields, especially via solid-state fermentation. Like bacteria, fungi can utilize inexpensive agro-industrial residues, though extraction and purification may increase costs. Fungal pigments cover a broad color spectrum: yellow, orange, red, purple, blue, brown, and black. Key producers include Monascus spp. (red, yellow, orange), Penicillium spp. (red, yellow, brown), Aspergillus spp. (yellow, brown), Fusarium spp. (pink, violet, red), and Blakeslea trispora (β-carotene). These natural fungal pigments serve as sustainable dye alternatives with added antimicrobial and antioxidant benefits, expanding their industrial uses. Numerous applications of fungal pigments have been discussed in recent articles. These publications offer a deeper understanding of specific fungal pigments; therefore, readers are encouraged to explore them further for additional insights [240,241,242,243,244,245,246,247,248,249,250,251,252].

Natural Algal dyes and pigments: Algae form a third important source of natural pigments, cultivated sustainably in open ponds or closed systems. Their synergy with wastewater treatment and use of sunlight enhances environmental benefits. Despite these advantages, achieving high pigment yields remains challenging and is an active research area. Large-scale cultivation requires significant upfront investment. Algal pigments include green chlorophylls; yellow, orange, and red carotenoids (notably astaxanthin); and blue phycocyanin. Algae are especially rich in carotenoids and phycobiliproteins, providing vivid color options. Important pigment producers are Dunaliella salina (β-carotene), Haematococcus pluvialis (astaxanthin), and Spirulina spp. (phycocyanin). Algal pigments are classified into chlorophylls, carotenoids, and phycobiliproteins. Chlorophylls are green photosynthetic pigments with a porphyrin ring and central magnesium ion, existing in forms a to f with differing light absorption. Carotenoids resemble those in bacteria and fungi, serving as antioxidants and photoprotectants. Phycobiliproteins (e.g., blue phycocyanin and red phycoerythrin) are water-soluble proteins involved in light harvesting, widely used in food and cosmetics. Structural pigment variations arise from chlorophyll side chains and functional groups on carotenoids and phycobiliproteins. The arrangement of conjugated double bonds governs their light absorption and colors. Commercial examples include β-carotene in food and nutraceuticals; astaxanthin in food, aquaculture, and cosmetics; and phycocyanin in food coloring and cosmetics. There are a number of applications of algae pigments that have recently been discussed in several articles. These readings will provide a deeper understanding of specific algal pigments; therefore, readers are encouraged to study them for further insights [30,235,253,254,255,256,257,258,259,260,261,262,263,264].

Table 5. Microbial pigments and dyes from various sources: bacteria [108,232,234,235,236,237,238,239], fungi [240,243,249,265,266], and algae [30,235,255,256,257,258,259,260,261].

Name	Source	Microorganism Name	Structural Specificity	Applications	Biological Activities
Astaxanthin	Microalgae	Haematococcus pluvialis	Carotenoid, xanthophyll	Food colorant, cosmetics, supplements	Antioxidant, anti-inflammatory, anticancer
Beta-Carotene	Fungi, Yeast	Blakeslea trispora	Carotenoid, provitamin A	Food colorant, supplements, cosmetics	Antioxidant, immune system booster, eye health
Lycopene	Bacteria, Fungi	Brevundimonas sp.	Carotenoid, unsaturated hydrocarbon	Food coloring, supplements, cosmetics	Antioxidant, anticancer, cardiovascular health
Canthaxanthin	Bacteria	Xanthomonadaceae	Carotenoid, xanthophyll	Food coloring, dietary supplements	Antioxidant, anti-inflammatory, immune system booster
Lutein	Algae	Asteraceae family	Carotenoid, xanthophyll	Food colorant, supplements, eye health	Antioxidant, anti-inflammatory, eye protection
Spirulina Blue	Cyanobacteria	Arthrospira platensis	Phycocyanin, protein-pigment complex	Food colorant, cosmetics, health supplements	Antioxidant, anti-inflammatory, neuroprotective
