# Valorization of Fruit Processing Wastes for Sustainable Biofuel Production: Composition, Co-Digestion Approaches for Enhanced Biogas Production, and Associated Challenges

Akash Phillip, Shravan Kumar, Sakshi Atulbhai Shah

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Akash Phillip, Shravan Kumar, Sakshi Atulbhai Shah. Valorization of Fruit Processing Wastes for Sustainable Biofuel Production: Composition, Co-Digestion Approaches for Enhanced Biogas Production, and Associated Challenges. European Journal of Scientific Research and Reviews, 2025, 3 (3), pp.191-203. 10.5455/EJSRR.20250626080851. hal-05290973

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# Valorization of fruit processing wastes for sustainable biofuel production: Composition, co-digestion approaches for enhanced biogas production, and associated challenges

Akash Phillip<fnref n="1" /> ID, Shravan Kumar<fnref n="2" />, Sakshi Atulbhai Shah<fnref n="3" />

## ABSTRACT

Fruit-processing wastes (FPWs) represent abundant, renewable substrates with significant potential for sustainable energy generation through anaerobic digestion. This review consolidates current knowledge on the composition, availability, and bioenergy potential of major FPWs, with a particular emphasis on biogas production. The high carbohydrate, pectin, and moisture contents of FPWs make them suitable for rapid microbial degradation, while co-digestion strategies, especially with nutrient-rich substrates such as cow dung, help optimize carbon-to-nitrogen ratios, enhance process stability, and improve methane yields. Comparative data from various FPW types demonstrate notable improvements in gas production and quality when synergistic feedstock combinations are applied. The review also examines pretreatment methods for overcoming lignocellulosic recalcitrance, identifies common inhibitory compounds, and discusses the environmental and economic implications of FPW-based biogas systems, including greenhouse gas mitigation and rural energy security. Challenges such as seasonal availability, collection logistics, and process scaling are highlighted alongside potential solutions. By integrating technical, economic, and environmental perspectives, this work underscores the role of FPW valorization in advancing circular bioeconomy goals and outlines research directions for improving efficiency and adoption of decentralized biogas technologies.

## ARTICLE HISTORY

Received June 26, 2025

Accepted August 15, 2025

Published September 12, 2025

## KEYWORDS

Anaerobic digestion;
methane yield; co-digestion;
waste valorization; circular
bioeconomy; renewable
energy

## Introduction

Biofuels have emerged as a pivotal alternative to fossil fuels, offering a sustainable pathway to reduce greenhouse gas emissions and alleviate the environmental burden of conventional energy systems [1]. With global energy demands continuing to rise, biofuels are becoming increasingly vital in the transition to low-carbon economies [2]. Recent estimates of global biofuel consumption increased significantly from approximately 187,000 barrels of oil equivalent per day (BOE/D) in 2020 to nearly 1.68 million BOE/D by 2024, driven by supportive policies and environmental imperatives [3]. The Biofuels Market was assessed at USD 148.83 Billion

in 2024, with total revenue projected to expand at a CAGR of 5% from 2025 to 2032, approaching nearly USD 219.90 Billion [3-5]. Nations such as the United States, Brazil, and India lead ethanol production, representing more than 70% of worldwide output, while the sustainable aviation fuel sector is anticipated to expand at a CAGR exceeding 15% from 2025 to 2035 [6]. These trends underscore the growing role of biofuels in global energy portfolios. Among the various types of biofuels, second-generation biofuels, derived from lignocellulosic and organic waste, have gained prominence due to their non-competitive nature with food crops [7]. These feedstocks, comprising agricultural residues, fruit and vegetable peels, industrial organic waste, and

<footnote n="1">¹Department of Biotechnology, IIMT University, Meerut, India</footnote>

<footnote n="2">²Department of Biochemical Engineering, Harcourt Butler Technical University, Kanpur, India</footnote>

