Current Review: Alginate in the Food Applications

Abstract

Due to global development and increased public awareness of food’s effects on health, demands for innovative and healthy products have risen. Biodegradable and environmentally friendly polymer usage in modern food products is a promising approach to reduce the negative health and environmental effects of synthetic chemicals. Also, desirable features such as flavor, texture, shelf-life, storage condition, water holding capacity, a decrease in water activity, and an oil absorption of fried food have been improved by many polysaccharides. One of the important polymers, which is applied in the food industry, is alginate. Alginates are a safe and widely used compound in various industries, especially the food industry, which has led to innovative methods for for the improvement of this industry. Currently, different applications of alginate in stable emulsions and nano-capsules in food applications are due to the crosslinking properties of alginate with divalent cations, such as calcium ions, which have been studied recently. The main aim of this review is to take a closer look at alginate properties and applications in the food industry.

1. Introduction

Currently, widespread concerns about health and diet have encouraged the food industry to use effective food additives and innovative approaches to raise the quality of functional food among commercial products [1]. There are different ranges of bioactive additives, which have been prepared to enhance physicochemical properties, such as flavor, taste, and texture. In the modern world, the food industry uses many additives to meet world population needs. Among polysaccharides, alginate has a specific usage in medicine, such as drug delivery, and various applications in the food industry [1]. Alginate is produced from the cell walls of brown algae in marine ecosystems that are purified by alkaline extraction conditions. Alginates comprise 1→4-linked α-L-guluronic acid (G), and β-D-mannuronic acid (M) pyranose remains in a straight chain. The important part of this molecule is the G blocks, since the calcium and hydrogen ions can form gels at low temperatures by forming a characteristic “egg-box” structure, and MG block also helps to reduce solution viscosity [1].

In recent years, the food and nutraceutical industry has studied the encapsulation of bioactive compounds as a tool for their application in foods and in the formulation of controlled-release preparations to design functional foods and food ingredients. Different works have studied the use of biopolymers as encapsulating agents and their effect on stability and polymer-active interaction [1]. These encapsulates must comprise biodegradable, non-toxic, and versatile matrices for human consumption and industrial use. This causes decomposition in nature and reduces environmental effects; it does not threaten consumer safety and can be used for various applications in the industry. These features maintain stability, safety, and compatibility in consumer products and industrial processes [2]. In this sense, alginate has been used due to its multiple advantages for human consumption and industrial applications [2], including highlighting the prebiotic effect of low molecular weight alginates, the benefits of their intake as daily fiber for the reduction in blood sugar, and cholesterol levels, as well as the ability to extend the useful life of products. New technological trends have focused on producing structured and functional foods by adding active compounds, such as antioxidants, vitamins, amino acids, minerals, and even small molecules such as cells, enzymes, and probiotic microorganisms beneficial to health [2]. These active compounds must be conserved, maintaining their beneficial properties in foods during processing and storage [3]. The gelled alginate can be used through encapsulation techniques, resulting in a final product that protects the encapsulated compounds from adverse factors, such as heat and humidity, thus improving their stability and bioavailability [1]. Additionally, encapsulation can be used as a means of masking or preserving flavors and aromas by acting as an insulator [4]. Alginate gel is commonly used in food applications to encapsulate plant polyphenols, which helps to enhance their performance and stability in food products. By encapsulating the aqueous extract of Thymus serpyllum L., alginate gel can prevent the degradation of its active substances without creating a chemical interaction between the active substances and alginate [5]. In a study, lemon balm extract was encapsulated using alginate gel. The study found that the antioxidant properties of the extract were preserved, and there was no chemical interaction between the two substances [6]. To minimize the powdery or grainy texture in foods such as yogurt and ice cream, gels with an average diameter of no more than 30 μm should be used [7]. By incorporating bioactive substances into alginate gel with diameters less than 30 μm, manufacturers can add functional properties to their products while preserving the original taste and texture [8]. Also, alginate plays a crucial role in producing substitute structures for saturated fats, which gives the product healthy attributes. This also preserves the functional properties of the original product to maintain its acceptability [8].

Alginates have been applied in different forms, such as alginate-based nanomaterial, alginate-based films, and thickening properties [8]. The present review provides an overview mainly focused on the application of alginates in food. An attempt was made to comprehend and discuss the most recent research works that have been conducted in the field of alginate application in the food industry.

2. Structures and Properties of Alginates

2.1. Structural Features

Alginate is a natural polysaccharide extracted from brown seaweeds. It is composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G), which are linked by 1–4 bonds. Alginate units are randomly arranged in a linear chain, with varying levels of L-guluronic acid and β-D-mannuronic, depending on the source and treatment [1]. In the formation of gel by calcium ions, four poly chains of L-glucuronic acid and four water molecules are connected in an octahedral coordination introduced as an egg-box model (Figure 1), so the hardness of the water used to make a solution of alginate is important [9]. Alginate has three different types of blocks: G, M, and MG. The average pKa value of alginate is 3.5, which explains that as the pH value decreases, the solubility of alginate decreases. This is because of the reduction in ionization caused by the precipitation of carboxylic acids in the molecules due to the loss of the negative charge of the colloid. [10].

2.2. Alginate Molecule Crosslinking

According to the low mechanical features of alginate studies of stable hydrogels, new methods (crosslinking and crosslinkers) have been developed in recent years. An alginate molecule has sections—uronic acids, specifically the guluronic acid, which releases H₃O⁺ in water-based solvents leading them to make the connection with Ca ions between two parts of the alginate molecule (egg-box model). The second section, which is mannuronic, is responsible for the viscosity of water-based solutions. In this regard, the crosslinking feature helps to use alginate as a delivery compound because it can be formed around an agent aimed to be carried and saved. These properties can be followed to protect bioactive and worthwhile compounds from heating, fluctuation of pH, oxidation, and reduction reactions (the reactions that decrease the bioactivity of compounds). In addition, due to its acidic nature, alginate destroys the alkaline conditions of the grafts and can lead to an acceptable carrier for drug delivery in the stomach [11].