Phycocyanin	Cyanobacteria	Spirulina	Phycobiliprotein, protein-pigment complex	Food coloring, supplements, cosmetics	Antioxidant, immune-boosting, anti-inflammatory
Violacein	Bacteria	Chromobacterium violaceum	Indole-derived, violacein	Bio-dye, antimicrobial, anticancer research	Antimicrobial, anticancer, anti-inflammatory
Prodigiosin	Bacteria	Serratia marcescens	Indole-derived pigment, prodigiosin	Cosmetics, research, antibacterial products	Antimicrobial, anticancer, anti-inflammatory
Xanthophylls	Yeast	Xanthophyllomyces dendrorhous	Carotenoid, oxygenated carotenoid	Food colorant, supplements, cosmetics	Antioxidant, anti-inflammatory
Astaxanthin	Yeast	Phaffia rhodozyma	Carotenoid, xanthophyll	Food colorant, aquaculture feed, cosmetics	Antioxidant, anti-inflammatory, neuroprotective
Carotenoids	Fungi	Mucor spp.	Carotenoid, unsaturated hydrocarbon	Food coloring, pharmaceuticals	Antioxidant, anticancer, immune-boosting
Curcumin	Microorganisms (Fermented)	Curcuma longa	Phenolic compound, diarylheptanoid	Food coloring, cosmetics, supplements	Antioxidant, anti-inflammatory, anticancer
Rutin	Fungi	Fusarium oxysporum	Flavonoid glycoside	Food and beverage colorant, health supplement	Antioxidant, anti-inflammatory, cardiovascular health
Betanin	Beetroot (Fermented)	Beta vulgaris	Betacyanin, betalain	Food coloring, pharmaceuticals	Antioxidant, anti-inflammatory, liver health
Safflor Yellow	Plants (fermented)	Carthamus tinctorius	Flavonoid-based, safflor yellow	Food and beverage coloring, dye industry	Antioxidant, anti-inflammatory
Carminic Acid	Insects (via fermentation)	Dactylopius coccus	Anthraquinone derivative	Food coloring, cosmetics	Antimicrobial, anti-inflammatory
Rubropunctatin	Fungus	Monascus purpureus	Anthraquinone derivative	Food and beverage coloring, medicinal products	Antioxidant, anticancer, anti-inflammatory
Rhamnetin	Fungi	Fusarium graminearum	Flavonoid-derived	Cosmetics, supplements	Antioxidant, anti-inflammatory
Indirubin	Plant-derived bacteria	Indigofera tinctoria	Indole-derived, derivative of indigo	Textile and food colorant	Anticancer, anti-inflammatory
Alizarin	Plants, Fermented Bacteria	Rubia tinctorum	Anthraquinone derivative	Textile and food coloring	Antioxidant, anti-inflammatory
Emodin	Plants, Fungi	Rheum spp.	Anthraquinone derivative	Food coloring, medicinal products, textiles	Anticancer, anti-inflammatory
Melanin	Fungi	Aspergillus niger	Polymorphic pigment	Cosmetics, medical applications, hair products	Anti-inflammatory, skin protection
Carmines	Insect-derived	Cochineal	Anthraquinone derivative	Food and cosmetics coloring	Antimicrobial, anti-inflammatory
Lichens Yellow	Lichens	Usnea	Lichen-derived, pulvinic acid	Textile coloring, potential use in food colorants	Antimicrobial, antifungal
Pseudomonas Yellow	Bacteria	Pseudomonas spp.	Quinone-based pigment	Food and industrial colorant	Antimicrobial, anti-inflammatory
Algal Red	Algae	Chlorella	Carotenoid, xanthophyll	Food, beverages, cosmetics, supplements	Antioxidant, immune-boosting
Rhodamine B	Bacteria	Rhodococcus	Xanthene dye (organic compound)	Food coloring, textile dyeing	Antimicrobial, anti-inflammatory
Riboflavin (B2)	Fungi	Aspergillus oryzae	Vitamin B2, flavin compound	Food and pharmaceutical industry, dietary supplements	Antioxidant, anticancer, anti-inflammatory
Methyl red	Bacteria	Micrococcus luteus	Azobenzene derivative	pH indicator, dye industry	Antimicrobial, diagnostic applications
Tyrosinase Inhibitors	Fungi	Cunninghamella elegans	Fungal-based inhibitors of tyrosinase	Cosmetics, skincare, food coloring	Skin whitening, anti-inflammatory
Xanthine	Penicillium	Penicillium	Purine derivative	Industrial colorant	Antioxidant, potential anticancer
Quinones	Bacillus	Bacillus spp.	Quinone pigments	Food and textile dyeing	Antioxidant, antimicrobial
Septempyrone	Actinomycetes	Streptomyces spp.	Pyrone-based pigment	Research, medical applications	Anticancer, antimicrobial
Curvularia Yellow	Fungi	Curvularia spp.	Polyketide-derived pigment	Textile, food colorant	Antimicrobial, antifungal