<footnote n="3">³Institute of Advanced Research, Gandhinagar, India</footnote>
forestry by-products, are rich in cellulose, hemi-cellulose, and fermentable sugars, which make them ideal substrates for biofuel production [8]. Unlike first-generation biofuels, second-generation processes rely on non-edible biomass, thereby supporting sustainability goals while addressing solid waste management issues [7]. Notably, fruit processing waste (FPW) has emerged as a promising candidate within this category, constituting between 20 and 50% of the total fruit weight after industrial processing [4,9]. FPWs are created in massive numbers and offer a largely unexplored resource for energy recovery [9]. Globally, fruit production reached over 93.7 million tonnes in 2017, generating an estimated 57 million tons of waste annually from peels, pomace, rinds, and seeds [3,4]. Unfortunately, a significant portion of this waste is either incinerated or disposed of in landfills, contributing to methane emissions and other forms of environmental degradation [9]. These conventional disposal practices not only pose ecological and public health risks but also miss the opportunity to harness the rich biochemical composition of fruit wastes [9]. Typically high in fermentable carbohydrates, dietary fiber, pectin, and trace lipids, these wastes are particularly well-suited for biochemical conversion into biofuels such as biogas [10] and bioethanol [11]. Utilizing fruit waste for biofuel production not only aligns with circular economy principles but also offers a dual benefit of energy generation and waste mitigation [12].

A particularly effective strategy to enhance the biofuel potential of fruit wastes is their co-digestion with animal manure, particularly cow dung (CD) [13]. CD, which is abundantly available in agrarian economies such as India, possesses a favorable microbial load and buffering capacity but suffers from a low carbon-to-nitrogen (C/N) ratio [14]. In contrast, fruit wastes typically have high C/N ratios and a rich carbohydrate profile. When co-digested, these two substrates complement each other, stabilizing the anaerobic digestion (AD) process and improving methane yields [13]. Furthermore, co-digestion helps dilute toxic compounds found in certain fruit wastes, such as limonene in citrus peels, thereby mitigating inhibitory effects on microbial communities [15]. From a logistical standpoint, the co-digestion approach also facilitates decentralized biogas production systems, making it an accessible and scalable solution for rural and peri-urban regions [10].

## Types of Fruit Wastes and Their Availability

Rapid growth in global fruit production, surpassing 887 million metric tons in 2020, has led to a parallel increase in FPW, particularly peels, pomace, rinds, and seeds [16]. Such wastes can represent 20%-70% of total fruit weight, depending on fruit type and processing techniques [17]. These by-products are often treated as low-value residuals, commonly disposed of via landfills or incineration, despite being rich in fermentable compounds such as cellulose, hemicellulose, pectin, sugars, proteins, and lipids [18]. Below is a detailed overview of fruit residues studied in biofuel research, their global output, and seasonal/geographical aspects.

### Overview of major fruit wastes

Table 1 presents the global production volumes, waste fractions, primary biochemical components, and seasonal or regional occurrence of key fruit wastes, highlighting their potential as feedstocks for biofuel production.

The generation of fruit-processing residues is highly dependent on both geographical production zones and seasonal harvest calendars. Major fruit wastes such as banana, mango, and pineapple peels predominantly originate from tropical and subtropical regions (e.g., Southeast Asia, Central America, and Africa), while temperate-zone fruits such as apple, pear, peach, and grape produce pomace in North America, Europe, and China [3,25]. Seasonal harvest cycles create distinct peaks of fruit waste generation, necessitating flexible and regionally adapted valorization strategies. Beyond availability, the chemical composition of fruit wastes determines their biodegradability and conversion efficiency into biofuels.

