Citrus Waste Valorization: Unconventional Pathways for Sustainable Biomaterials and Bioactive Products

Abstract

Citrus fruits are among the most important global crops, with annual production exceeding 160 million tons. Processing produces significant waste, mainly peels, seeds, and pulp, which can make up to fifty percent of the fruit’s mass. This review critically examines innovative ways to valorize these byproducts. Recent research shows that peels, seeds, and pulp can be converted into high-value materials, including biocomposites and biomaterials, marking a shift from traditional uses like animal feed and biogas production. Notable innovations include smart packaging, pectin-based wound dressings, and biodegradable polymers for sustainable electronics. Advanced green extraction methods, such as deep eutectic solvents, have achieved extraction yields over 85% for flavonoids. Additionally, multifunctional biorefineries processing citrus and olive residues have increased biogas yields by 38–42%. The review explores emerging applications in nanotechnology, nutraceuticals, biodegradable polymers, and functional coatings, all aligned with principles of circular economy and green chemistry. These advances suggest that citrus waste can play a significant role in sustainability efforts and new market development. The review also discusses barriers to adoption, including scalability challenges, regulatory limits, and consumer acceptance, from both global and regional viewpoints.

1. Introduction

Citrus fruits, members of the Rutaceae family, encompass numerous genera and species with shared characteristics; include major species such as orange (Citrus sinensis), lemon (C. limon), mandarin (C. reticulata), and grapefruit (C. paradisi). They are the third most economically significant fruit crops globally. The increasing demand for citrus in juices, jellies, jams, fresh produce, essential oils, flavorings, and fragrances [1,2] has led to notable environmental challenges. Approximately one-third of global production is destined for the juice industry, generating 40–60% of waste—mainly peels, seeds, pulp, and pomace. This waste stream contributes to methane emissions, groundwater contamination, and landfill overcapacity [3,4].

Historically, citrus by-products have been utilized predominantly in low-value applications such as animal feed or compost, often overlooking their substantial biochemical content. Citrus peels contain 10% to 20% pectin, essential oils like D-limonene, and polymethoxylated flavones, all of which possess well-documented antioxidant and anti-inflammatory properties [5].

Recent advances in green chemistry and materials science have increased the value of citrus waste, making it a source for innovative, high-value products. Developments include using nanocellulose in flexible electronics and creating biodegradable polymers from limonene. Citrus residues are becoming more important in the circular bioeconomy. Despite progress, key questions remain: Can green extraction and biotechnological methods replace traditional techniques at scale? How can nanotechnology improve product functionality without risking safety? This review suggests that (i) integrated biorefinery models are more effective than single-product approaches, (ii) nanostructured materials derived from citrus waste can meet performance standards for packaging and medical uses, and (iii) regulatory harmonization is crucial for market acceptance. The main goal is to provide a thorough overview of unconventional valorization strategies. Specific aims include: (a) mapping emerging technologies, (b) evaluating new applications, (c) identifying barriers to circular integration, and (d) developing actionable roadmaps for stakeholders.

1.1. The Global Citrus Waste Challenge

Over the past two decades, the global citrus industry has grown significantly, driven by rising demand for juice, fresh fruit, and essential oils. Figure 1 shows global citrus production increasing from 106.9 million tons in 2003 to 169.4 million tons in 2023 [6,7,8,9], a compound annual growth rate (CAGR) of 2.3%. This expansion indicates that by 2023, citrus waste will reach approximately 13 to 33 million tons, and, due to population growth and food demand, we can only expect this to increase unless decisions and actions are taken to promote a circular economy for these crops. The accumulated waste poses environmental risks, such as soil acidification due to limonene leaching and methane emissions from decomposition [3]. Note: Compound annual growth rate (CAGR) calculated using endpoint method: [(2023 value/2003 value)^(1/20)] − 1.

Figure 1. Annual output of oranges, lemons, mandarins, grapefruit and other citrus fruits. Source: Own elaboration with data from FAOSTAT Crops and Livestock Products database [6], accessed January 2025. Methodology: Production data (metric tons) for all citrus categories were extracted for the period 2003–2023. Values represent official country-reported statistics aggregated by FAO. No smoothing or interpolation was applied. Stacked area chart created using R ggplot2 package. Note: “Other citrus” category includes lime, pomelo, and minor citrus species not separately reported.

The concentration of citrus production exacerbates waste management challenges in specific regions. Figure 2 presents the leading producers of the four major citrus varieties. China accounts for 35% of total output and supplies 59% of global mandarin demand. Brazil constitutes 34% of the orange market, while India produces 22% of the global lemons. These and other regions depicted in Figure 2 experience significant pollution resulting from inadequately managed citrus waste. In many countries, particularly in rural areas with limited waste management infrastructure, citrus by-products are disposed of in landfills, dumped in agricultural fields, or openly burned [10]. These practices release substantial quantities of greenhouse gases (CH₄, CO₂, NO_x), fine particles (PM₂.₅), volatile organic compounds, and dioxins, all of which degrade air quality and pose risks to human health [11,12,13]. The accumulation of organic waste in soil and water promotes eutrophication, generates foul odors, increases the prevalence of disease vectors, and contributes to biodiversity loss [4,10]. While China and other leading producers have made notable progress in reducing the environmental impact of citrus cultivation over the past decade [14], efforts have primarily focused on production, with waste management remaining insufficiently addressed.

Figure 2. Geographic Dominance in Citrus Production (2019–2023 Average): Top four producing nations for each variety. Own analysis using data from FAOStat (2023) [7]. Methodology: Production data for oranges, mandarins, lemons, and grapefruits were collected from 156 countries/territories (2019–2023). Five-year averages were calculated to reduce yearly fluctuations. The top four producers for each variety are identified by their average annual production. Percentages show each country’s share of the global output for that specific variety. A sunburst chart was created to visualize the hierarchical market structure. Note: Some countries produce multiple citrus types; China dominates mandarin (59% of global share), Brazil leads in orange production (34%), and India accounts for 22% of lemon output.

In the absence of biotechnological innovations, citrus waste generation could reach 80–120 million tons annually by 2030, posing risks to Sustainable Development Goals 12 (Responsible Consumption and Production) and 13 (Climate Action). Emerging biotechnologies may offer scalable and eco-friendly applications for citrus by-products, with their success highly contingent on policy frameworks, particularly in regions with severe waste management challenges [15].

It is important to note that citrus peels may contain pesticide residues, which can limit direct use in animal feed and require detoxification or controlled processing steps [16,17].

1.2. Beyond Conventional Uses: The Need for Innovation

Traditionally, citrus waste has been used for animal feed, organic fertilizer, or, in some cases, biogas production. These approaches reduce disposal costs and can limit environmental impacts. However, they don’t fully harness the value of citrus residues as a bioresource. Recent research shows citrus waste contains bioactive compounds, fibers, and nutrients. This makes it suitable not just for animal feed but also for high-value food, agriculture, and energy products. Advanced processing techniques can boost digestibility and protein content in citrus-based animal feed, thus improving livestock health and productivity [2,18,19,20,21,22,23]. Adding citrus waste as an organic amendment can improve soil quality and crop yields, supporting sustainable agriculture [24,25].

Interest in utilizing citrus waste for renewable energy is increasing. Modern biorefineries convert citrus residues into biogas and biofuels, providing sustainable alternatives to fossil fuels and supporting the circular bioeconomy [26,27,28,29]. Biogas production from citrus waste reduces greenhouse gas emissions and generates digestate, a nutrient-rich byproduct that serves as a biofertilizer and facilitates nutrient cycling in agricultural systems [30,31].

Several challenges persist. Citrus waste contains anti-nutritional factors, necessitating preservation steps such as drying or ensiling, which increase costs and may reduce nutrient content. Determining optimal inclusion rates for different animal species and developmental stages is also complex. Technical and economic barriers remain, including the need for practical preservation, improved processing, and enhanced valorization strategies to ensure scalability and profitability [19,28,32].

The valorization of citrus processing residues is increasingly influenced by market demand for natural antioxidants, antimicrobials, and biodegradable materials, particularly in specialized sectors such as food packaging, cosmetics, and pharmaceuticals, as well as in emerging areas such as wearable electronics. Citrus residues are rich in bioactive compounds, including polyphenols, flavonoids, and essential oils, which possess potent antioxidant and antimicrobial properties. This bioactivity positions citrus by-products as promising candidates for natural additives and functional ingredients in high-value applications [2,33]. Furthermore, the development of biodegradable polymers and biofilms from citrus-derived biopolymers addresses environmental concerns and regulatory pressures to replace conventional, non-degradable plastics. Recent studies have demonstrated the feasibility of producing biodegradable films and composites with inherent antioxidant and antimicrobial functions from citrus waste. These materials are suitable for active packaging and have potential applications in wearable electronics and other niche markets requiring sustainable, multifunctional materials [34,35]. Additionally, the unique microstructure and chemical composition of citrus residues facilitate the design of innovative bioinspired materials, such as fibers, fabrics, and nanostructured components, supporting the transition toward a circular economy and zero-waste manufacturing models [34]. As consumer awareness and environmental regulations advance, the integration of citrus waste-derived materials into specialized industries is expected to increase, driven by the need for enhanced product functionality and environmental sustainability.