Anthraquinone	Plant-related bacteria	Averrhoa carambola	Quinone derivative	Textile, food coloring	Anticancer, anti-inflammatory
Helianthin	Sunflower	Helianthus annuus	Carotenoid-based, xanthophyll	Food and beverage coloring, industrial use	Antioxidant, immune-boosting
Torulene	Yeast	Torula yeast	Carotenoid, red pigment	Food, beverages, cosmetics	Antioxidant, anti-inflammatory, immune-boosting
Capsanthin	Capsicum	Capsicum annuum	Carotenoid, xanthophyll	Food coloring, supplements	Anti-inflammatory, antioxidant
Rhizobium Yellow	Bacteria	Rhizobium spp.	Bacterially derived yellow pigment	Food and textile dyeing	Antioxidant, antimicrobial
Aureusidin	Fungus	Penicillium aureus	Flavonoid derivative	Textile, food colorant	Antioxidant, anti-inflammatory
Bacterioruberin	Halophilic bacteria	Halobacterium	Red carotenoid derivative	Food and industrial coloring	Antioxidant, anti-inflammatory
Marinol	Yeast	Phaffia rhodozyma	Carotenoid, xanthophyll	Food coloring, nutritional supplements	Antioxidant, anti-inflammatory, neuroprotective
β-Damascenone	Bacteria	Bacillus spp.	Terpenoid-derived pigment	Industrial coloring, research	Anticancer, anti-inflammatory
Carotene	Yeast	Saccharomyces cerevisiae	Carotenoid, beta-carotene	Food coloring, dietary supplements	Antioxidant, immune boosting
Astaxanthin	Microalgae	Chlorophyta	Carotenoid, xanthophyll	Aquaculture feed, food colorant	Antioxidant, anti-inflammatory, neuroprotective
Phaeophytin	Microalgae	Chlorella vulgaris	Chlorophyll derivative	Food colorant, cosmetics	Antioxidant, detoxifying
Torulin	Yeast	Torulaspora delbrueckii	Carotenoid derivative, torulene	Food coloring, dietary supplements	Antioxidant, immune system booster
Annatto	Plant-derived bacteria	Bixa orellana	Carotenoid, bixin	Food coloring, cosmetics	Antioxidant, anti-inflammatory
Astaxanthin	Yeast	Xanthophyllomyces dendrorhous	Carotenoid, xanthophyll	Food colorant, cosmetics	Antioxidant, anti-inflammatory, immune-boosting
Alpha-carotene	Bacteria	Brevundimonas sp.	Carotenoid derivative	Food coloring, supplements	Antioxidant, anticancer
Fumonisin	Fungus	Fusarium verticillioides	Mycotoxin-derived pigment	Food colorant, agricultural use	Antifungal, anti-inflammatory
Lentinan	Fungus	Lentinula edodes	Polysaccharide, non-carotenoid	Food coloring, supplements, pharmaceuticals	Immunomodulatory, anti-inflammatory
Pterin	Bacteria	Brevundimonas spp.	Pteridine-derived pigment	Bio-dye, food coloring, textile dyeing	Antioxidant, anti-inflammatory
Biliverdin	Bacteria	Pseudomonas spp.	Porphyrin-derived pigment	Bio-dye, research	Anticancer, anti-inflammatory
Phycourobilin	Cyanobacteria	Spirulina platensis	Phycobiliprotein, biliverdin-like	Food colorant, supplements, cosmetics	Antioxidant, neuroprotective, anti-inflammatory
Indole Red	Bacteria	Bacillus subtilis	Indole-derived pigment	Food coloring, research	Anticancer, anti-inflammatory
Erythrosine	Bacteria	Erythrobacter spp.	Xanthene dye (organic compound)	Food coloring, textile dyeing	Antioxidant, anti-inflammatory
Rhizobium Red	Bacteria	Rhizobium spp.	Quinone derivative	Food and textile dyeing	Antioxidant, anti-inflammatory
Fungal Melanin	Fungi	Aspergillus spp.	Polymorphic pigment	Medical applications, cosmetics, dyeing	Anti-inflammatory, anticancer
Neosartoryamine	Fungi	Aspergillus niger	Alkaloid-derived pigment	Food and textile coloring	Antioxidant, anti-inflammatory
Laccase	Fungi	Trametes versicolor	Enzyme-derived pigment	Food and textile coloring, bioremediation	Antimicrobial, antioxidant
Violacein	Bacteria	Chromobacterium violaceum	Indole-derived pigment	Bio-dye, antimicrobial, anticancer	Antimicrobial, anticancer, anti-inflammatory
Prodigiosin	Bacteria	Serratia marcescens	Indole-derived pigment, prodigiosin	Antimicrobial, research	Antimicrobial, anticancer, anti-inflammatory
Monascus Red	Fungus	Monascus purpureus	Anthraquinone derivative	Food and beverage coloring, medicinal products	Anticancer, anti-inflammatory
Carotenoids	Fungi	Aspergillus spp.	Carotenoid derivatives	Food coloring, pharmaceutical	Antioxidant, immune boosting