### Chemical composition of fruit wastes

The chemical composition of FPWs significantly influences their suitability for biofuel production [26]. Key parameters include carbohydrate content (simple sugars, cellulose, and hemicellulose), lipid content, lignin, pectin, protein content, C/N ratio, moisture content, and pH [9]. These factors determine the biodegradability, energy yield, and compatibility of FPWs as co-substrates in AD processes [26]. Table 2 summarizes critical compositional parameters, such as carbohydrates, lignin, pectin, lipids, protein content, C/N ratio, and pH, that influence the biofuel potential and biodegradability of various fruit residues.
<table><caption>**Table 1.** Overview of global production, composition, and seasonal availability of major fruit processing wastes.</caption><thead><tr><td>Fruit waste</td><td>Global production (Year, MMT)</td><td>Waste fraction</td><td>Primary components</td><td>Season/region</td><td>References</td></tr></thead><tbody><tr><td>Banana peel</td><td>119.8 MMT bananas in 2020 → ~40% peel ≈ 48 MMT</td><td>35%–40% of fresh fruit</td><td>Carbohydrates, lignocellulose, pigments, catecholamines</td><td>Year-round in the tropics (Asia, Africa, LATAM)</td><td>[3,19]</td></tr><tr><td>Citrus peels (orange, lemon, grapefruit)</td><td>75.5 MMT oranges; 16 MMT peel/year from orange processing</td><td>30%–50% by weight</td><td>Cellulose, hemicellulose, limonene, PMFs</td><td>Winter harvest in the Mediterranean, Central America, and India</td><td>[3,20,21]</td></tr><tr><td>Mango peel/ pomace</td><td>55-60 MMT mangoes; waste ≈ 15-20% of fruit weight</td><td>~15%–20% peel; pomace rich in seed/peel</td><td>Carbs, polyphenols, seed lipids, cellulose</td><td>Summer in the tropics, seasonal at-processing</td><td>[9,21]</td></tr><tr><td>Apple pomace</td><td>86.4 MMT apples; 3-4.2 MMT pomace/year</td><td>~30% of fruit weight</td><td>Pectin, phenolics, fiber</td><td>Temperate harvest, Q3–Q4 (North America, Europe, China)</td><td>[3,19,20]</td></tr><tr><td>Pineapple peel/core</td><td>27 MMT fruit consumption; peel ~25-30%</td><td>25%–30% of fruit weight</td><td>Cellulose, bromelain, ferulic acid, and antioxidants</td><td>Tropical; seasonal peaks in Southeast Asia, LATAM</td><td>[19,21]</td></tr><tr><td>Grape pomace</td><td>78 MMT grapes; pomace ~20-30% depending on process</td><td>~20%–30% pomace -peels, seeds, stems</td><td>High fiber (46%), lipid (8%), protein (8%)</td><td>Fall/winter in wine regions (France, Italy, USA, Chile)</td><td>[3,21]</td></tr><tr><td>Tomato pomace</td><td>242 MMT tomatoes; 4 MMT pomace</td><td>~1%–2% by weight processed; large-scale fabs</td><td>Fiber ~59%, lipid ~5.9%, protein ~19%</td><td>Global, peaks Q2–Q3 in processing regions</td><td>[19,21]</td></tr><tr><td>Papaya peel</td><td>~13.9 MMT</td><td>20%–25%</td><td>Carbohydrates (cellulose, hemicellulose), pectin, moisture, and low lignin</td><td>Tropical &amp; subtropical regions (India, Brazil, Indonesia); peak in spring-summer</td><td>[22]</td></tr><tr><td>Avocado seed</td><td>~8.4 MMT (mostly from pulp)</td><td>13%–18%</td><td>Lipids, starch, lignin, polyphenols, cellulose, hemicellulose</td><td>Grown in Mexico, Peru, Kenya, Colombia, peaks in summer-autumn</td><td>[23]</td></tr><tr><td>Watermelon rind</td><td>~103 MMT (FAOSTAT, 2021)</td><td>30%–35%</td><td>Cellulose, hemicellulose, pectin, sugars, moisture, trace lignin</td><td>China, India, Iran, Brazil, Turkey; peak in late spring to early summer</td><td>[24]</td></tr><tr><td>Peach pomace</td><td>~25.8 MMT (FAOSTAT, 2021)</td><td>10%–15%</td><td>Cellulose, hemicellulose, sugars, pectin, phenolics, lignin</td><td>Temperate climates (e.g., USA, China, Italy, Spain); peak in late spring-summer</td><td>[21]</td></tr><tr><td>Pear pomace</td><td>~24.7 MMT</td><td>20%–30%</td><td>Simple sugars, cellulose, hemicellulose, pectin, and moisture</td><td>Grown in China, Argentina, Italy, Turkey; harvested in late summer-early autumn</td><td>[20]</td></tr><tr><td>Passion fruit peel</td><td>~2.0 MMT (estimated from pulp ratio)</td><td>45%–60%</td><td>Pectin, cellulose, hemicellulose, phenolic compounds, waxes</td><td>Tropical &amp; subtropical areas (e.g., Brazil, Colombia, Kenya, India); year-round, peaks in summer</td><td>[21]</td></tr></tbody></table>

MMT = million metric tons.