In regions such as Brazil, India, and parts of the Mediterranean, citrus residues are incorporated into animal feed after drying or ensiling [2,16,17,18,19]. However, the presence of pesticides remains a concern, reinforcing the need for innovative processing methods, such as green solvents, enzymatic hydrolysis, and microbial fermentation [36,37,38]. Countries including Spain, Italy, and China are leading research on green extraction and nanotechnology-based valorization [15,38,39].

1.3. Objectives

This review seeks to deliver a comprehensive and critical synthesis of recent advances in citrus waste valorization, with particular emphasis on bridging the gap between laboratory-scale innovation and industrial-scale implementation. The specific objectives are as follows:

Map Emerging Technologies: Systematically catalog and evaluate advanced extraction, processing, and functionalization techniques—including green solvents, nanofabrication, and enzymatic hydrolysis—used to convert citrus residues (peels, seeds, pulp) into high-value biomaterials. Emphasis will be placed on optimizing yields, improving energy efficiency, and minimizing environmental impacts.

Evaluate Unconventional Applications: Assess the technical feasibility and commercial potential of novel products derived from citrus waste, such as smart packaging films, biomedical hydrogels, biodegradable electronics, and nutraceutical encapsulates. These applications will be benchmarked using key performance indicators, including barrier properties, mechanical strength, and bioactive compound retention.

Identify Barriers to Circular Integration: Analyze the main technical, economic, and regulatory challenges hindering the integration of citrus waste valorization into circular economy models. Pair each challenge with the specific ‘take-make-reuse’ loop it disrupts and propose a restorative action to close that loop. For example, addressing seasonal variability in waste composition could involve establishing a consistent supply chain protocol, aligning processes with a steady resource input (take), and reducing disruptions in biorefinery operations. Scalability constraints in biorefinery processes can be addressed by optimizing production methods (make) to enhance throughput and efficiency. The lack of robust certification frameworks for new biomaterials disrupts the ‘reuse’ loop. It can be mitigated by establishing standardized certification processes that facilitate market entry and longevity for sustainable products. These mitigation strategies will be proposed in alignment with Sustainable Development Goals 9 (Industry, Innovation and Infrastructure) and 12 (Responsible Consumption and Production).

Formulate Roadmaps for Stakeholders: Develop actionable recommendations and strategic roadmaps for researchers (e.g., AI-driven process optimization), industry stakeholders (e.g., pilot-scale industrial symbiosis networks), and policymakers (e.g., incentives for ISO-compliant circular systems) to accelerate the market adoption of sustainable citrus waste-derived products.

1.4. Review Methodology

To compile this review, a comprehensive literature search was conducted using the academic databases Scopus, Web of Science, and Google Scholar. The search employed specific keywords, including “citrus waste,” “valorization,” “biomaterials,” “circular economy,” “pectin,” “limonene,” “nanocellulose,” combined with Boolean operators. The search covered the period from 2015 to 2024, with particular emphasis on publications from the last five years (2020–2024) to capture the most recent technological advances. Inclusion criteria prioritized peer-reviewed articles, reviews, and book chapters reporting innovative extraction methods, novel applications, or critical analyses of challenges in citrus waste valorization. Exclusion criteria removed studies focused only on traditional uses (e.g., animal feed without value-added processing) or those with inadequate methodological detail.

A bibliometric analysis of the 165 ma references included in this review reveals distinct temporal and thematic patterns in the field’s evolution:

Temporal Distribution:

• 2015–2019: 38% of references—This period established foundational work on green extraction methods and fundamental compositional analysis.
• 2020–2024: 62% of references—Marked acceleration in research output, with 2023–2024 alone contributing 28% of total references, reflecting the field’s rapidly growing importance.

Predominant Research Themes:

The cited literature clusters around several key thematic areas:

• Green Extraction Technologies (32% of references): Dominated by supercritical CO₂, deep eutectic solvents, and enzyme-assisted extraction.
• Nanotechnology Applications (24%): Focusing on nanoencapsulation, nanocellulose, and carbon quantum dots from citrus waste.
• Biomaterial Development (22%): Particularly pectin-based hydrogels, biocomposites, and biodegradable packaging.
• Circular Economy Integration (15%): Emphasizing biorefinery concepts, life cycle assessment, and industrial symbiosis.
• Bioactive Compounds and Nutraceuticals (7%): Primarily focused on hesperidin, limonoids, and antioxidant applications.

The geographical distribution of research shows strong contributions from Europe (particularly Italy and Spain), Asia (China and India), and Latin America (Brazil and Argentina), with emerging research clusters in North America and Africa.

This methodological approach ensures a comprehensive, contemporary, and critically balanced perspective on citrus waste valorization, capturing both established knowledge and cutting-edge innovations driving the field forward.

2. Citrus Waste Composition: A Treasure Trove of Bioactive Resources

2.1. Key Components by Waste Type

Citrus processing waste is primarily composed of three anatomical regions: peel (flavedo and albedo), the pulp partition (segment membrane), and seeds. Each of these components exhibits distinct structural and chemical characteristics, which determine their potential for high-value applications. Figure 3 shows the parts of a citrus fruit and a brief description of the main bioactive components in each one.

Figure 3. Anatomical distribution of bioactive compounds in citrus fruit (C. sinensis): valorization potential of peel, pulp, and seed residues. Own elaboration with data from Afifi et al., 2023 [40].

The flavedo is the outermost, colored layer of the citrus peel. It is characterized by a dense cellular structure with minimal intercellular space, providing mechanical protection and reducing water loss. The flavedo is particularly rich in essential oils, carotenoids, and polymethoxyflavonoids (PMFs), as well as coumarins and various phenolic acids. Studies have shown that the flavedo contains higher concentrations of phenolic acids, flavonols, flavones, and especially PMFs compared to the albedo, contributing to its potent antioxidant activity and characteristic citrus aroma [5,40,41,42]. For example, in Citrus sinensis and Citrus limon, the flavedo is a significant source of nobiletin, hesperidin, quercetin, and essential oils, with vitamin C content also higher than in the albedo [34,35]. The presence of these bioactive compounds makes the flavedo a valuable raw material for the extraction of natural antioxidants, flavorings, and functional ingredients [41,42,43,44].

The albedo is the inner, white, spongy layer of the peel, located between the flavedo and the pulp. It has a high proportion of intercellular spaces, which confer excellent water- and oil-holding capacities and make it an effective shock absorber. The albedo is especially rich in dietary fiber, pectin, hemicellulose, and polysaccharides, as well as flavanones (such as naringin and hesperidin), phenolic acids, and small amounts of vitamin C [40,41,45,46,47,48].

The pulp partition, also known as the segment membrane or tabique, is the internal membrane that separates the citrus segments. This component is a notable source of dietary fiber (up to 57 g/100 g), cellulose, pectin, and residual sugars, as well as appreciable amounts of polyphenols and flavonoids, though generally at lower concentrations than in the flavedo and albedo [45,48]. The segment membrane also contains significant levels of antioxidant compounds, contributing to the overall functional potential of citrus waste streams. Its structural polysaccharides and fiber content make it suitable for use in prebiotic and functional food formulations.

Citrus seeds are notable for their lipid content, particularly rich in linoleic acid, as well as phenolic compounds, limonoids, dietary fibers, vitamins, and carotenoids. The composition varies with citrus variety, maturity, and extraction method, but seeds consistently demonstrate antioxidative, anti-inflammatory, and anticancer activities, supporting their use in functional foods, nutraceuticals, and cosmetics [5,49,50].

2.2. Comparative Analysis of Citrus Varieties

Variability in the bioactive compound composition among major citrus species, such as orange, mandarin, lemon, and grapefruit, is a fundamental factor in developing effective strategies for valorizing agroindustrial waste. Recent research confirms that several factors, including cultivar, species variety, growing conditions, fruit ripeness, and processing techniques, influence this variability. Table 1 presents updated quantitative data on the content of key bioactive compounds predominantly present in citrus peels; these compounds are currently highlighted for potential applications in biomaterials, nutraceuticals, and environmentally friendly technologies.

Table 1. Quantitative composition of key bioactive compounds in citrus peel residues under different extraction methods.