The polymers that have the same alginate characteristics, like nano-crystal cellulose (due to an increase in the O–H bond), chitosan, locust bean gum, pectin [12], xanthan [13], and gelatin, which have good biocompatibility and help to form a hydrogel, increase the hydrophobic reaction, which amplifies gelation. The combination of cellulose and gelatin polymers forms by two positive-charged ions like Zn²⁺ (every divalent cation could be possible, for example, Ba²⁺, Sr²⁺, Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, Mn²⁺, and mostly calcium Ca²⁺), which acts as an intermolecular amplifier that reduces and corresponds to link polymers. Then, the hydrogel appeared homogeneous-dispersed and contained high water levels [14]. The use of glycerol is examined, indicating alginate can effectively interact with O–H groups of polyalcohol molecules [15]. Additionally, montmorillonite microbeads, which comprise inorganic materials, have tetrahedral silica sheets between alumina layers, which have been observed for its formation of alginate beads. This material releases cations, helping the formation of crosslinking [16]. The pectin mixture with alginate depends on the methoxylation degree; if it is low, pectin gels form in a viscose and hard texture, which is not suitable for binding Ca²⁺ and charging negatively [17]. The alginate is crosslinked by Ca²⁺, forming in two methods, diffusion and internal setting. In the first method, the ions are added to the solution from outside, and in the second one, ions exist in the structure. The diffusion gels are made with CaCl₂ and Na–alginate; consequently, internal gels are formed by CaCO₃ as calcium ions [18].

Also, alginates merged with propylamine were developed, which means that alginate carboxylic groups (–COOH) and amine (–NH₂) groups of propylamine (C₃H₇NH₂) interact perfectly (the functional group of NH of propylamine makes the bond with –COO⁻ parts) and form a hydrophobic compound, which can embrace hydrophobic agents as a carrier [19]. The proteins have special impacts on the gelation properties process of calcium–alginate gels. Moreoever, positively charged proteins (soy and whey proteins) have different interactions with alginate, such as hydrogen bonds and electrostatic absorptions of cationic sections [19].

2.3. Stability of Alginate Solutions

Pure sodium alginate powder can have a shelf life of several months if stored in a dry, cool place without exposure to sunlight. Frozen alginate can even be preserved for several years without significantly reducing molecular weight. In contrast, dry alginic acid has limited stability at ordinary temperatures due to its weak acid status, which catalyzes its degradation. Regarding aqueous alginate solutions, their strength is limited by various causes of degradation. For example, at a very acidic or alkaline pH, a severe reduction in viscosity occurs, which is also observed with the presence of radicals that oxidize the polymer [20].

Furthermore, because alginates are natural food products, microorganisms can attack them for digestion. The use of thermal and sterilization treatments also promotes the depolymerization process, which is evident not only in the reduction in the relative viscosity but also in the loss in gel resistance if there has been gelation [20]. Neutral alginate solutions of low to medium viscosity can be stored at 25 °C for many years without a loss in viscosity as long as an antimicrobial agent is added. Adding small amounts of calcium can increase the stability of sodium alginate solutions. A combination with strong acid can cause precipitation of alginic acid, while the presence of strong alkalis facilitates the breaking of the polymer chain, degrading it. Propylene glycol alginate solutions are stable at room temperature at pH levels between 3 and 4. They lose viscosity rapidly at pH levels below 2 and above 6 [21].

2.4. Alginate Gelling Properties

The gelation process occurs in the presence of multivalent cations (except magnesium), of which the food industry uses the calcium ion most. Gelation happens when G-blocks in alginate molecules form a union zone through calcium ions, linking physically with G-blocks from other alginate molecules. This interaction creates a loop-type spatial configuration, with the calcium ions fitting into the cavities of the divalent cations. This chelate-type bond thus forms a matrix-type intermolecular network. The M-blocks and MG-blocks will not participate in the bonding zones but form the so-called elastic segments in the gel network (Figure 2). Therefore, the proportion of G-blocks determines the gel’s stiffness, as mentioned above (Figure 2) [17]. The visualization of the physical structure, called the “eggbox” model by Draget (2000) [20], is shown in Figure 3.

The “eggbox” model can be modified during the gel formation process due to the proximity of the two G-blocks [21]. This union would give rise to areas of multiple layers, as seen in Figure 1. Therefore, the gel formed would be more resistant. In this sense, a higher G content and longer G blocks make the alginate more reactive and, therefore, can accept a more significant amount of calcium. Consequently, it is essential to consider the availability of calcium ions and even the molecular weight of the chain to form more or less intense gels, as appropriate [2].

2.5. Alginate Gel Stability

In contrast to most gelling polysaccharides, alginate gels have the particular characteristic of being prepared cold. Furthermore, alginate gels are thermo-irreversible. However, they resist relatively high temperatures (≈100 °C) well. This resistance to temperature, thickening power, and rapid gelation has allowed for its wide use in producing food products, such as baking creams, pastries, frozen desserts, and jellies [16]. Alginates are subject to chemical degradation processes. Prolonged heat treatment at low or high pH can destabilize the gel [22].

On the other hand, since the gel retains water through hydrogen bonds, once it contracts, it then expels water. This effect, called syneresis, is commonly observed in various gel systems obtained from biopolymers. In alginate gels, syneresis depends on parameters such as the mannuronic/guluronic acid (M/G) ratio, calcium concentration, gelation mechanism, and molecular weight. The control of these factors is essential for avoiding or reducing an undesirable syneresis phenomenon [2]. Gels prepared from lower molecular weight alginates show less syneresis than higher molecular weight gels. This is probably due to a lower number of intact elastic segments between junction zones, resulting in a lower ability of the network to reorganize and contract during the gelation process. Alginates with high MG-block levels exhibit more syneresis because they have more flexible elastic segments [20]. Syneresis is usually negligible in a balanced formulation (calcium/alginate) with enough calcium to saturate all the G-blocks, while excess calcium can aggravate the phenomenon. The internal gelation mechanism tends to give gels with less syneresis than those prepared externally, mainly because the calcium/alginate ratio is easier to distribute more homogeneously [2].