## Biofuel potential of individual fruit wastes

The biofuel potential of FPWs is substantial, offering diverse pathways such as AD for biogas, fermentation for bioethanol, tranesterification for biodiesel, and thermochemical processes like pyrolysis [40]. The following table summarizes

the biofuel yields and relevant data for various fruit wastes, followed by detailed explanations. Table 3 details the types of biofuels derived from FPWs along with their associated yields and process descriptions, covering AD, fermentation, and thermochemical conversions.
## Co-Digestion Strategies for Improved Biogas Production

The co-digestion of FPWs with CD has emerged as a promising approach to enhance the efficiency and stability of AD processes [53]. This strategy not only improves methane yield but also addresses the nutritional imbalances commonly associated with mono-digestion of high-carbon fruit wastes [54]. Studies have demonstrated that the synergistic combination of fruit wastes and CD leads to a more balanced substrate composition, enhanced microbial activity, and better process stability [55]. One of the foremost advantages of co-digestion is the improvement in the C/N ratio, which is critical for the metabolic functioning of anaerobic microorganisms [56]. Fruit wastes typically possess high carbon content but are deficient in nitrogen, leading to an elevated C/N ratio, which can cause microbial inhibition and reduced gas production [57]. CD, being nitrogen-rich, helps in adjusting this ratio to an optimal range of 20-30:1, thus supporting balanced microbial growth and enhanced biogas output [56]. For instance, the co-digestion of mango peels with CD has been shown to adjust the C/N ratio effectively, producing maximum methane

yields of up to 3.581 m³ CH₄/kg VS degraded at a 1:10 ratio of mango peel to CD [58]. In addition to improving nutrient balance, CD contributes to the buffering capacity of the digestive system [59]. The high ammonia and bicarbonate concentrations in CD help in maintaining a stable pH during AD [59]. This is particularly important when dealing with fruit wastes like citrus peels or pineapple residues that may lead to rapid acidification due to their high sugar and organic acid content. A stable pH environment is vital for the activity of methanogens, the archaea responsible for the final step of methane production [60].

Moreover, synergistic microbial interactions in co-digestion systems also contribute significantly to the enhanced performance [61]. CD introduces a diverse microbial community that complements the microbial consortia already present in the fruit waste substrate [62]. This microbial diversity facilitates the breakdown of complex polymers such as lignin, cellulose, and hemicellulose, often found in fruit peels and pomaces [62]. The co-digestion of banana peels with CD at a 50:50 ratio resulted in a 2.38-fold increase in methane yield compared to mono-digestion of banana peels alone [63]. Optimization of