Compoud	Orange	Lemon	Tangerine, Mandarin or Clementine	Grapefruit	Extraction Method	Reference
Pectin (% DW)	23.02 ± 2.12 b 18.73 ± 1.19 c	16.61 a 13.00 ± 1.06 b	21.95 b 15.23 a	17.92 b 8.5 a	AE a UAE b MAE c	[36,51,52,53,54,55,56]
Cellulose (% DW)	37.08 ± 3.10	23.06 ± 2.11	17.29 ± 1.02	26.57	NDF/ADF	[22,36,51,52,57]
Hemicellulose (% DW)	37.2 ± 3.3	8.09 ± 0.81	11.38 ± 0.98	5.60	NDF/ADF	[22,36,51,52,57]
Lignin (% DW)	7.52 ± 0.59 4.18 ± 0.69	7.56 ± 0.54	8.8 0.56	11.6	ADL	[36,39,51,52,56]
Sugars (% DW)	9.6 a 21.06 ± 1.93 b	6.5 a	10.1 a	8.1 a	SE a SE + C	[51,52]
Carotenoids (μg/g β-caroten)	50.94 ± 2.28 a 6.30 b	11.09 ± 0.47 a	98.80 ± 2.95 a	3.61–61.42 c	R-ph Ch a UAE b SE c	[54,58,59]
D-Limonene (% EO)	94.88 a	94.427 a	97.38 a	90.92–93.98 b	MAE	[54,60]
Hesperidin (mg/g DW)	20.7 ± 0.38 a 2.052	0.84 ± 0.08 a 0.07 d	29.50 ± 0.32 a 13.74 b 64.45 b 58.60 c	0.071 b 0.62–1.09 b	SE a UAE b MAE c AMSE d	[54,58,61]
TPC (mg GAE/g)	12.20 b 66.36 a 12.59 a	15.74 b 18.12 a 15.22 a	30.84 a 152.57 a 58.68 a	21.17 a 12.48 a	UAE a MAE b	[36,54]
TFC (mg QE/g)	36.29 ± 5.69 b	30 ± 3 a	25.40 c 7.93 a 6.38 a	13.09 ± 0.06 a	SE a SC-CO2 b UAE c	[54,62,63,64]

Abbreviations: DW: dry weight; EO: essential oil. TPC: total phenolic content expressed as mg gallic acid equivalent per g dry weight (mg GAE/g DW). TFC: total flavonoid content expressed as mg quercetin equivalent per g dry weight (mg QE/g DW). AE: Acid Extraction. UAE: Ultrasound Assisted Extraction. MAE: Microwave Assisted Extraction. SE: Solvent Extraction. SE + C: Solvent Extraction + Centrifugation. R-ph Ch: Reversed-phase HPLC with UV detector. NDF/ADF: Neutral Detergent Fiber/Acid Detergente Fiber (Van Soest Method). ADL: Acid Detergent Lignin. AMSE: deep eutectic solvent-based automated solvent extraction. SC-CO₂: Supercritical CO₂ extraction. Note: The superscript letters in the numerical data relate to the extraction or quantification methods used for each compound, which are indicated for each row.

The composition of citrus residues varies significantly between species, cultivation conditions, and extraction methods. This variability is primarily attributable to three factors: (1) cultivar/species differences—genetic variation in secondary metabolite biosynthesis pathways results in distinct phytochemical profiles between oranges, lemons, mandarins, and grapefruit; (2) maturity stage—bioactive compound accumulation is highly developmental-stage dependent, with flavonoid content typically peaking at physiological maturity but declining post-harvest; and (3) extraction methodology—yields and selectivity vary substantially with solvent polarity, temperature, pressure, and auxiliary technologies (ultrasound, microwaves, enzymes). For example, hesperidin recovery from mandarin peel ranges from 13.74 mg/g DW with ultrasound-assisted extraction to 64.45 mg/g DW with microwave-assisted extraction [54,58,61]. This variability underscores the importance of standardizing extraction protocols and reporting units to enable meaningful comparisons across studies and optimize biorefinery design for specific product targets.

Pectin, with 13% to 23% DW content in peels, is widely used in the food and biomaterials industries for its gelling properties. Structural polysaccharides (cellulose, hemicellulose, lignin) are precursors to second-generation biofuels through hydrolysis and fermentation. Lignin content was primarily determined using the Van Soest detergent fiber analysis method (Acid Detergent Lignin—ADL) for references [36,51,52,56]. Values from Panwar et al. [39] are compiled from the literature using various analytical methods.

D-limonene is likely the main component of essential oils derived from citrus residues, and yields vary mainly with the extraction technology used. Flavonoids, such as hesperidin, dominate in mandarin, which is also the species that has been most extensively studied. The total phenolic and flavonoid content (TPC, TFC) can vary greatly depending on the species and extraction method, and advanced techniques—like high hydrostatic pressure or pulsed electric fields—substantially increase yields. This variability underscores the importance of standardizing extraction protocols and reporting units to enable comparisons across different studies. Suggested standardization strategies include using consistent reporting units, such as mg/g DW for phenolic content, referencing specific extraction techniques, such as high hydrostatic pressure or ultrasonication, and establishing global guidelines for measurement units and yield reporting. Additionally, the unique profiles of each citrus species point to the need for customized biorefinery approaches to maximize value within a circular economy framework.

3. Emerging Extraction and Processing Technologies

Emerging extraction and processing technologies are revolutionizing the valorization of citrus waste, enabling the sustainable recovery of high-value bioactive compounds and advanced biomaterials. Recent research highlights the integration of green chemistry, nanotechnology, and biotechnology to maximize yield, efficiency, and environmental benefits.

3.1. Green Solvent Systems

A eutectic solvent is a multicomponent mixture characterized by a melting point lower than that of any of its individual pure components. This phenomenon is observed in systems with high stability in the liquid phase, where the components are virtually insoluble in the solid state. In this context, Deep Eutectic Solvents (DES) have emerged as promising, tailor-made green solvents with a wide range of applications. Formulations such as choline chloride/levulinic acid/acid/acid/N-methylurea, glycerol, and ethylene glycol have demonstrated high efficiency and selectivity in the extraction of flavonoids—including polymethoxylated flavonoids and glycosides—as well as in the recovery of D-limonene from citrus peel waste. These systems have achieved extraction yields exceeding 85%, outperforming conventional solvents in both efficiency and environmental sustainability [37,65,66,67]. Furthermore, DES have proven effective in the extraction of microcellulose, which can subsequently be converted into cellulose nanocrystals [38], thus adding value to lignocellulosic biomass residues.

Supercritical carbon dioxide (scCO₂) extraction is a highly efficient green technology that utilizes CO₂ above its critical point (31.1 °C and 7.38 MPa), where it exhibits properties intermediate between those of a gas and a liquid. In this state, CO₂ possesses high diffusivity, low viscosity, and a tunable solvating power that can be adjusted through pressure and temperature, making it an ideal solvent for nonpolar and moderately polar compounds. This technique enables the efficient recovery of essential oils from citrus waste without exposure to high temperatures, thereby preventing thermal degradation and preserving both the bioactivity and aromatic profile of the extracted compounds [68,69,70,71].

Process efficiency and scalability considerations: scCO₂ extraction has demonstrated 80% reduction in organic solvent use compared to conventional methods [71], with CO₂ recovery rates exceeding 95% through pressure reduction and recycling systems [68,69]. D-limonene extraction yields reach approximately 50% from freeze-dried citrus peels [66], with higher yields (up to 75%) achieved when using propane as co-solvent [69]. Response surface methodology optimization identified 28.7 MPa and 60 °C as optimal conditions for maximizing recovery of flavonoids, total phenolics, and antioxidant activity [68].

However, energy requirements remain a limiting factor for widespread adoption: maintaining supercritical conditions requires continuous energy input (estimated at 15–25 MJ/kg extracted product), and capital costs for high-pressure equipment can exceed $500,000 for pilot-scale systems [72]. Despite these challenges, the elimination of solvent residues in final products and CO₂’s Generally Recognized As Safe (GRAS) status make this technology particularly attractive for food-grade and pharmaceutical applications. Figure 4 presents a schematic diagram illustrating the effectiveness of eutectic solvents and carbon dioxide extraction.

Figure 4. Schematic comparison of two green extraction technologies for citrus bioactive recovery. Left: Hydrogen-bond network in a deep eutectic solvent (DES) composed of choline chloride:glycerol (1:2 molar ratio), which lowers the melting point and enhances cell wall penetration, enabling efficient extraction of flavonoids and D-limonene from citrus peel waste. DES formulations have achieved extraction yields exceeding 85%, outperforming conventional organic solvents in both efficiency and environmental sustainability [37,38,65,66,67]. Right: Supercritical carbon dioxide (scCO₂) extraction, illustrating the CO₂ phase diagram and the supercritical region with optimal conditions (28.7 MPa; 60 °C) for maximizing flavonoid and essential oil recovery. This technique achieves D-limonene yields of approximately 50% from freeze-dried citrus peels, with higher yields (up to 75%) when propane is used as a co-solvent [68,69,70]. scCO₂ reduces organic solvent consumption by 80% and enables CO₂ recovery rates above 95%, although its widespread adoption is limited by high energy requirements (15–25 MJ/kg) and capital costs for high-pressure equipment [68,69,72].

3.2. Nanotechnology-Driven Approaches to Enhance Bioactive Compound Delivery

Nanotechnology-based strategies, specifically nanoencapsulation and nanoemulsification, have emerged as practical tools to mitigate the inherent limitations of bioactive compounds derived from citrus residues. These limitations include volatility, low water solubility, and susceptibility to degradation by UV radiation, oxygen exposure, and pH fluctuations. These advanced methods outperform conventional encapsulation techniques, such as spray drying or simple emulsification, which, while industrially viable, often suffer from high material loss and reduced long-term stability due to their larger particle size, leading to reduced overall functional efficiency and uncontrolled release profiles [73]. A comprehensive review by Oprea et al. [74] elucidates the efficacy of lipid nanostructures (liposomes, solid lipid nanoparticles, emulsions) and polymeric systems in stabilizing citrus essential oils against these adverse conditions. This stabilization process demonstrably improves biocompatibility and functional availability for food and cosmetic applications.

Collectively, these findings highlight the critical technological considerations for successful nano-delivery systems, including: formulation method (ultrasonication, emulsion phase inversion, polymeric coatings, electrospinning), bioactive compound characteristics (hydrophobicity, heat sensitivity, etc.), droplet or particle size and distribution, physical stability (PDI, zeta potential, particle size after storage), and preservation of bioactivity (antioxidant, antimicrobial). These general technical principles are established here; specific applications in health, nutrition, and functional foods are further elaborated in Section 4.1.