2.6. Gelation Mechanisms of Alginate

After the dispersion of the alginate sol within the droplets, the next step is gelation by divalent cations. The bivalencies join the guluronate blocks to form the structure called the “egg box.” The degree of affinity of alginate towards divalent cations varies as follows: Pb, Cu, Cd, Ba, Sr, Ca, and Co. However, the calcium cation (Ca²⁺) is the most used due to its low toxicity compared to the other cations [23]. Different types of calcium salts can be used to obtain alginate gel; these salts are divided into three categories: soluble, partially soluble, and insoluble. Calcium chloride (CaCl₂) is soluble in water (74.5 g/100 mL) and causes instant gelation. On the other hand, calcium sulfate (CaSO₄), which is partially soluble (0.205 g/100 mL of water), allows the slow dissociation of Ca²⁺ ions, but the control of gelation kinetics is difficult; finally, calcium carbonate (CaCO₃) is practically insoluble (0.00066 g/100 mL of water), so it is used when you want gradual cross-linking.

2.6.1. External Gelation

External gelation is the classic and most widely used process for forming alginate hydrogels [24]. In this process, Ca²⁺ ions are introduced into the alginate drops formed by liquid or air–liquid dispersion methods. Ca²⁺ ions diffuse inward into the interstitial spaces between the alginate polymer chains to initiate cross-linking [24,25] (Figure 4).

2.6.2. Internal Gelation

In the internal gelation method, an insoluble calcium salt such as CaCO₃ is used, an alginate sol containing calcium carbonate is emulsified in an oil phase [24]. Then, acetic acid is added to reduce the pH and, thus, induce the dissociation of CaCO₃ into Ca²⁺ and carbon dioxide. It is carbon and water with which gelation begins. Since cross-linking and gelation begin inside the drop, the method is called internal gelation [24,25] (Figure 4).

2.6.3. Inverse Gelation

In the inverse gelation method, an aqueous or water/oil solution containing Ca²⁺ ions is extruded drop by drop into an alginate solution [26]. After contact, the Ca²⁺ ions diffuse out of the periphery of the droplet and cross-link with the alginate polymer layers surrounding the droplet. The ionotropic gelation process continues until the free Ca²⁺ ions are exhausted; at the end, the semipermeable Ca–alginate membrane engulfs the solution drop of Ca²⁺ ions. This method is only used when liquid core droplets (aqueous or oil) are generated by the air–liquid method. The main characteristic of this method is using a simple apparatus to produce the capsules. A concentric nozzle is not required. However, the capsules produced do not always obtain the spherical shape due to the deformation they suffer when colliding with the bottom of the capsule [26].

2.6.4. Interfacial Gelation

In the interfacial gelation process, oil is emulsified in an aqueous dispersion of CaCO₃ nanoparticles [26]. The nanoparticles can self-assemble at the interface to form a Pickering water/oil emulsion. The Pickering emulsion separates before it is collected (rich in oil droplets) and dispersed in alginate sol. Acetic acid is subsequently added to the emulsion system to induce the dissolution of the CaCO₃ nanoparticles at the water/oil interface. After dissolution, the Ca²⁺ ions are released and cross-linked with the alginate polymers at the water/oil interface to form the gel membrane. Interfacial gelation is better for forming oil-core alginate capsules than other processes [26].

2.6.5. Interrupted Gelation in Multi-Step

In this mechanism, the hydrogel is prepared by interrupting and repeating an external gelation mechanism in which membrane layers are formed sequentially from the outer layer to the inner layer of the core by alternating immersion in a solution of CaCl₂ and water [26].

2.7. Extraction of Alginate

Generally, alginate is extracted from seaweeds (Ascophyllum nodosum, Laminaria sp., Lessonia nigrescens, Ecklonia maxima, Macrocystis pyrifera, and Durvillaea Antarctica) in salt forms, for example, sodium, potassium, and calcium; at the same time, it can be derived from some bacteria capsules. The extraction of sodium alginate traditionally is achieved (Figure 5) through a seven-step process (dried seaweed formaldehyde preprocess, acidic process, alkali treatment, bleaching, precipitation, and drying) which is called conventional extraction (CE), and CE is affected by various conditions, such as pH, process time, process temperature pressure, particle size, the solvent type used, the ratio of sample the solvent used, the ratio of stirring, etc. [27]. The most vital step is alkaline extraction, which is associated with the physicochemical features of the final product, and also bleaching, which is considerable for color [27]. Several methods and techniques can be used to evaluate the productivity of extraction results, which are crucial for subsequent applications. For example, microwave-assisted extraction (MAE) is a rapid technique that promotes fast release of compounds, while ultrasound-assisted extraction (UAE) results in shorter molecular chains. Pressurized liquid extraction (PLE) and enzyme-assisted extraction (EAE) break down cell walls, facilitating the release of intracellular components [28]. Although alginate is not digestible in the human gut, it can be fermented by intestinal bacteria and used as a carbon source [29].

Two main purposes are considered before the main extraction of the algal biomass and polysaccharides: (i) to avoid interference and simultaneous extraction of other components which are active and of the same solubility; (ii) to obtain better extraction yields due to wall destruction and enhanced transfer of polysaccharides into the solvent. For example, the mixture of methanol/chloroform/water in a ratio of 4:2:1 was developed to remove lipids, terpenes, and phenols [30]. There are other substances that help the process of extraction, such as formaldehyde, which renders phenols insoluble; acetone, which is used to wash algae to remove pigments like chlorophyll; and activated charcoal, which can adsorb many substances [3].

2.7.1. Producing Low Molecular Weight Alginate

Low molecular weight products may increase the functional properties of alginate by the correction in molecular weight. Low molecular weight alginate shows more physicochemical and biological activities. For example, a light alginate with a low M/G ratio indicates better protection of probiotics against unstable environmental conditions, and also, the hydrogels from light alginate show great radical absorption features [31]. In the different experiments, it is demonstrated that light alginate (oligosaccharide alginate) in the post-harvest dip of 25, 50, or 100 mg/L oligosaccharide alginates for ten minutes significantly inhibits post-harvest deterioration such as gray mold (B. cinerea), blue mold (P. expansum), and black rot (A. alternata) in kiwis stored at 25 °C. However, the 25 mg/L had no considerable effects against gray mold. Also, the incidence of all three pathogens after 4 days was 100%, while, in 50 mg/L treatment after 4 days was 66% gray mold, 57% blue mold, and 71% black rot. Additionally, no notable inhibitions were seen against spore germination. Oligosaccharide alginate saves desirable properties during storage time and encourages the expression of enzyme genes and antioxidant compounds in stored fruit. The genes which encode antioxidant enzymes against post-harvest plant pathogenic fungi are CAT and SOD. CAT gene expression was regulated in a treatment that contained oligosaccharide alginate [32]. CAT and SOD detoxification are the reactive oxygen species that occur in response to infection [33].