<table><caption>**Table 2.** Comparative chemical composition of fruit wastes.</caption><thead><tr><th>Fruit waste</th><th>Carbohydrates (% dry weight)</th><th>Lipids (% dry weight)</th><th>Lignin (% dry weight)</th><th>Pectin (% dry weight)</th><th>Protein (% dry weight)</th><th>C/N ratio</th><th>Moisture content (% fresh weight)</th><th>pH</th><th>References</th></tr></thead><tbody><tr><td>Banana peel</td><td>14.6</td><td>1.6</td><td>6-12</td><td>10-20</td><td>5.3</td><td>25-30</td><td>50.5</td><td>5.0-5.2</td><td>[27]</td></tr><tr><td>Citrus peel</td><td>60-70</td><td>1.0-2.0</td><td>5-10</td><td>20-30</td><td>6.0</td><td>30-35</td><td>70-80</td><td>3.5-4.5</td><td>[28]</td></tr><tr><td>Mango peel</td><td>20-30</td><td>2.0-4.0</td><td>5-10</td><td>15-20</td><td>3.0-5.0</td><td>25-30</td><td>70-80</td><td>4.0-5.0</td><td>[29]</td></tr><tr><td>Apple pomace</td><td>60-70</td><td>1.1-3.6</td><td>15-20</td><td>10-15</td><td>2.0-3.0</td><td>20-25</td><td>75-80</td><td>3.5-4.0</td><td>[30]</td></tr><tr><td>Pineapple peel</td><td>50-60</td><td>0.2-0.5</td><td>5-10</td><td>10-15</td><td>2.0-3.0</td><td>25-30</td><td>80-85</td><td>3.5-4.5</td><td>[31]</td></tr><tr><td>Grape pomace</td><td>29.2</td><td>8.5</td><td>30-35</td><td>10-15</td><td>8.5</td><td>20-25</td><td>60-70</td><td>3.8</td><td>[32]</td></tr><tr><td>Tomato pomace</td><td>10.6-38.5</td><td>18.3-22.6</td><td>5-10</td><td>10-15</td><td>12.9-16.2</td><td>15-20</td><td>13-22</td><td>5.8-6.1</td><td>[33]</td></tr><tr><td>Papaya peel</td><td>50-65</td><td>1-2</td><td>4-6</td><td>10-13</td><td>5-6</td><td>25-30</td><td>85-90</td><td>5.8-6.2</td><td>[34]</td></tr><tr><td>Avocado seed</td><td>40-50</td><td>10-15</td><td>10-12</td><td>3-5</td><td>3-5</td><td>35-45</td><td>45-55</td><td>6.0-6.5</td><td>[35]</td></tr><tr><td>Watermelon rind</td><td>40-55</td><td>&lt;1</td><td>3-5</td><td>12-16</td><td>4-6</td><td>30-35</td><td>90-93</td><td>5.5-6.0</td><td>[36]</td></tr><tr><td>Peach pomace</td><td>54.0</td><td>3.0</td><td>5-10</td><td>10-15</td><td>7.5</td><td>20-25</td><td>70-80</td><td>3.5-4.5</td><td>[37]</td></tr><tr><td>Pear pomace</td><td>50-60</td><td>2.0-3.0</td><td>5-10</td><td>10-15</td><td>2.0-3.0</td><td>20-25</td><td>75-80</td><td>3.5-4.5</td><td>[38]</td></tr><tr><td>Passion fruit peel</td><td>49.0</td><td>0.1</td><td>5-10</td><td>10-15</td><td>2.0-3.0</td><td>20-25</td><td>70-80</td><td>3.5-4.5</td><td>[39]</td></tr></tbody></table>