3.3. Biotechnological Innovations

Biotechnological innovations are at the forefront of citrus waste valorization, enabling the transformation of residues into high-value products through enzymatic hydrolysis and microbial fermentation. These approaches not only enhance resource efficiency but also support the development of sustainable bioproducts for diverse industries.

Enzymatic hydrolysis uses specific enzymes—such as cellulases, pectinases, and hemicellulases—to break down complex polysaccharides in citrus waste into fermentable sugars and bioactive oligosaccharides. This process is highly efficient and environmentally friendly compared to chemical hydrolysis. For example, optimized enzymatic cocktails can significantly increase the release of fermentable sugars, which are then used as substrates for further bioprocessing [57,75,76]. Enzymatic hydrolysis of citrus pectin generates low-molecular-weight oligosaccharides with prebiotic activity, promoting beneficial gut microbiota and increasing the production of short-chain fatty acids, which have recognized health benefits [77]. Additionally, enzymatic hydrolysis can enhance the nutritional and functional properties of citrus-derived products by increasing the content of amino acids, vitamin C, and bioactive compounds [76].

Enzymatic hydrolysis of citrus residues—Unlike acid/base hydrolysis, the enzymatic route operates under mild conditions (≈35–55 °C; pH 4.5–6.5) and exhibits high specificity toward defined glycosidic linkages, thereby reducing inhibitor formation and facilitating integration with downstream fermentations (Figure 5). In cellulose, the synergistic action of endo-β-1,4-glucanases, cellobiohydrolases, and β-glucosidases accelerates glucose release; in hemicelluloses, xylanases/mannanases and accessory enzymes enhance depolymerization; in pectins, polygalacturonases, pectin lyases, and pectin methylesterases dismantle homogalacturonan and RG-I regions, decreasing viscosity and liberating galacturonic acid and pectooligosaccharides. Key performance determinants include pretreatment strategy, enzyme loading, solid-to-liquid ratio, pH and temperature control, and mitigation of inhibitory compounds (e.g., terpenes). This approach enables co-production of fermentable sugars for L-lactic acid, bioethanol, and other bioproducts, while generating oligosaccharides with added functionality [27,36,74,75,76,78,79].

Figure 5. A comparison of acid/base chemical hydrolysis versus enzymatic hydrolysis of citrus residues. Chemical hydrolysis: takes place under harsh conditions (≈120–250 °C; pH < 1 or >10), has low selectivity, and produces sugars along with degradation by-products (e.g., HMF, furfural). Enzymatic hydrolysis: occurs under mild conditions (≈35–55 °C; pH 4.5–6.5), offers high linkage specificity, and yields fermentable sugars and functional oligosaccharides with minimal inhibitor formation. Adapted from recent studies on citrus waste bioprocessing [27,36,74,75,76,80,81].

Microbial fermentation leverages the metabolic capabilities of bacteria and yeasts to convert citrus waste hydrolysates into valuable biochemicals. Recent advances include the use of engineered strains, such as Weizmannia coagulans and Saccharomyces cerevisiae, for the efficient one-step conversion of citrus waste into L-lactic acid and meso-galactaric acid, respectively [78]. These processes combine enzymatic saccharification and fermentation, achieving high product yields and reducing process costs. Fermentation is also used to produce bioethanol, biosurfactants, and high-quality animal feed, with lactic acid bacteria improving the nutritional profile and shelf life of the resulting products [57,79]. Furthermore, fermentation can increase the antioxidant activity and bioactive content of citrus-derived products, making them suitable for functional food and nutraceutical applications [76,79]. Table 2 summarizes the main emerging technologies for the valorization of citrus waste.

Table 2. Key Technologies for Citrus Waste Valorization.

Technology	Target Product(s)	Key Advantage	Author(s)
Deep eutectic solvents (DES)	Flavonoids, microcellulose	high selectivity, green chemistry	[37,38,67]
Supercritical CO2 extraction	Limonene, phenolics	no thermal degradation, high purity	[68,70,82]
Ultrasonication	Pectin, CNCs	energy efficient, high yield	[38,83]
Nanoencapsulation	Essential oils	controlled release, enhanced food safety	[2,5,84]
Enzymatic hydrolysis	Prebiotic oligosaccharides, sugars	mild conditions, prodution of functional products	[80,81,85]
Microbial fermentation	Biosurfactants, L-lactic acid, bioethanol, feed	Biochemical diversity, sustainability, high yield	[2,57,75,79,86,87,88,89]
Enzyme production	Cellulases, pectinases, xylanases	Supports biorefineries, circular economy	[28,90,91]

4. Less Conventional Applications: From Lab to Market

This section synthesizes recent advancements in unconventional applications of citrus waste, highlighting a transition beyond traditional uses toward innovative solutions with market potential. These applications encompass the development of bioactive materials, advanced biomaterials, sustainable consumer goods, and novel agritech synergies. Progress in these areas is supported by an expanding body of research addressing technical, economic, and regulatory barriers to market adoption.

4.1. Citrus Waste Valorization: Bioactive Innovations for Health, Wellness, and Sustainable Food Systems

Recent investigations, building on established principles of nanoencapsulation and nanoemulsification (as detailed in Section 3.2), highlight the significant potential of citrus-derived bioactive compounds to improve health and well-being and to support sustainable food systems.

These investigations are exemplified by the incorporation of various citrus essential oils (bergamot, mandarin, orange, grapefruit, and lemon) into nanoemulsions using Tween-80 and ultrasonication, which resulted in an average droplet size of less than 50 nm [92]. The resulting nanoemulsions largely retained their antioxidant and antimicrobial activity after 30 days of storage, although some mandarin-based formulations exhibited reduced stability.

The application of lemon essential oil (LEO) nanoemulsions to fresh-cut kiwis not only reduced weight loss and enhanced firmness but also preserved nutrient content and antioxidant activity to levels comparable to 10 mg/L of vitamin C [93] Similar research found that LEO nanoemulsions effectively inhibited the growth of Phomopsis sp., a significant cause of postharvest decay in kiwis, thereby contributing to food security and reducing economic losses [94].

Nanoencapsulation techniques have demonstrated the ability to improve the bioaccessibility of citrus pulp polyphenols. By encapsulating these compounds within various colloidal systems (liposomes, emulsions, etc.), researchers have enhanced their solubility, stability, and subsequent absorption within the body. Furthermore, active coatings combining LEO nanoemulsions with gelatinous matrices show promising barrier properties against water loss and browning in fresh produce [95,96], potentially prolonging shelf life, reducing food waste, and simultaneously delivering beneficial bioactive compounds directly to the food surface. A study optimizing these parameters found that using a specific protein-based encapsulation matrix and maintaining a Polydispersity Index (PDI) below 0.3 increased antioxidant activity in fruit coatings by 20%. This underscores the practical impact of meticulous parameter optimization, ensuring product efficacy and wider consumer acceptance.

However, careful monitoring of several critical parameters is essential to maximize the functional effects of nanoencapsulated citrus bioactives. These include the selection of the encapsulation matrix (proteins, polysaccharides, biodegradable polymers), the bioactive compound loading, the particle/droplet size and Polydispersity Index (PDI), the zeta potential for colloidal consistency, stability during storage (temperature, time, light exposure), retention of biological activity (antioxidant, antimicrobial), and the sensory properties within the food matrix. Meticulous optimization of these variables is crucial for ensuring both the efficacy and consumer acceptance of these innovative products.

In the field of functional material development to extend food shelf life, antimicrobial films infused with citrus essential oils (CEOs) are gaining traction. These films, typically combining citrus-derived pectin with complementary biopolymers such as xanthan gum, alginate, chitosan, or starch (all Generally Recognized As Safe, GRAS), offer desirable mechanical and barrier properties. CEOs, primarily those rich in D-limonene, provide broad-spectrum antimicrobial and antioxidant effects while enhancing the film’s ability to modulate gas and moisture exchange. Fabrication methods, such as solution casting, extrusion, spraying, and dipping, allow for tailoring to specific applications, while ultrasound-assisted hydrodistillation provides an environmentally friendly alternative for extracting active volatile compounds [97,98,99].

The versatility of citrus-derived compounds extends beyond food applications, offering promising avenues for promoting overall health and well-being. Citrus peels serve as a rich reservoir of bioactive compounds with diverse therapeutic potential. Among these, hesperidin (Hesp), a glycosylated flavanone, has garnered significant attention for its multi-faceted properties, including anti-inflammatory, antioxidant, anti-cancer, and neuroprotective effects. Several studies have demonstrated hesperidin’s efficacy in mitigating inflammation and oxidative stress in various models, such as cisplatin- and methotrexate-induced nephrotoxicity [10,53,100]. In these studies, hesperidin enhanced antioxidant capacity, reduced lipid peroxidation, and preserved renal architecture by upregulating cytoprotective proteins such as Nrf2 [101]. Novel nanoformulations, such as bilosomes (HES-BS), have further improved bioavailability and systemic efficacy, demonstrating their potential in advanced drug delivery systems for targeted therapeutic interventions [102].