2.7.2. Ultrasound-Assisted Extraction (UAE)

Among the new methods, ultrasound-assisted extraction is more practical for industrial applications due to reasons such as process simplicity, higher extraction speed, improved performance, as well as reduced cost and time of the extraction process [34]. Ultrasound-assisted extraction can also be used in conjunction with other conventional methods such as enzymatic extraction or microwave extraction [35]. Ultrasound extraction produces physical forces such as shear force, shock waves, micro-jets, and sound currents which destroy cell walls, reduce particle size, and better combine solvent and target compounds [35]. In addition, in an ultrasound-assisted extraction, a series of rapid jumps are formed and destroyed, which in the liquid medium under treatment causes severe stress and an irreversible chain gap [36]. From the green method point of view, it is important to decrease the amount of chemical usage because they can change aqua ecosystems’ pH; the use of the enzymatic process is safe and produces less protein and polyphenol residues compared to other chemical methods. For example, the conditions of UAE for the bio-extraction of Sargassum muticum are ultrasonic bath 1.5 A, 150 W, and 40 Hz at 25 °C for 5–30 min [37].

2.7.3. Microwave-Assisted Extraction (MAE)

Microwave extraction is one of the most efficient methods used for extraction and can overcome the disadvantages of conventional methods, including low selectivity, long extraction procedures, and degradation of heat-labile compounds. During the microwave extraction process, heat is generated directly inside the material (volumetric heat distribution) and through ionic conduction of soluble ions and/or bipolar rotation of the polar solvent. Therefore, heat is not generated in non-polar compounds that are exposed to microwaves. This rapid internal heating during microwave use leads to better cell wall destruction and the release of intracellular compounds into the extracted solvent [38]. According to Yuan and Macquarrie’s (2015) [39] method, the highest yield was carried out after 120 °C for 15 min extraction. This means that the temperature plays an important role in the fucoidan yields. Microwave radiation also can stimulate the destruction of the cuticle layer, which is present on the surface of very rough algae; after using the microwave at high pressure (120 psi), many holes are observed in the cuticle layer [40]. Microwave extraction has been used successfully to extract various bioactive compounds from seaweed, such as α-amylase, α-glucosidase, pancreatic lipase, and tyrosinase inhibition activities that were extracted by 300 mL of 70% methanol and 2.45 GHz microwave radiation for 15 min at 110 °C [40,41], as well as polysaccharides from other plants, indicating that the highest yield belonged to 70 °C and 600 W of microwave power [41]. In the extraction of polysaccharides from Cyphomandra betacea, the best result was from the combination of 400 W of microwave power, 60 min, and 60 °C conditions [42].

Moreover, the extraction of sodium alginate from the alga Nizimuddinia zanardini was successfully performed in the microwave for the first time. The results of the study showed that the use of the microwave has a great effect on the extraction performance and uric acid content of sodium alginate [43]. According to the results, the optimal conditions were determined as follows: temperature of 67 °C, extraction time of 19 min, microwave power of 400 watts, and solvent-to-algae ratio of 29 mL/g. Under these optimal conditions, the extraction yield and uronic acid content of sodium alginate were 31.39 ± 0.52% and 62.33 ± 0.30%, respectively.

2.7.4. Pressurized Liquid Extraction (PLE)

Liquid extraction under pressure is a new extraction method based on the use of high temperature and pressure to extract target compounds in an oxygen-free and light-free environment in a short time and with minimal solvent use [44]. The high temperature causes the sample to dissolve better and increase its diffusion rate, while the high pressure causes the solvent to remain below the boiling point [45]. Depending on what solvent we use in what conditions, the pressurized liquid extraction method can have different names, such as pressurized fluid extraction (PFE), pressurized solvent extraction (PSE), accelerated solvent extraction (ASE), critical water extraction (SWE), or hot water extraction (HWE). To extract polysaccharides from brown algae, various types of static [46] or dynamic [47,48] extraction equipment with pressurized liquid have been used. The equipment used was laboratory-scale commercial equipment commonly used for pressurized liquid extraction developed by Dionex Corporation in 1995, and it can only be used in static (batch) mode, while some indoor equipment (off-scale laboratory) can be used in dynamic mode (continuous flow) [49].

2.7.5. Enzymes-Assisted Extraction (EAE)

The use of enzymes to break down the cell wall of polysaccharides is one of the most useful extraction techniques to improve the efficiency of extracted bioactive compounds, which are mostly used for terrestrial plants and not widely used for seaweed [50]. Enzyme-assisted extraction increases extraction performance, speeds up the process, consumes less energy during the process, and reduces solvent consumption compared to conventional methods, which ultimately results in easier recovery of the target compounds such as polyphenols, oils, polysaccharides, flavors and pigments, and proteins [50]. Enzyme-assisted extraction of polysaccharides uses enzymes capable of degrading the cell wall or enzymes that cause the relative degradation of desirable polysaccharides into smaller fragments to facilitate extraction. Various enzymes, including carbohydrate hydrolytic enzymes and proteases, are utilized to extract polysaccharides for industrial and commercial applications due to their cost-effectiveness. However, access to specific polyhydrolytic enzymes can be more limited.. Enzyme extraction reduces the molecular weight of polysaccharides extracted from Ecklonia radiata algae by 20 to 50% compared to the conventional method, indicating that enzymes can hydrolyze specific bonds in alginate molecules [51]. In one study, the lowermost alginate extraction yield (3.30%) was observed in the water extraction method, while a slight increase in the yield of the extraction process was observed after alkalase (3.5%) and cellulase (3.47%) treatments [52]. Similarly, the yields of sulfated polysaccharides extracted from the alga Turbinaria turbinate, which were extracted using the enzymes cellulose, amyloglucosidase, and vicozyme, were higher than those obtained without the enzyme-assisted extraction processes and the optimum conditions for extraction of sulfated polysaccharide from Turbinaria turbinata are a cellulase concentration of 1.5 µL/mL and a hydrolysis time of 19.5 h, which yielded 25.13% [53]. Also, the alginate yield increased up to 6.60% after cellulase use, while alkalase did not improve the alginate yield compared to normal water extraction without enzymes (3.8%). In addition, the use of alkalase and cellulase enzymes produces alginates with the least chemical contamination with proteins and polyphenols and with the lowest M/W ratio [54].