The compositional characteristics outlined above translate into varying biofuel yields depending on the type of fruit waste and conversion pathway.
<table><caption>**Table 3.** Biofuel yields from fruit wastes.</caption><thead><tr><th>Fruit waste</th><th>Biofuel Type</th><th>Yield</th><th>Process Details</th><th>References</th></tr></thead><tbody><tr><td rowspan="2">Banana peel</td><td>Biogas</td><td>403 l/kg VS</td><td>Continuous AD with mixed cultures</td><td>[41]</td></tr><tr><td>Bioethanol</td><td>5.4% (v/v)</td><td>Direct fermentation, enhanced to 7.14% with nutrient supplementation</td><td></td></tr><tr><td rowspan="3">Mango peel</td><td>Biogas</td><td>0.21–0.54 m³/kg VS; 50%–64% methane</td><td>Thermophilic AD</td><td>[42]</td></tr><tr><td>Bioethanol</td><td>8.45 g/l</td><td>Fermentation with <i>Saccharomyces cerevisiae</i></td><td></td></tr><tr><td>Biodiesel</td><td>98% yield</td><td>Transesterification using mango peel ash as a catalyst</td><td></td></tr><tr><td rowspan="2">Apple pomace</td><td>Biohydrogen</td><td>60–83 l/kg TS</td><td>Batch fermentation with defined mixed cultures</td><td>[41]</td></tr><tr><td>Bioethanol</td><td>15–20 g/l</td><td>Fermentation after enzymatic hydrolysis</td><td></td></tr><tr><td rowspan="2">Pineapple peel</td><td>Bioethanol</td><td>21.3 g/l</td><td>Fermentation with <i>Saccharomyces cerevisiae</i></td><td>[43]</td></tr><tr><td>Biogas</td><td>285.7 ml/g</td><td>AD at 350°C with sewage sludge</td><td></td></tr><tr><td rowspan="2">Grape pomace</td><td>Biohydrogen</td><td>384 l/kg VS</td><td>Continuous fermentation with mixed cultures</td><td>[41]</td></tr><tr><td>Bioethanol</td><td>10–15 g/l</td><td>Fermentation after acid hydrolysis</td><td></td></tr><tr><td>Papaya peel</td><td>Bioethanol</td><td>4.19 g/l</td><td>Fermentation with <i>Saccharomyces cerevisiae</i></td><td>[4]</td></tr><tr><td>Avocado seed</td><td>Biodiesel</td><td>90%–95% yield</td><td>Transesterification of extracted oil</td><td>[44]</td></tr><tr><td rowspan="2">Citrus peel</td><td>Biogas</td><td>348 Nml/g</td><td>AD, methane yield of 128 NmL/g</td><td>[43]</td></tr><tr><td>Bioethanol</td><td>5–10 g/l</td><td>Fermentation after hydrolysis</td><td></td></tr><tr><td>Passion fruit peel</td><td>Biodiesel</td><td>Catalyst for biodiesel production</td><td>Used as a heterogeneous catalyst in transesterification</td><td>[45]</td></tr><tr><td rowspan="2">Tomato pomace</td><td>Biogas</td><td>340–420 ml CH₄/g VS</td><td>AD, pre-digestion drying, size reduction, and co-digestion with manure enhance stability.</td><td>[46,47]</td></tr><tr><td>Bioethanol</td><td>12–20 g ethanol/100 g dry biomass</td><td>Enzymatic hydrolysis followed by fermentation (<i>S. cerevisiae</i>), high cellulose, and fermentable sugars</td><td></td></tr><tr><td rowspan="2">Peach pomace</td><td>Biogas</td><td>310–400 ml CH₄/g VS</td><td>AD with inoculum from dairy digesters, high biodegradability due to sugar and pectin content</td><td>[37,48]</td></tr><tr><td>Bioethanol</td><td>~18–24 g ethanol/100 g dry pomace</td><td>Acid or enzymatic hydrolysis + fermentation, pectin and glucose-rich matrix</td><td></td></tr><tr><td>Pear pomace</td><td>Biogas</td><td>290–350 ml CH₄/g VS</td><td>AD benefits from co-digestion with cow manure, a balanced C/N ratio.</td><td>[49]</td></tr><tr><td rowspan="2">Watermelon rind</td><td>Biogas</td><td>250–370 ml CH₄/g VS</td><td>AD, co-digested with CD or poultry manure to balance the C/N ratio and enhance buffering. Pre-treatment includes chopping and pH adjustment (optimal around 7.0).</td><td>[50–52]</td></tr><tr><td>Bioethanol</td><td>~14–18 g ethanol/100 g dry rind</td><td>Enzymatic saccharification followed by fermentation using <i>Saccharomyces cerevisiae</i>, substrate rich in fermentable sugars, especially glucose and fructose. Hydrothermal pretreatment enhances sugar release.</td><td></td></tr><tr><td>Biohydrogen</td><td></td><td>~65–90 ml H₂/g VS</td><td>Dark fermentation using mixed microbial cultures or specific strains (e.g., <i>Clostridium spp.</i>). Requires pH ~5.5 and heat treatment of the inoculum. Inhibitor removal is recommended for higher yields.</td><td></td></tr></tbody></table>