Beyond nephroprotection, hesperidin and its derivatives (e.g., α-glucosyl hesperidin) have shown promise in ophthalmological applications by promoting corneal regeneration without inflammatory infiltration [103]. They have also demonstrated efficacy in managing metabolic disorders such as non-alcoholic fatty liver disease, where supplementation reduced key inflammatory markers (TNF-α, CRP, NF-κB) [101]. Emerging evidence even suggests a role for hesperidin in sports medicine, attenuating exercise-induced inflammation during post-exertion recovery [104]. These findings, coupled with improved formulation strategies, position hesperidin as a promising candidate for nutraceuticals and therapeutic biomaterials targeting both chronic and acute inflammatory conditions.

Collectively, these innovations underscore the growing potential of citrus by-products to advance sustainability and promote overall well-being.

4.2. Advanced Biomaterials

Citrus-derived biomaterials have garnered increasing attention for their potential to replace synthetic counterparts in biomedical and sensing applications, aligning with sustainability and circular economy principles. Among them, pectin-based hydrogels are emerging as promising candidates for 3D-printed wound dressings. They have emerged as versatile candidates for advanced wound care systems [38]. Pectin, a heteropolysaccharide, contains reactive carboxyl and hydroxyl groups that facilitate ionic cross-linking (e.g., with Ca²⁺) and chemical modifications for targeted functions.

The incorporation of auxiliary biopolymers—such as gelatin, carboxymethylcellulose (CMC), or chitosan—improves the mechanical integrity and aqueous stability of these hydrogels, expanding their therapeutic utility. Innovative formulations combining pectin with ZnO nanoparticles and chitosan have demonstrated accelerated wound healing and intrinsic antimicrobial activity without cytotoxic effects [105]. Moreover, oxidation-mediated self-healing pectin hydrogels are being explored for dynamic tissue regeneration applications [106]. Recent advances include bio-inks integrating pectin, CMC, and nanoparticles, enabling patient-specific dressing fabrication via 3D printing [105] These findings underscore the promise of pectin hydrogels as scalable, eco-friendly platforms in next-generation biomedical materials.

4.2.1. Carbon Quantum Dots: Characterization and Functional Utility

In parallel, carbon quantum dots (CQDs) are a class of zero-dimensional carbon nanomaterials that exhibit unique properties such as intense fluorescence, biocompatibility, and tunable emission wavelengths and can be synthesized from citrus peel waste. These biodegradable nanomaterials are gaining ground for sensing and smart packaging. These plant-derived CQDs, usually obtained by hydrothermal or microwave-assisted carbonization, offer advantages such as photostability, tunable fluorescence, and surface functionalization potential, without the toxicity associated with heavy metal-based QDs [107,108,109]. Citrus peels are rich in phenolic precursors (e.g., flavonoids, limonoids) that facilitate CQD synthesis under green conditions.

Critical characterization data: A thorough understanding of their physicochemical properties is crucial for industrial applications. Citrus peel-derived CQDs typically range from 2 to 10 nm in diameter, with Polydispersity Index (PDI) values between 0.15 and 0.35, indicating relatively narrow size distributions [107,108,109]. Regarding surface charge, CQD generally exhibit negative ζ-potential values ranging from −15 to −35 mV due to surface carboxyl and hydroxyl groups. Since values above ± 20 mV are typically considered stable against aggregation, this negative charge significantly contributes to their colloidal stability in aqueous systems. Properly formulated CQD nanoemulsions retain over 90% of their initial fluorescence intensity after 30 days at 4 °C, demonstrating good storage stability, although mandarin-based formulations may show accelerated degradation.radation [92].

Functionally, these CQDs show vigorous antimicrobial activity and have been embedded in biopolymer matrices like chitosan-pullulan or CMC-starch films, achieving over 99% inhibition against E. coli and S. aureus [110]. In food systems, citrus CQDs enable the development of intelligent packaging films responsive to volatile spoilage compounds and are incorporated into edible coatings to prolong shelf life [108,109]. Specific applications include fluorescent sensors for detecting bacterial contamination in milk and pesticide residues in citrus juices [111].

4.2.2. Regulatory Landscape and Future Priorities

Despite promising laboratory-scale performance, challenges remain in terms of synthesis reproducibility, scalability, quantum yield enhancement, and biosafety validation under real-use conditions [112,113]. The current regulatory frameworks for food-contact nanomaterials are fragmented and inadequate for naturally-derived nanostructures. The European Food Safety Authority (EFSA) published guidance on nanomaterial risk assessment in 2021 [114] establishing criteria based on particle size (<100 in at least one dimension), but the implementation pathways for agro-waste-derived nanomaterials remain undefined. The U.S. FDA maintains a more flexible, product-by-product approach but lacks specific guidelines for citrus-derived nanocellulose or CQD in food packaging applications [115,116].

Key regulatory gaps include: (1) absence of standardized toxicity testing protocols for plant-derived nanomaterials; (2) unclear migration limits for nanoparticles from packaging into food; (3) lack of harmonization between regional authorities (EU, US, Asia); and (4) insufficient long-term environmental fate data. These uncertainties create market entry barriers and slow the commercialization of otherwise promising technologies [114,116].

Addressing these limitations through standardized protocols and comprehensive toxicological assessments will be critical for translating citrus-derived CQDs into market-ready, sustainable diagnostic platforms. The most urgent research gaps include developing industrial-scale synthesis processes that ensure batch-to-batch consistency, conducting long-term ecotoxicity studies to understand potential environmental impacts, and refining quantum yield enhancement techniques. By focusing on these priorities, the transition from laboratory success to widespread adoption in various applications can be significantly expedited.

4.3. Sustainable Consumer Goods

Leveraging the inherent biocompatibility, biodegradability, and functional properties of citrus-derived compounds, emerging innovations are enabling the creation of eco-friendly textiles, smart packaging, and biodegradable electronics that align with circular economy principles and low-impact manufacturing.

4.3.1. Citrus Fiber Composites in Sustainable Textiles

Citrus-derived fiber composites are gaining momentum in the textile industry as biodegradable, low-toxicity alternatives to petroleum-based materials. Citrus sinensis peels serve as a rich source of microcrystalline cellulose (MCC), which can be extracted using environmentally benign protocols—alkaline pretreatment, acid hydrolysis, and chlorine-free bleaching—to yield material with high crystallinity (~72%) and thermal stability (~308 °C). Scanning electron microscopy reveals a rod-like morphology with microfibrillar architecture, while elemental analysis confirms a high carbon and oxygen content, consistent with desirable reinforcement properties [117].

This citrus-derived MCC has been effectively incorporated into biopolymer-based composites, enhancing mechanical strength and thermal resistance while enabling circular use of agro-industrial waste [99]. Additionally, the transformation of citrus pulp into textile fibers has been industrialized by companies such as Orange Fiber S.r.l. (Catania, Italy), demonstrating the viability of citrus-based fabrics with aesthetic, durable, and sustainable performance [99,118]. Since its founding in 2014, Orange Fiber has produced over 500,000 m of citrus-derived textile, partnering with major fashion brands and achieving mechanical properties comparable to silk (tensile strength: 200–250 MPa, elongation: 15–20%) [118]. The process requires approximately 15 kg of citrus pastazzo (wet waste) to produce 1 kg of dry cellulose suitable for textile spinning, with water consumption reduced by 40% compared to conventional cotton processing.

Beyond structural materials, citrus essential oils—including lemon, orange, and lime—are being utilized in functional textiles via microencapsulation. This strategy preserves volatile compounds and facilitates controlled release of bioactives with antimicrobial, antioxidant, insect-repellent, and aromatic properties. When these oils are incorporated into coating matrices such as chitosan, alginate, or gum arabic and applied by padding to cotton, wool, and polyester, they create multifunctional textiles with improved UV resistance and biocidal properties. The addition of plant-derived nanomaterials further provides self-cleaning and energy-harvesting abilities, while reducing environmental impact [119]. These innovations make citrus fibers and oils key drivers of sustainable, high-performance textiles in textile ecosystems.

4.3.2. Limonene as a Green Solvent in Biodegradable Electronics

Limonene is emerging as a bio-based, non-toxic, and biodegradable alternative to hazardous solvents in organic electronics fabrication. Its potential for low-carbon production through algae or microbial biosynthesis aligns with circular bioeconomy principles [120]. While its integration offers low environmental impact and compatibility with certain conjugated polymers, challenges persist concerning solvent–polymer miscibility and large-scale formulation. Binary solvent systems like limonene:indane have shown promise, enabling the processing of semiconductors like PM6 and BTP-eC9 and achieving power conversion efficiencies in OPVs approaching those of traditional toxic solvents [120]. Ongoing optimization and formulation strategies are crucial to expanding its applicability across various material systems. Interdisciplinary research integrating biochemistry, materials science, and green chemistry is key to designing tailored terpene solvents. Comprehensive life cycle assessments and toxicological evaluations are essential to support the industrial transition towards green electronics based on citrus-derived inputs [120,121].

4.4. Agri-Tech Synergies

Biochar derived from citrus waste is emerging as a multifunctional input within sustainable agriculture, offering a scalable solution for soil remediation, nutrient management, and carbon sequestration. Produced through controlled pyrolysis (300–500 °C) or vermicomposting of citrus peels, this biochar exhibits high porosity, chemical stability, and surface functionality—primarily hydroxyl and carboxyl groups—making it an effective soil conditioner and adsorbent for heavy metals such as Pb, Cu, and As [122,123,124]. Its agronomic benefits include increased cation exchange capacity, pH regulation in acidic soils, enhanced retention of Ca, Mg, and K, and stimulation of microbial activity, all of which contribute to improved plant growth and root development [122].