2.8. Application of Alginate in the Food Industry

Alginate is especially important, from the preservation of compounds in ice cream to that of microencapsulated probiotic microorganisms [55]. Adequate viscosity is necessary to improve the texture of ice cream. This fundamental property of good ice cream can be achieved by a mixture of gelatin and sodium alginate, among other compounds [56]. Furthermore, using alginate in ice cream considerably improves the taste of vanilla in ice [57]. If crystallization needs to be delayed, sodium isothiocyanate could be considered a suitable option. Additionally, rhodamine could be added to a solution containing alginate and sucrose to potentially achieve the same effect [58,59]. In addition to the ice cream industry, alginate is also used in cooking to produce edible spherical structures and hydrogels with considerable resistance [60].

Molecular gastronomy, a culinary science that aims to study the mechanisms of chemicals that operate at the molecular level when we cook, is rising [61]. Within this science, spherification is a process in which liquefied foods are mixed with sodium alginate and immersed in a solution containing calcium ions that create different spheres [62]. In this way, spherification with alginate has been used to encapsulate different foods, such as caviar [63]. One of the first applications of the spherification technique occurred when Ferran Adrià put it into practice at Bulli in 2003 to make fruit caviar and false gnocchi [64]. Reverse spherification is also commonly used. In this case, burdock leaf extracts mixed with calcium ions are dropped into sodium alginate to produce liquid core hydrogel microspheres [61]. Also, with this culinary technique, it is possible to improve both the flavor and the texture, and the alginate gives the food an original texture and stickiness [65]. Some studies focus on creating spheres with freshwater alginate formed by changing specific physical properties to different alginate salt concentrations [66]. Due to its unique properties of agglutination, thickening, gelation, formation film, and stabilization, alginate and its many derivatives have a long history of use in food and even as a binder in aquaculture. Its thickening property is helpful in sauces, syrups, and ice cream toppings. Gelation is required in desserts and instant jellies from milk, cream of bakery filling, and fruit pies. General colloidal properties are essential when sodium alginate is added to ice cream, and propylene glycol alginate (PGA) stabilizes beer foam or suspends solids in fruit drinks. Adding alginate improves ice formation, making them non-sticky [67]. Sodium alginate has been used to encapsulate various agents, including living cells, protein drugs, enzymes, food ingredients, volatile compounds, and catalysts. Immobilized systems have been used in various applications, including tissue engineering, controlled drug delivery, biocatalysis for the production of chemical products, stabilization of food ingredients, adsorption of contaminants, and energy storage [68].

2.8.1. Thickening Properties

Alginate use in the food industry (Table 1) is wide due to its compatibility for the thickening of marmalade, jars, savory sauces, and desserts, which allows it to provide the viscose matrix for introducing desired features like low-fat products, advanced water storage ability of processed food, and organoleptic characteristics. Different important points are vital to estimate product quality and acceptance. For example, the molecular weight of the alginate affects the viscosity of solutions related to chain involvement. It means that involvement increases when the molecular weight of the alginate is high. For instance, if the concentration of alginate increases, the viscosity increases, while the temperature elevation significantly impacts the viscosity decrease due to changes in chain structure. The high salt and pH (under 3 and above 11) lead to a decrease in electric charges and produce weak viscosity [69]. Generally, alginate can show synergistic effects with polysaccharides. Alginate has interactions with gums because gums have flexibility and charges. However, satisfactory properties depend on the calcium concentration. The gums used were from barks of T. cordifolia and seeds of I. gabonensis; they interacted with 2 mM of calcium ions and showed the best compact gel [70].

As the confectionery sector has been growing every year, sustainable ingredients and efficient packaging are vital. Jelly candy production is based on gelling agents, such as starch, agar, pectin, and many others. However, they require the heat process to form gels. Among different polymers, the combination of alginate with pectin can reduce the energy for the gel formation process [71].

Table 1. Different types of alginate application.

Alginate Use Section	Type of Additives	References
Active food packaging	Alginate films with cottonseed protein hydrolysate.	[72]
	Alginate-based edible film of fruits and vegetables.	[73]
	Alginate and carrageenan selective barrier to CO2 and fats.	[74]
	Sulfur–alginate films.	[69]
	Alginate films based on soybean oil were applied to sweet cherries.	[75]
	The casing of dry fermented sausage (different cations).	[76]
	Probiotic edible films.	[77]
	Rainbow trout (Oncorhynchus mykiss) filet coting with alginate/tannins.	[78]
	Green edible alginate/pectin chocolate and vegetable puff packages.	[79]
	Edible coating of alginate/thyme oil effects in fresh cantaloupe.	[80]
	UV wall and oxygen permeability of alginate/Cellulose nanocrystals in chicken breast.	[81]
Thickening application	Gelling agents instead of the back-fat ingredient of low-fat frankfurters sausage.	[82]
	Cold gelation for jelly candy products.	[68]
	Using grape oil in alginate/gelatin emulsion instead of pork fat in meat products.	[83]
	Delivering cells.	[84]
Capsulation	Betacyanins and polyphenols encapsulation.	[85]
	Alginate/κ-carrageenan gel beads to release egg yolk immunoglobulin Y (antibody).	[86]
	Alginate beads combined with different compounds to deliver tea polyphenols.	[87]
	Whey protein and soy protein combination with alginate to encapsulate lycopene.	[88]
	Encapsulation of assai pulp oil in chitosan/alginate complexes.	[65]
	The natural colorant of anthocyanin and phenolic compounds in alginate beads for beverage application.	[89]
Hydrogel	Linseed oil nano-emulsion to prohibit oxidation.	[90]
	Selective solid phase extraction of lead ions in food and water samples.	[91]
	Imitated fruit pieces for various food products like dairy and bakery products or as topping in food products.	[92]
	Colorimetric hydrogel indicator for visual food spoilage monitoring	[93]
	extrusion-based 3D food printing	[94]

Table 1. Different types of alginate application.