Nml = Nml refers to milliliters at standard temperature and pressure (0°C, 1 atm); TS = total solids; VS = volatile solids. While mono-digestion of fruit wastes is feasible, co-digestion, particularly with nutrient-rich substrates like CD, can significantly enhance biogas yields.

mixing ratios is a critical factor in co-digestion strategies. The appropriate proportion of fruit waste to CD varies depending on the specific fruit type, its chemical composition, and the desired methane yield [64]. A study found that a 70:30 ratio of mango peel to CD produced 1,590 ml of gas over a 30-day

retention period, underscoring the significance of fine-tuning substrate proportions [65]. Similarly, pineapple waste co-digested with CD at a 1:1.5 ratio not only increased methane content from 54.14% to 63.81%, but also reduced the hydraulic retention time (HRT), thus improving process efficiency [66].
The co-digestion approach is not limited to tropical fruits. For temperate fruits such as apples, apple pulp incorporated at 5% of the total substrate mix has demonstrated stable biogas production (~400 l/kg VS) with methane concentrations around 52% [67]. These studies collectively affirm that co-digestion enhances not only the quantity but also the quality of biogas produced by improving methane content.

## Environmental and Economic Impacts

Anaerobic co-digestion of FPW with CD transforms low-value organic residues into valuable biogas and nutrient-rich digestate, exemplifying circular economy principles [68]. In a case study at Yogyakarta's Gemah Ripah fruit market, a mix of 10 t/day of FPW (primarily orange and mango peel) achieved approximately 1,075 m³n/day of biogas, with a methane content of 54%, powering a 733 kWh electrical generator [69]. By converting FPW from a disposal issue into a renewable energy source, co-digestion both diverts waste from landfills and captures energy that would otherwise be lost [68]. Landfill disposal of FPW not only occupies space but also generates methane (CH₄) and carbon dioxide (CO₂) [70]. Even though FPW contributes less than 5% to global GHG emissions directly, the environmental burden, especially through leachate and methane release, is significant [71]. Compared to landfilling, AD reduces environmental impacts. Life cycle assessment studies have consistently shown lower greenhouse gas emissions when FPW is processed in digesters rather than dumped [72]. Biogas generated via co-digestion serves as a carbon-neutral energy carrier; CO₂ released upon combustion is offset by CO₂ previously absorbed during biomass growth [73]. Additionally, capturing organic carbon helps prevent the release of potent CH₄ emissions from decomposition [73,74]. Farm-level and decentralized AD systems further reduce emissions associated with waste transport, which may contribute up to 70% of greenhouse gas impacts in centralized scenarios [75]. Decentralized, community-based digesters near waste sources yield enhanced sustainability outcomes [75].

### Economic feasibility for decentralized bioenergy production

Co-digestion plants, particularly small-scale ones, have been demonstrated as economically superior to landfill options due to lower processing costs and added revenue from energy generation [76]. In Yogyakarta, a biogas plant using FPW produced

electricity and sold digestate back into agriculture, scoring a sustainability index of 4.11/5, indicating strong environmental and economic performance [69]. Over 1,075 Nm³/day biogas generated would produce ~733 kWh/day, offering a consistent local power supply [69]. Small-scale anaerobic digesters (<1,000 t/year or <15 kW capacity) mitigate the need for costly centralized infrastructure and transport logistics [75]. Metrics from sub-Saharan Africa spotlight rolling out micro-digesters that harness both CD and local organic waste, yielding positive net present value, reasonable IRR, and pay-back periods under 6 years [77]. Digestate by-products serve as nutrient-rich fertilizers, reducing dependence on synthetic fertilizers and boosting crop yields by 10–20% [78]. When integrated with feed-in tariffs for electricity and the sale of carbon or waste management credits, small farm-based digesters become financially compelling [79].