Applications in crop systems have demonstrated promising results, including yield enhancement in sandy soils and disease suppression in tomatoes and peppers at application rates of 1–5% [125]. Beyond productivity, citrus biochar supports climate-smart agriculture by sequestering atmospheric CO₂ in stable carbon forms and reducing emissions of N₂O and CH₄, thanks to its high surface area and adsorptive capacity [126,127]. In citrus orchards, it has been shown to improve soil organic carbon dynamics, increasing resilience to water stress and environmental degradation.

These synergistic effects position citrus-derived biochar as a strategic tool within the agri-biotechnological framework, bridging technological innovation and circular economy principles. By transforming citrus residues into value-added amendments, this approach enables waste valorization, carbon mitigation, and agroecosystem restoration, especially in climate-vulnerable regions. As such, citrus biochar exemplifies how agri-tech integration can drive sustainable development at both farm and landscape scales.

The type of treatment citrus waste undergoes determines the products that can be obtained. A summary of this is shown in Figure 6.

Figure 6. Conceptual map illustrating the valorization pathways of citrus waste, including direct uses and biorefinery processes.

5. Challenges and Unresolved Issues

The valorization of citrus waste offers significant benefits but also presents many challenges. The efficient and sustainable conversion of this type of waste into high-value products is challenged by technical, economic, regulatory, and environmental factors. Addressing these challenges is crucial for the development of integrated biorefinery models and the transition to a circular bioeconomy in citrus-producing regions [27,32]. The following section analyzes the main barriers and unresolved issues currently limiting the large-scale implementation of citrus waste valorization strategies.

5.1. Technical Barriers

Despite noteworthy advancements in the upcycling of citrus by-products, especially their conversion into bio-functional and value-added compounds, considerable engineering and scientific hurdles remain. Principal limitations include the temporal and compositional variability of citrus feedstocks, as well as the susceptibility of bioactive constituents to degradation during processing. These combined challenges hinder process standardization, the achievement of reproducible results, and the successful large-scale implementation required for industrial adoption [16].

Seasonal Variability in Citrus Waste Composition

The biochemical composition of citrus by-products, particularly peels and pulps, is significantly influenced by factors such as varietal genotype, climatic conditions, and harvest practices [16,17]. D-limonene exhibits substantial variability (30–90%) based on citrus species, developmental stage, and geographic origin [44]. This variability significantly affects both the extraction yield and the inhibitory effects on microbial fermentation processes used in bioethanol and biogas production [27,32]. This compositional variability directly impacts the suitability of limonene as a green solvent, thereby affecting both extraction yields and performance in electronic applications. For example, fluctuations in limonene content will affect its solvent properties, impacting polymer dissolution and device performance. Therefore, understanding and managing seasonal variability is crucial for ensuring consistent limonene quality and optimizing its effectiveness in biodegradable electronics and other applications.

5.2. Economic and Regulatory Hurdles

The implementation of sustainable technologies for the valorization of citrus byproducts—particularly for the isolation and modification of functional biomolecules and nanoscale materials—faces significant financial and policy constraints. Despite their potential to substitute synthetic alternatives and reduce ecological footprint, these methods remain economically unfeasible and are frequently obstructed by the lack of harmonized regulatory standards, especially within emerging bio-economies [72,115,128].

5.2.1. High Costs of Green Extraction Methods

Despite their clear environmental and functional advantages, advanced green extraction methods—such as supercritical carbon dioxide (SC-CO₂), deep eutectic solvents (DES), ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and enzymatic hydrolysis—face substantial economic and regulatory barriers when compared to conventional solvent-based techniques. These technologies align closely with green chemistry principles by reducing solvent use, enhancing the recovery of thermolabile bioactives, and improving process safety and selectivity. However, their adoption at an industrial scale is often limited by high capital and operational costs. The need for specialized equipment, precise process calibration, and elevated energy input results in significant financial burdens, particularly for small- and medium-sized enterprises (SMEs) [27,54]. Additionally, infrastructural gaps, restricted access to technology, and underdeveloped markets in citrus-rich regions—such as Latin America, Africa, and Southeast Asia—further hinder scalability and commercialization [27]. To overcome these challenges, early-stage integration of techno-economic assessments and life-cycle costing is critical. Strategies such as co-location with existing agro-industrial facilities, fostering industrial symbiosis, and developing integrated multi-stream biorefineries can enhance cost-effectiveness and promote the sustainable valorization of citrus waste.

5.2.2. Lack of Standardized Regulations for Citrus-Derived Nanomaterials

The rapid development of citrus-based nanomaterials—including nanocellulose, nanoemulsions, and other functional nanotech—has surpassed the creation of consistent regulatory standards. These nanomaterials have strong potential for applications such as smart packaging, biomedical coatings, targeted drug delivery, and biosensing. However, their legal and safety status, particularly for food-contact uses, remains largely undefined. Major regulatory agencies such as EFSA, FDA, and OECD often do not adequately address naturally sourced nanoparticles derived from agro-industrial waste, such as citrus byproducts. This results in notable gaps in understanding toxicity levels, environmental effects, and long-term safety [54,114].

The lack of globally standardized criteria for safety assessment, labeling, and approval processes creates legal ambiguity and significantly increases the time and cost of compliance, particularly for small and medium-sized enterprises (SMEs) and research-focused projects operating in regions with fragmented or evolving regulatory environments. These challenges hinder the widespread adoption and commercialization of potentially safe and beneficial nanotechnologies [114,116].

To ensure responsible integration and commercialization, internationally harmonized frameworks are crucial. These frameworks should incorporate evidence-based toxicity criteria, clear definitions, and streamlined approval pathways for sustainable nanomaterials. Simultaneously, developing open-access data repositories and fostering interdisciplinary collaborations between academia, industry, and regulatory agencies are essential to promote regulatory convergence and accelerate the translation of citrus-based nanotechnologies from the laboratory to the Marketplace [114,116].

5.3. Environmental and Social Trade-Offs

Transforming citrus waste into nanomaterials and sophisticated bioproducts offers exciting opportunities to promote a regenerative, eco-conscious bioeconomy. Nonetheless, a careful evaluation of both ecological and societal repercussions is crucial. Two primary challenges emerge: the cradle-to-grave environmental footprint linked to the fabrication of nanostructures from citrus remnants, and the growing tension with nutritional-grade uses—especially concerning pectin, a staple in the global food industry [129].

5.3.1. Life Cycle Analysis (LCA) to Obtain Derivatives from Citrus Waste

Given these considerations, sustainability cannot be assumed solely on the basis of the renewable origin of raw materials. Accurate and regionally contextualized life cycle assessments (LCAs) are imperative. These assessments must incorporate the boundaries of multifunctional systems, reflecting real-world variables such as local electricity grids, transportation logistics, and waste management infrastructure. It is recommended that future research prioritize transparency and comparability in LCA practices by adhering to established standards such as ISO 14040/44 [130]. These standards provide a universally recognized framework for conducting LCAs, ensuring methodological consistency and facilitating cross-study comparisons. Currently, comprehensive and standardized LCA data for the production of citrus-derived nanomaterials remain limited, hindering informed decision-making by technology developers and policymakers. Transparent and comparative LCA frameworks that consider environmental and social impacts across the entire product life cycle should be prioritized, thereby supporting the responsible advancement of citrus waste valorization [131,132].

Pectin and limonene recovery have been the subject of extensive LCA assessments, indicating that pectin extraction entails a significantly higher environmental burden than limonene recovery, mainly due to the high electricity demand and the use of ethanol as a solvent. A recent study revealed that 300 g of orange peel produces 8.22 g of pectin and 0.14 g of limonene; by replacing ethanol with more sustainable solvents, the environmental footprint of pectin production was reduced by 73.4% [133]. From a market perspective, the global pectin sector is projected to grow at a compound annual growth rate (CAGR) of approximately 7% between 2023 and 2030, driven by demand from the food, pharmaceutical, and cosmetics industries. Similarly, the limonene market is exhibiting an upward trend due to its applications in cleaning products, fragrances, and bioplastics [133]. However, the economic viability of citrus waste valorization ultimately depends on process optimization and environmental impact minimization, and LCA plays a crucial role in identifying critical points and directing improvements toward sustainable industrial practices [27,32,133].

Table 3 presents a comprehensive comparison of environmental performance across major citrus valorization technologies, revealing that multifunctional biorefinery approaches with cascading extraction can achieve 60–65% reductions in both GHG emissions and energy consumption compared to single-product processes [68,133,134].

Table 3. Comparative environmental performance of citrus waste valorization technologies based on life cycle assessment studies.