Chlorophyll a	Algae	Chlorella vulgaris	Chlorophyll derivative	Food colorant, supplements	Antioxidant, detoxifying
Bacteriochlorophyll	Bacteria	Rhodopseudomonas spp.	Chlorophyll derivative	Bioremediation, food coloring	Anti-inflammatory, antioxidant
Mycosporine-like Amino Acids	Fungi	Cladosporium spp.	Amino acid derivatives, UV-protective pigments	Cosmetics, pharmaceutical	UV-protection, antioxidant
Cytochrome C	Fungi	Fusarium spp.	Protein-derived pigment	Medical and research	Antioxidant, anticancer
Luteoxanthin	Algae	Chlamydomonas spp.	Carotenoid, xanthophyll	Food and beverage colorant	Antioxidant, anti-inflammatory
Echinocandin B	Fungus	Glarea lozoyensis	Terpenoid-derived pigment	Pharmaceutical use, medical colorants	Antifungal, antimicrobial
Rhamnose-pyranose	Bacteria	Bacillus spp.	Sugar-based pigment	Food and industrial coloring	Antioxidant, anti-inflammatory
Polyketide Pigments	Fungi	Aspergillus nidulans	Polyketide-derived pigment	Food coloring, medicinal products	Anti-inflammatory, antioxidant

presents microbial pigments and dyes from various sources: bacteria, fungi, and algae.

4. Conclusions

Synthetic colorants have played a significant role in revolutionizing industries by offering vibrant hues and efficient coloring solutions. However, their widespread use comes with notable drawbacks, including occupational hazards and environmental toxicity. As global demand for synthetic dyes continues to grow, these challenges intensify, calling for sustainable alternatives and stricter regulatory measures. Plant-based colorants, on the other hand, are gaining attention due to their eco-friendly nature and diverse applications. These natural compounds, such as anthocyanins, carotenoids, flavonoids, and betalains, offer a wide range of distinct colors and possess properties that make them valuable in food, textiles, and cosmetics. However, challenges remain in maximizing their stability and usability, as factors like oxidation, pH sensitivity, and light exposure can compromise their effectiveness. Some of the main strategies attempted to enhance the stability of natural compounds, notably including encapsulation or inclusion into supramolecular structures [226,267,268,269]. For example, various research groups have used chitosan as an encapsulating agent for colorants, while the use of cyclodextrin derivatives is also well documented. However, the use of such stability additives can also increase the overall cost. Despite their potential, natural colorants face a range of structural issues that hinder their commercial viability. For example, anthocyanins are sensitive to environmental conditions like temperature, pH, and light, which can cause degradation and instability. Similarly, betalains, while offering unique colors, suffer from issues such as thermal degradation and solubility challenges. Carotenoids, although valuable for their health benefits and color range, are prone to oxidation and light-induced degradation, limiting their application. Flavonoids, too, are susceptible to oxidation and other environmental factors that impact their stability. Addressing these structural challenges through innovative formulations, stabilization techniques, and careful environmental control is essential for enhancing the effectiveness and commercial viability of these natural colorants. As the demand for sustainable and non-toxic alternatives grows, plant-based colorants are becoming more prevalent in various industries, with brands like Earthues and Dharma Trading Co. leading the way. At the same time, technological advancements in biotechnology, such as lab-grown carmine, microbial pigments, and algae-based dyes, are offering promising alternatives to synthetic and animal-based colorants. These innovations are paving the way for more sustainable, ethical, and environmentally friendly options. As research and development continue, the future of colorants looks brighter, with the potential for new, eco-friendly solutions that not only reduce our environmental impact but also ensure the health and safety of consumers.