Alginate Use Section	Type of Additives	References
Active food packaging	Alginate films with cottonseed protein hydrolysate.	[72]
	Alginate-based edible film of fruits and vegetables.	[73]
	Alginate and carrageenan selective barrier to CO2 and fats.	[74]
	Sulfur–alginate films.	[69]
	Alginate films based on soybean oil were applied to sweet cherries.	[75]
	The casing of dry fermented sausage (different cations).	[76]
	Probiotic edible films.	[77]
	Rainbow trout (Oncorhynchus mykiss) filet coting with alginate/tannins.	[78]
	Green edible alginate/pectin chocolate and vegetable puff packages.	[79]
	Edible coating of alginate/thyme oil effects in fresh cantaloupe.	[80]
	UV wall and oxygen permeability of alginate/Cellulose nanocrystals in chicken breast.	[81]
Thickening application	Gelling agents instead of the back-fat ingredient of low-fat frankfurters sausage.	[82]
	Cold gelation for jelly candy products.	[68]
	Using grape oil in alginate/gelatin emulsion instead of pork fat in meat products.	[83]
	Delivering cells.	[84]
Capsulation	Betacyanins and polyphenols encapsulation.	[85]
	Alginate/κ-carrageenan gel beads to release egg yolk immunoglobulin Y (antibody).	[86]
	Alginate beads combined with different compounds to deliver tea polyphenols.	[87]
	Whey protein and soy protein combination with alginate to encapsulate lycopene.	[88]
	Encapsulation of assai pulp oil in chitosan/alginate complexes.	[65]
	The natural colorant of anthocyanin and phenolic compounds in alginate beads for beverage application.	[89]
Hydrogel	Linseed oil nano-emulsion to prohibit oxidation.	[90]
	Selective solid phase extraction of lead ions in food and water samples.	[91]
	Imitated fruit pieces for various food products like dairy and bakery products or as topping in food products.	[92]
	Colorimetric hydrogel indicator for visual food spoilage monitoring	[93]
	extrusion-based 3D food printing	[94]

2.8.2. Alginate as a Raw Material for Food Packaging: Edible Film and Coating

Due to the upward trend in plastic use, scientists are trying to find compatible and biodegradable ingredients to change packaging materials [95]. Alginate can form films, that are water-resistant, antimicrobial, and decrease bacterial growth [95]. Bioactive packaging could improve product quality and shelf-life duration without the direct addition of additives. According to different experiments, peptides after enzymatic hydrolysis from different sources released to film matrices, such as silver carp protein hydrolysate and by-products of cottonseed in alginate-based films [96]. In another research [97], the mixture of ascorbic acid, citric acid, and alginate was introduced and controlled by glycerol levels (36.6, 54.8, and 109.6 g/100 g alginate); the result illustrates that citric acid levels did not vary during film shelf-life, although film-browning reactions and color changes associated with ascorbic acid degradation were observed. In different results, cellulose (as a barrier of water transition) and copper oxide nanoparticles (as antimicrobial) were applied in an alginate-combined film, which provided promising antimicrobial and antioxidant activities [73]. Also, the biodegradable bioplastic developed from the composite of alginate, starch, carboxymethyl cellulose, sorbitol, and polyethylene glycol prevented the migration of additives into a simulated aquatic food system for 10 days. These films were completely decomposed in 14 days and had better oxygen permeation and higher water-vapor permeation [78].

In recent years, many reports have claimed that foodborne illnesses are associated with Salmonella and Listeria. Although the coating method is not an adequate preservation technique to eradicate these bacteria, the combination of coatings and other antimicrobial agents, such as essential oils and biochemical preservatives, can be more effective [77]. The alginate combination with pea protein and polyglycerol esters of fatty acids coating on drying fermented sausage was not successful enough to control the properties of alginate coating [77]. In recent attempts, edible coatings for preservation were observed widely [77]. Fruits and vegetables are an important group of food for human consumption. However, creating safe barriers to reduce losses and their desired characteristics is challenging. In this regard, chitosan-based alginate coatings form in the interaction of –NH³⁺ on chitosan molecules and –COO⁻ on alginate, which is a multilayer coat. This kind of coating controls water vapor diffusion, and the gas interchange has an esthetic appearance [79]. Other anti-microbial agents that have been used in coating and films are ascorbic acids, tea tree essential oils, thyme, and many others [79]. Moreover, the effects of tannic acid and quebracho tannin combined with alginate as a backbone of coating have been applied in rainbow trout for cold storage, showing decreased microbial counts and lipid oxidation [98].

Currently, there is great motivation through the desire for a sustainable environment to adopt new green approaches to prohibit the production of plastic bags and other plastic polymers. So, biodegradable and biodegradable and edible packages are very considerable. Polysaccharides are associated with the raw material of green packages and edible films. Among others, alginate plays a dominant role in the packaging industry [99]. Also, some different additives are applied in edible packages, for example, antioxidants, essential oils, antimicrobial compounds, and many others, to enhance desirable properties [100]. A combination of alginate and pectin from pineapple rind was modified with organic acids like citric and tartaric acid, which improved chemical stability and thermal resistivity. Alginate films are edible (proven by mice-feeding observations) and environmentally safe (it is thrown in the soil to witness plant growth), and they are used as a wrapping material for chocolate and Indian vegetable puff [100].

2.8.3. Alginate as a Chelating Agent

Alginate is more stable than other polymers. In this regard, an alginate crosslink with chitosan/activated carbon has great potential to absorb heavy metals [83]. Due to heavy metal emission from the industrial zone, it seems considerable to eradicate Pb, Ni, Cd, As, and Hg, which are poisonous for living creatures because they can make strong bonds with important proteins and biochemicals [83]. In recent years, different techniques have been produced to absorb these metals from the environment. Among different discoveries, sodium alginate can remove heavy metals (chemical and physical absorption) and has an uptake capacity by its cross-linking ability. Some modification is needed to enhance the characterization of alginate in absorption capacity, for example, chemical changes are associated with functional groups [101].