Technology/Product	Functional Unit	Primary Environmental Hotspot	GHG Emissions (kg CO2-eq)	Energy Consumption (MJ)	Key Mitigation Strategy	Reference
Pectin extraction (conventional, ethanol-based)	1 kg pectin	Ethanol production and electricity use	8.5–12.3	145–180	Solvent substitution with DES (73% reduction)	[133]
D-limonene extraction (scCO2)	1 kg limonene	High-pressure equipment operation	3.2–4.8	85–110	CO2 recycling (>95% recovery); renewable electricity	[68,133]
Nanocellulose production (acid hydrolysis)	1 kg CNC	Sulfuric acid use and neutralization	15–22	200–280	Enzymatic pre-treatment; closed-loop acid recovery	[131,132]
Biogas production (anaerobic digestion)	1 MJ biogas	Methane leakage; digestate management	0.02–0.08	−5 to −15 (net energy producer)	Co-digestion with olive waste; fugitive emission control	[134]
Biochar production (pyrolysis, 450 °C)	1 kg biochar	Pyrolysis energy input	0.5–1.8 (net negative with C sequestration)	25–45	Integration with energy recovery from syngas	[122,126]
Active packaging film (pectin + CEO)	1 m2 film	Biopolymer extraction; essential oil distillation	0.8–1.5	12–25	Cascading valorization (oils first, then pectin)	[97,133]
Multifunctional biorefinery (cascading extraction)	1 ton citrus waste processed	Transportation (>15 km); electricity mix	120–180 (conventional) vs. 45–70 (optimized)	850–1200 vs. 350–550	Industrial symbiosis; solar energy; <15 km collection radius	[133,134]

Abbreviations: GHG: Greenhouse gas; DES: Deep eutectic solvents; scCO₂: Supercritical CO₂; CNC: Cellulose nanocrystals; CEO: Citrus essential oil. Notes: Negative energy values indicate net energy production. GHG emissions include cradle-to-gate impacts unless otherwise specified. Values represent ranges from multiple studies with varying system boundaries. Optimized biorefinery scenarios assume renewable electricity, industrial symbiosis, and cascading valorization.

5.3.2. Competition with Food-Grade Applications

The rising demand for citrus-derived pectin, once primarily used in food products such as jams and beverages, is now surging as it expands into bioplastics, edible films, and biomedical applications. This diversification, while showcasing innovation in bio-based materials, has led to resource competition, supply strain, and significant price volatility in the global pectin market—impacting mainly small-scale food processors in low and middle-income countries [135]. With prices increasing over 25% in the past five years, access to this key polysaccharide is becoming increasingly constrained due to competition from non-food sectors [16].

The shift toward high-value biomaterials from citrus waste risks undermining socially significant food uses—like artisanal jams and infant nutrition—by marginalizing smallholder farmers and regional processors who lack the means to compete with industrial actors [136]. To balance innovation with equity, scholars recommend a tiered valorization strategy that reserves premium citrus fractions for food and pharmaceutical applications while assigning lower-grade byproducts to industrial uses [137]. Inclusive innovation frameworks are also essential to ensure that citrus bioeconomy initiatives reflect local needs and safeguard fair access to vital food-grade resources for vulnerable communities [138].

5.4. Consumer Acceptance & Market Integration in Latin America

Consumer acceptance of citrus waste-derived products in Latin America is influenced by cultural, regulatory, and market factors, resulting in a duality. While a historical preference for synthetic additives persists, particularly in processed foods, there is a growing demand for natural and sustainable products driven by perceived health and environmental benefits. This shift is evidenced by recent surveys in Brazil that highlight high valuation of sustainable packaging and sustainability labels. Front-of-package nutrition labeling in several Latin American countries has reinforced this sensitivity to health and transparency, increasing interest in “clean” and natural ingredients and packaging.

Nevertheless, significant barriers remain, including caution regarding nanotechnologies in food, sensory sensitivity to essential oils in coatings, and skepticism towards unverifiable environmental claims, which emphasize the importance of robust certifications and traceability. The adoption of international certifications, such as GRAS and EFSA “novel food” approvals, although still emerging in the region due to a lack of regulatory harmonization, is becoming a key requirement for accessing global markets.

At the public policy level, weak and fragmented regulatory frameworks in major producer countries such as Brazil, Mexico, and Argentina, coupled with limited enforcement and incentives for waste valorization, constrain investment and diminish the sector’s competitiveness. Therefore, to foster consumer acceptance and ensure the viability of citrus waste valorization in Latin America, it is advisable to prioritize sensory-neutral designs, clear labeling, and performance validation aligned with industry standards. Additionally, strengthening public policies, creating fiscal incentives, and promoting the adoption of international certifications are recommended strategies. It is also crucial to promote regulatory harmonization and traceability measures to prevent greenwashing.and traceability to avoid greenwashing [139].

5.5. Public Policy & Regulatory Landscape in Latin America

The regulatory landscape in Latin America is evolving towards circular economy and extended producer responsibility (EPR) models, directly impacting active/bio-based packaging and food contact materials. Chile is leading the region, having implemented the first mandatory collection and recycling targets for packaging under Law 20.920 (EPR Law) since 2023, expanding categories in 2025 (including textiles) and reinforcing reporting and management obligations [140,141,142]. Mexico has enacted the General Law on Circular Economy at the federal level (with updates between 2023 and 2025) and state laws (such as in Mexico City, 2023), promoting the reduction, reuse, recycling, and valorization of waste [143]. In Brazil, ANVISA updated its guide on food contact materials in 2024, incorporating criteria for chemical recycling and biodegradable/compostable materials, aligned with the National Solid Waste Policy [144,145], and has announced increased inspections and specific restrictions for substances in contact with food [146,147]. Colombia has consolidated a National Circular Economy Strategy [148,149,150] with inter-institutional governance and sector-specific roadmaps, while Peru has approved Circular Economy Roadmaps for industry [151,152], and specific sectors between 2020 and 2025. Collectively, these regulatory instruments create opportunity windows for citrus waste valorization through green procurement, EPR targets for packaging, and incentives for recyclable and bio-based materials, while also demanding compliance with specific requirements (positive listing, substance migration, technical performance), thereby favoring the scaling up of biopolymers, active coatings, and compostable packaging with robust LCA evidence.

Global Policy Landscape—Comparative Analysis

While Section 5.5 emphasizes Latin American contexts given their significant citrus production and emerging circular-economy frameworks, a global comparative perspective is essential for understanding policy best practices and technology-transfer opportunities.

European Union: The EU leads in comprehensive circular economy policy through the European Green Deal and the Circular Economy Action Plan (2020) [153], which mandates member states to achieve 65% packaging recycling rates by 2025 and prioritize bio-based materials in public procurement [EU Policy]. The EU Novel Foods Regulation (2015/2283) [154] provides a centralized approval pathway for citrus-derived ingredients, though approval timelines typically exceed 18–24 months. Germany’s Packaging Act (VerpackG, 2019) [155] imposes extended producer responsibility with financial incentives for recyclable and bio-based packaging, creating market pull for citrus-derived materials.

United States: U.S. policy is more fragmented, with state-level initiatives driving innovation. California’s SB 54 (2022) [156] requires a 25% reduction in single-use plastics by 2032, opening opportunities for bio-based alternatives. The USDA BioPreferred Program certifies bio-based products and favors procurement, but it does not have mandatory standards. FDA regulation of food-contact materials relies on a Generally Recognized As Safe (GRAS) self-determination process, which speeds market entry but offers less regulatory certainty than EU pre-market approvals [115,157].

Asia: China’s 14th Five-Year Plan (2021–2025) [158] prioritizes the circular economy and sets targets for valorizing agricultural waste, with specific subsidies for biorefinery infrastructure in central citrus-producing provinces (Guangdong, Hunan, Jiangxi). Japan’s Sound Material-Cycle Society promotes cascading use of biomass through tax incentives and R&D funding. India focuses on waste-to-energy policies but lacks specific frameworks for high-value bioactive extraction, representing an opportunity gap.

Comparative strengths and opportunities for knowledge transfer:

• EU regulatory science leadership → applicable standardization protocols for Latin America and Asia
• U.S. market-driven innovation → business model insights for cooperative biorefineries
• Asian infrastructure investment → scalability lessons for regional processing hubs
• Latin American EPR expansion → inclusive models balancing industrial growth with smallholder equity

Policy harmonization priorities include: (1) mutual recognition of safety assessments for GRAS/novel food ingredients; (2) standardized LCA methodologies with regional adjustment factors; (3) technology transfer mechanisms linking EU/US research with citrus-producing developing economies; and (4) global certification schemes for sustainable bio-based materials.

6. Future Perspectives: Roadmaps for Sustainable Valorization

The valorization of citrus waste is currently transitioning toward more advanced, circular industrial models, driven by technological innovations, novel materials, and evolving regulatory frameworks. Current trends show a convergence between biotechnology, materials engineering, circular economy principles, and digitalization, opening significant opportunities to maximize the added value of these by-products while contributing to the United Nations Sustainable Development Goals (SDGs), particularly SDG 9 (Industry, Innovation, and Infrastructure) and SDG 12 (Responsible Consumption and Production).

6.1. Cross-Cutting Trends and Integration Opportunities

Convergent process design. Cascading schemes that sequence volatile recovery, pectin valorization, and carbohydrate conversion can unlock higher aggregate value while lowering impacts; these should be demonstrated under continuous operation with interoperable mass, energy, and LCA datasets [38,132,133,134].

Digital enablement. AI-assisted optimization and blockchain-based traceability will support adaptive control, certification, and transparent claims of circularity across distributed CR supply networks [38,39,132,134].

Materials translation. Citrus-derived nanocellulose, limonene-based polymers, and active/functional coatings offer promising platforms; market translation depends on reproducible scale-up, safety substantiation, and alignment with food-contact/biomedical standards [15,39,99,105,121,159].