2.8.4. Alginate as an Emulsifier

It has been discovered that proteins and surfactant-based emulsions are unstable over time, so modified alginate-based emulsion can be a promising agent to deal with emulsifiers’ challenges in the food industry, providing controlled release and stable emulsions. Alginate emulsion has some advantages such as high maintenance of oil and decreasing unsaturated fatty acids oxidation. Then, it was discovered that alginate can be applied to water/oil solutions as an emulsifier [102]. Alginate can disperse oil and water phases in food systems. This feature in the production process of salad dressings, mayonnaise, and margarine can stabilize the emulsion and prevent the oil and water phases from separating [102]. Also using alginate modified with Dodecenyl Succinic Anhydride (DSA) improves its hydrophobic characteristics and makes it a good candidate as an emulsifier [102]. Alginate emulsions’ stability at different ranges of temperature, pH, and ionic strength make it a potential emulsifier in the food industry. In particular, the stability and high emulsifying activity of alginate at acidic pH values can promote its use in food products containing citric acid or ascorbic acid, which require gelation [102].

2.8.5. Emulsified Meat

Animal fat is added to meat products to act as an emulsifier, leading to easy handling, raising yields of cooking, better flavor and texture, and water holding capacity, but it causes problems for health. Thus, using alternatives is necessary. Grape seeds have a high level of unsaturated fatty acids, which is very compatible with alginate as a gelling and emulsifier agent, and gelatin as an edible protein gel. This combination showed it is an effective method for reducing animal fat ingredients [103]. Tiger nut (Cyperus esculentus L.) oil was replaced by animal fat in beef burgers using oil emulsion gel obtained from alginate. The final product had health properties for the consumer while being generally acceptable [104]. Using alginate to prepare oily emulsion gel and replacing it with animal fat in fresh low-fat sausages increased its physical and health characteristics [103].

2.8.6. Encapsulation Properties

Complex Coacervation in Encapsulation of Alginate

The technology of encapsulation has been developed to improve the stability and bioactivity of essential components in adverse environmental conditions to achieve targeted release. The complex coacervation method has been applied for sensitive nutrients, which comprise three fundamental stages: emulsification, coacervation (meaning the separation into two liquid phases), and formation of shell. The coacervation method is a result of the reaction between oppositely charged polymers, like colloids, proteins, and surfactants, and polysaccharides; this method has positive impacts, such as high efficiency of encapsulation and integrity of materials [104]. Complex coacervation was discovered for encapsulation, the establishment of packing films, and food emulsions or gels. In general, alginate has a negative charge in a wide range of pH, the charge of proteins also changes with the pH, so the pH plays an important role in the formation of complexes.

Probiotic Encapsulation

Functional food is developing currently to regulate human health. Probiotics play a vital role in the enrichment of some products to increase the stability of intestine microbes and microbial flora. Some of the benefits of probiotics include improving lactose intolerance, cancer defense, cholesterol levels, immune system health against pathogenic bacteria, and the production of some nutrients and vitamins [105]. However, there are natural carriers of probiotics, such as dairy products. As these products go through the digestive system, probiotics decrease. Alginate can provide a stable environment due to its insoluble properties in the low and high pH of the gastrointestinal tract. Different studies have investigated the use of alginate for encapsulation of probiotics to increase their survival in gastrointestinal tract conditions [105]. Among different methods to encapsulate probiotics such as coacervation, freeze-drying, spray drying, and emulsion, the extrusion method has been presented to protect probiotics [105]. The capsules of alginate are formed through crosslinking solutions in the extrusion method. Others, for instance, the encapsulation (electrospun nanofiber mats) of probiotics with polyvinyl alcohol/alginate walls, showed successful encapsulation and had a high melting point (it could be applied in bakery and heat-processed products) [106].

Flavor and Aroma Encapsulation

The flavor and aroma stability of food are among the main concerns of food processors, while, due to the volatile nature of these components, it is hard to protect and preserve them in the food matrix. The encapsulation method can help maintain these chemicals due to their role in the desired food. Encapsulation creates layers that control the release of compounds from walls because they can be degraded by light, temperature rise, oxygen radicals, and moisture. The diffusion of chemical compounds related to flavor and odor correlates to molecular weight and boiling points [107]. Essential oils have been used as flavor agents in foods, and they are considered safe additives [107]. Essential oils are rich in phenolic compounds that have antimicrobial effects, for example, oregano, clove, rosemary, and sage. Among biochemical compounds which are applicable, cyclodextrins, which are oligosaccharides, encapsulate flavor and aroma molecules. Also, trehalose is taken into account as a promising capsulation agent with a glassy state at high temperatures and considerable power to stabilize proteins, lipids, and carbohydrates [108].

Encapsulation of Oils, Enzymes, and Sensitive Chemicals

Due to the fast oxidable nature of unsaturated oils, it is reasonable to find new sources to protect valuable oils from oxidation [109]. There are two major attitudes toward this purpose. Firstly, it is important to limit oxygen and oxidants transmission on the surface of products. Secondly, applying the encapsulation method provides a situation that can conduct electrons and act as an antioxidant and bacterial prohibition. Combining alginate with other agents such as κ-carrageenan helps to increase its structural properties. In addition, the addition of chitosan to the alginate and κ-carrageenan solution in the final step creates strong encapsulated particles containing ginger oil that are not easily oxidized [109].

Assai fruit’s pulp has around 12.5% polyunsaturated linoleic acids. Assai pulp oils can absorb radicals such as radical oxygen, which leads to the degradation of polyphenols, so it is reported that these oils can be protected by the encapsulation method of chitosan–alginate-based polymers [110]. The essential oil of black pepper has antimicrobial activity and phenolic compounds destroyed by high temperature, light, and low pH, requiring a preservation process. Encapsulation by the coacervation method has been applied by lactoferrin (a protein which is extracted from bovine milk) and alginate, as the wall material showed electrostatic interaction in pH = 3 and stronger capsules to protect black pepper oil. Additionally, alginate could prevent pepsin action, meaning that capsules can successfully carry essential oils, and the main terpene preserved was β-caryophyllene [111].

The encapsulation of Cinnamomum zeylanicum essential oil (antimicrobial) in alginate was tested to keep it safe from external agents. The antimicrobial potential of capsules in the vapor phase was observed, which provides results that claimed that, in higher temperatures, the rate of essential oil release increases, and this application could be an alternative for bioactive packaging [70]. It is reported that the combination of Ca–alginate microcapsules with sunflower oil (average size, 2 mm) improves the self-curing feature of the bituminous compound [112].

Enzymes are natural catalysts that speed up chemical reactions [89]. They are widely used in the food and beverage industry because of their catalytic efficiency [90]. However, enzymes have some drawbacks, including low thermal stability, narrow optimal pH range, and low tolerance to proteases and most organic solvents [90]. When exposed to high temperatures, enzymes change their 3D structure, which can cause them to denature more easily. Enzyme use is often limited due to their instability [92]. To overcome this challenge, a new method of stabilizing enzymes using biodegradable natural polymers is gaining popularity. Alginate is a commonly used natural polymer that has been approved by the Food and Drug Administration (FDA) for use in food and medicine [92]. With its wide range of applications in the food industry, alginate is generally regarded as safe (GRAS) [92]. Alginate-based materials have been reported to improve enzyme reusability, thermal stability, and substrate affinity when used for enzyme immobilization [92].

2.8.7. Heat Distributor

Using susceptors is an effective strategy for producing successful microwaved battered and breaded products. Susceptors are the latest development in microwave food packaging; they create local high-temperature zones during cooking, reaching up to 200–260 °C. This effectively crisps and browns the food. Edible susceptors made from food compounds containing solvents and plasticizers with high boiling temperatures and a positive temperature loss coefficient have been developed to replace metallic susceptors. A new approach involves the use of alginate gel containing all salt formulations to cover food before battering and baking as a susceptor for microwave waves. Therefore, alginate gel was used to cover microwave chicken nuggets, which were effective as a receiver and distributor layer of heating [84].

2.8.8. Fat Absorber

Deep-fried products are very popular in the modern world, but they are associated with health problems like high blood pressure, heart disease, diabetes, and so many others. Scientists are fascinated to observe and examine how they can decrease drawbacks and effects. In recent years, fried potato has been used more than other products; therefore, surface-coating edible hydrocolloids were introduced. It was explained that hydrocolloids (like alginate) form the hard surface in products during frying because of the thermos-gelling ability to keep water in the inner texture of products and decrease the number of oil absorptions [85]. An alginate hydrocolloid used as a coating in fried shrimp had a good performance in maintaining moisture and reducing fat [85].

2.8.9. 3D Printing

Three-dimensional food printing technologies involve adding beneficial compounds to enhance nutritional value and structural features in food products [86]. Creating unique tissues using plant cells in hydrogel structures is an application of this method where the alginate hydrogel is an ideal candidate due to its excellent properties, including favorable printability, biocompatibility and low toxicity, low cost, low gelation time in the presence of Ca²⁺ cross-linking [86]. Rice has a cohesive network structure and strong stability. At the same time, the content of protein and gluten are low, so adding alginate to the pate of rice can improve printability [86]. Various studies have been conducted with the aim of 3D printing cereals, lemon juice gel products, potato/corn starch, etc. Here, due to the limitation of 3D printing in the distortion of geometry, an additive layer manufacturing (ALM) simulation is a promising approach. In this regard, studies were carried out on different ratios of pea protein/alginate to achieve the optimal ratio and investigate the structural characteristics [86]. Also, bio-scaffolds were prepared using extrusion-based 3D printing and hybrid gelatin/alginate, which can be used to transport and deliver bioactive substances such as probiotics and vitamins [86].

2.8.10. Antioxidant and Antibacterial Potential

Oligosaccharides are low MW that can be extracted from fungi, bacteria, algae, and plants. They typically contain between 2 and 10 monosaccharide units whose properties depend on their composition, MW, and chemical properties. Oligosaccharides can also be produced by breaking down larger monosaccharide molecules, which is a simpler and less expensive method that is often used on an industrial scale [87]. This process can be used to modify the physical properties of alginate by reducing its MW. This improves the functional properties of alginate, which has no biological activity on its own and is therefore limited in its potential applications [87].

The process of depolymerization of sodium alginate results in the formation of oligosaccharides which cannot form a gel. However, they exhibit other significant biological activities including promoting the rapid growth and development of human cells, covering epidermal cells, and preventing oxidative stress. Oligosaccharides of sodium alginate are also beneficial in promoting the growth of plants including rice, lettuce, wheat, and tobacco. Studies have shown that alginate oligosaccharides have various health benefits such as immunomodulatory, antitumor, antioxidant, prebiotic, antidiabetic, antihypertensive, and other miscellaneous biological activities [88].

3. Conclusions and Future Trend

Alginate is a safe and natural food additive that is widely used in the food industry due to its excellent functional properties, including biodegradability, biocompatibility, and ionic cross-linking. The use of eco-friendly methods in alginate extraction can contribute to its usage sustainability. Alginate is considered an agent for trapping something inside the 3D shape of the molecules and as a carrier and barrier to deliver chemicals constantly or protect them from water evaporation from the surface of the products and prohibit bacterial growth. The health-promoting and antibacterial properties of alginate oligosaccharides make it an appropriate candidate for various applications in the food industry. Albeit alginate is the backbone of edible and safe additives, the presence of alginate is not adequate for food application; its use is associated with the combination of other polymers. Different compounds and biochemical mixtures enhance alginate use in food.

Currently, due to carbon dioxide emissions and global warming effects worldwide, scientists tend to discover new ways to produce safe food and enough valuable nutrients. This trend leads to emerging new ideas around the decrease in carbon emissions to the atmosphere; therefore, cell-based and plant-based food is mentioned. These products are provided indirectly, meaning that they decrease the harsh effects of slaughtering animals. Especially, cell-based food needs beds to connect produced cells in the scaffolding process, which needs more experiments in this area. Additionally, alginate can be used in plant-based products to stabilize compounds (odor and taste) and help with mouthfeel properties. However, alginate has several limitations due to its characteristics, such as poor stability or low mechanical and barrier properties, incompatibility with heavy metals, and heat treatment instability, some of which need improvement research.

Although there have been adequate studies on the production and application of alginate in the food and nutraceutical industries, more research is needed to enhance our apprehension of its biological activities and potential applications. Undoubtedly, these studies will lay a better foundation for the production and usage of alginate in the future.