6.2. Standardization for Comparability and Investment

Minimum reporting standards will accelerate progress: (i) feedstock definition (species, maturity, storage), (ii) unified yield units (e.g., mg g⁻¹ DW; wt% of original CR), (iii) unit-operation mass/energy balances, and (iv) inhibitor profiles relevant to downstream steps. Harmonized disclosures enable robust TEA/LCA and de-risk scale-up decisions [24,38,131,132].

6.3. Prioritized Knowledge Gaps and Actionable Research

Industrial-scale nanocellulose production from CR—Focuses on scale, reproducibility, and environmental impact. Action: Launch a pilot process using closed-loop reagents and enzyme-assisted pretreatments; publish batch quality metrics (crystallinity, DP, surface chemistry) and cradle-to-gate lifecycle inventories. Milestone: Achieve 6–12 months of continuous operation with accessible data on mass, energy, and LCA, along with quality control protocols [131,160].

Long-term toxicology and ecotoxicology—Safety for food-contact/biomedical use. Action: Execute chronic and environmental fate studies for nanocellulose/CQDs and active coatings under realistic conditions (storage, migration, disposal); report harmonized endpoints to build weight-of-evidence dossiers [106,107,108,109,110,111,117,161,162]. Milestone: Public, cross-study comparable datasets covering migration limits, bioaccumulation, and biodegradation relevant to regulatory review.

Regulatory harmonization and approval pathways—From fragmented guidance to aligned compliance. Action: Map intended uses to EFSA/FDA/OECD guidance; define standard test batteries (toxicology, migration, labeling) and create shared data repositories to enable mutual recognition and faster approvals for bio-based nanomaterials [114,115,116]. Milestone: Consensus test panels and templates for food-contact/biomedical submissions, co-developed with regulators and industry.

6.4. Implications for Industry and Policy

For industry, the near-term priority is integration over intensification: build cascading, modular biorefineries co-located with CR sources, run continuously, and validate products in specific use-cases (e.g., L-lactic acid, active films) with prespecified performance/safety criteria [38,99,132,133,134]. For policymakers, auditable evidence—transparent LCA, standardized safety dossiers, and certification schemes—will be decisive to attract investment and ensure credible circular outcomes in diverse regional contexts [114,115,116,129,131,132].

6.5. Short-Term Agenda (12–24 Months)

Demonstrate two or more end-to-end pilot lines (cascading extraction + enzymatic conversion) with continuous operation and open TEA/LCA inventories [38,132,133,134].

Publish minimum-standard datasets (feedstock, yields, balances, inhibitors) to enable meta-analysis and benchmarking [16,131].

Execute targeted toxicology/ecotoxicology programs on priority materials (nanocellulose, CQDs, active coatings) with harmonized endpoints for regulatory use [105,106,107,108,110,111,117,125,161,162].

Co-create with regulators aligned test panels and labeling/migration criteria for citrus-derived, bio-based nanomaterials to reduce time-to-market [114,115,116].

By focusing on integration, standardization, safety validation, and regulatory alignment, citrus residues can evolve from an environmental liability into a reliable industrial platform for sustainable biomaterials and bioactive products—supporting SDG 9 and SDG 12 while enabling region-appropriate circular bioeconomy models [27,38,39,132,133,134].

Table 4 summarizes the state of the art and future research perspectives or recommendations for the valorization of citrus waste.

Table 4. State of the art and future directions in citrus waste valorization.

Focus Area	Recent Advances	Key Challenges	Strategic Priorities	Timeline	References
Digital Technologies & AI Integration	Machine learning for process optimization; blockchain traceability systems; IoT-enabled waste monitoring	Lack of standardized protocols; limited open-access datasets; high implementation costs for SMEs	Development of open-source AI algorithms for seasonal variability management; pilot-scale demonstrations with 18–24 month ROI targets	Short-term (1–3 years)	[16,17,38,39,133]
Multi-Output Biorefineries	Cascading extraction systems (oils→pectin→nanocellulose→biochar); industrial symbiosis networks co-processing citrus-olive waste	Scale-up barriers; regional infrastructure gaps; techno-economic modeling limitations	Regional demonstration hubs in major producing countries; policy incentives for co-located facilities; modular biorefinery designs	Medium-term (3–5 years)	[27,28,32,38,133,134,137]
Nanomaterial Standardization	Carbon quantum dots for biosensing; nanocellulose for flexible electronics; advanced characterization methods	Regulatory uncertainty; lack of ISO-compliant protocols; limited long-term ecotoxicity data	Harmonized LCA frameworks (ISO 14040/44) [130]; comprehensive safety assessments; batch-to-batch quality control standards	Short to medium-term (1–5 years)	[15,38,107,108,109,110,111,112,113,114,131]
Green Extraction Scale-Up	DES, scCO2, enzymatic hydrolysis at pilot scale; hybrid extraction strategies	High capital costs; technical complexity; energy intensity; limited industrial expertise	Modular/containerized processing units; open innovation consortia; hybrid technology integration; subsidized access for SMEs	Medium-term (3–7 years)	[27,38,54,66,75,76,77,115,128]
Feedstock Variability Management	Compositional databases for varieties; controlled storage protocols	Seasonal/varietal inconsistency; lack of real-time analysis; process standardization difficulties	Data-driven blending protocols; low-cost on-site sensors (NIR, e-noses); post-harvest stabilization methods	Short-term (1–3 years)	[16,17,32,38,39,133]
Policy & Regulatory Frameworks	Chile’s EPR Law 20.920; Colombia/Peru circular economy strategies; Brazil’s updated food contact regulations	Fragmented regulations across regions; slow approval pathways for novel materials; limited fiscal incentives	Fast-track GRAS approvals; green procurement mandates; international standards harmonization; EPR expansion to bio-based materials	Short to medium-term (2–5 years)	[114,116,142,144,145,147,148,151,152,157]
Business Model Innovation	Cooperative biorefinery ownership; product-as-a-service models; blended finance instruments	Traditional linear thinking; risk-averse investment; unclear ROI timelines	Risk assessment frameworks for bioeconomy ventures; impact investment instruments; transparent market mechanisms	Medium-term (3–7 years)	[136,137,138]
Food Security vs. Industrial Use	Tiered valorization hierarchies; market transparency initiatives	Pectin price volatility (>25% increase); competition between food/non-food applications; smallholder marginalization	Preferential access programs for smallholders; commodity exchanges for citrus derivatives; quality-based allocation systems	Ongoing priority	[16,135,136,137,138]
Consumer Acceptance	Growing demand for sustainable packaging (76% in Brazil); certification systems (B Corp, C2C)	Nanotechnology skepticism; sensory concerns with EO coatings; greenwashing distrust	Sensory-neutral product design; third-party certifications; transparent labeling; performance validation vs. benchmarks	Short to medium-term (2–5 years)	[95,139,157,163,164,165]
Advanced Biomaterials	4D-printed pectin hydrogels for drug delivery; self-healing limonene vitrimers; mycelium-citrus composites	Scalability of advanced materials; cost competitiveness with petrochemicals; limited clinical validation	Industrial-scale production without toxic byproducts; comprehensive material performance testing; carbon-negative pathways	Long-term (5–10 years)	[15,100,105,121,127]
Biochar & Soil Applications	Carbon sequestration in citrus orchards; heavy metal remediation; nutrient retention enhancement	Pyrolysis process standardization; long-term agronomic trials needed; application rate optimization	Controlled pyrolysis protocols; multi-year field studies across soil types; integration with precision agriculture	Medium to long-term (3–10 years)	[122,124,125,126]
Ecotoxicity & Environmental Impact	Initial LCA studies for limonene/pectin extraction; citrus CQD antimicrobial efficacy	Limited cradle-to-grave assessments; unknown chronic toxicity of nanomaterials; regional LCA variations	Comprehensive long-term ecotoxicity studies; standardized LCA methodologies with regional contextualization; transparent reporting	High priority (1–5 years)	[112,113,132,133,161,162]
Enzymatic & Microbial Processes	One-step lactic acid production; prebiotic oligosaccharide generation; enzyme production from citrus waste	Enzyme stability; process optimization; scale-up complexity; feedstock variability	Thermophilic enzyme development; synthetic biology approaches; integrated saccharification-fermentation systems	Medium-term (3–7 years)	[75,76,77,78,91]

7. Conclusions

Citrus residue valorization must progress from isolated laboratory advances to integrated, scalable solutions. Future research should prioritize decision frameworks that enable context-specific technology selection rather than yield-based comparisons. Establishing minimum reporting standards for composition, energy balances, and inhibitor profiles will improve comparability and accelerate techno-economic and life cycle assessments.

Process integration—such as cascading extraction combined with enzymatic hydrolysis—offers greater sustainability gains than intensifying single steps. Demonstrating pilot-scale continuous operations and publishing open-access datasets will be critical to de-risk industrial adoption. Regulatory readiness should be embedded early in product design to shorten approval timelines, while modular biorefineries and cooperative models can ensure equitable value distribution in citrus-producing regions.

Citrus residue valorization is pivotal for advancing circular bioeconomy models. This review demonstrates that green extraction, nanotechnology, and biotechnological innovations can transform citrus waste into high-value biomaterials and bioactive products, reducing environmental burdens and creating new economic opportunities. Future efforts should prioritize industrial-scale nanocellulose production, long-term toxicological validation of citrus-derived nanomaterials, and regulatory harmonization to accelerate market adoption. By integrating these priorities, citrus waste can evolve from an environmental liability into a strategic resource for sustainable industries, supporting SDG 9 and SDG 12.