## Challenges and Pretreatment Requirements

The AD of FPWs, despite its promise for renewable bioenergy, is hindered by several compositional and process-related challenges. One of the foremost obstacles is the high lignin content present in many FPWs, especially those derived from peels and pomace [80]. Lignin is a complex, recalcitrant aromatic polymer that encrusts cellulose and hemicellulose, thereby reducing the accessibility of hydrolytic enzymes required for AD [81]. Its structural resistance significantly lowers the biodegradability of the substrate, leading to inefficient methane yields [81]. It was demonstrated that even after extended digestion, a considerable fraction of the lignin remains undegraded, thereby limiting the overall conversion efficiency [82]. While mechanical pretreatment, such as grinding or milling (to sizes <2 mm), is widely used to increase surface area, it accounts for a significant share of operational energy, up to 33% of the total electricity input in some digestion systems [83]. Biological pretreatments, involving lignin-degrading microorganisms like white-rot fungi, offer a low-energy solution but suffer from slow kinetics and scalability issues [84].

To overcome lignin-related resistance and improve overall biodegradability, a range of pretreatment methods, mechanical, thermal, chemical, and biological, have been developed. Mechanical pretreatment alone, while increasing surface accessibility, is insufficient for complete delignification [83]. Thermal approaches, including steam explosion, liquid hot water, and wet oxidation at 160°C–250°C,
disrupt the lignocellulosic matrix but may also lead to the formation of inhibitory compounds such as furfural and hydroxymethylfurfural [85]. Similarly, chemical pretreatments, especially alkaline methods like NaOH treatment, effectively solubilize lignin and improve enzymatic digestibility [86]. However, they also introduce cost and safety concerns related to chemical handling and reactor corrosion [87]. Acid pretreatments, though effective, generate inhibitors like acetic acid and phenolics that require detoxification [87]. Biological pretreatment, while eco-friendly and selective for lignin removal, remains underdeveloped at an industrial scale due to its slow processing time and the need for controlled environmental conditions [88]. Therefore, hybrid approaches combining mechanical and chemical or thermal methods are often employed to balance cost, time, and efficacy [89].

An additional challenge in the AD of FPWs arises from the presence of inhibitory compounds, particularly in citrus-derived wastes [90]. Citrus peels, especially those from oranges and lemons, contain high concentrations of limonene, an essential oil compound with well-known antimicrobial properties [91]. Limonene can severely inhibit methanogenic archaea, thus reducing biogas yields or causing complete process failure [91]. Additionally, limonene concentrations above 400–900 µl/l in semi-continuous anaerobic systems have been reported to significantly suppress microbial activity [91]. To mitigate this, various limonene-removal strategies have been investigated [92]. Solvent extraction using hexane has been shown to increase methane yield by over three times, from 0.061 to 0.217 m³ CH₄/kg VS [92]. Similarly, steam distillation and ethanol extraction methods have achieved near-complete removal of limonene, significantly enhancing anaerobic performance [93]. More innovative techniques, such as membrane bioreactors equipped with protective layers (e.g., PVDF membranes), have demonstrated tolerance to limonene concentrations as high as 378 g/m³, compared to 60 g/m³ in conventional systems [94–96]. Biological detoxification using limonene-degrading fungi is another promising route, though the resulting metabolites may still be partially inhibitory and require further treatment [97].

## Future Prospects and Research Needs

The sustainable and scalable conversion of FPWs into biofuels calls for a multidimensional research strategy that integrates advanced technologies

such as artificial intelligence (AI), biorefinery frameworks, and synthetic biology. While conventional AD and fermentation technologies offer promising routes, limitations in substrate heterogeneity, bioconversion efficiency, and process optimization necessitate innovation beyond traditional methods.

## Role of AI and modeling in substrate prediction

AI and machine learning algorithms are poised to revolutionize biofuel research by enabling accurate substrate behavior prediction, biogas yield forecasting, and process optimization [98]. AI models such as artificial neural networks, support vector machines, and decision trees have been used to predict methane yields based on complex feedstock compositions [99]. These models account for parameters such as C/N ratio, lignin content, moisture, pH, and volatile solids, allowing researchers to simulate optimal co-digestion scenarios without extensive experimentation [100]. Moreover, deep learning tools can integrate heterogeneous datasets, including weather, regional crop yields, and fruit-processing industry outputs, to estimate temporal availability and regional supply of FPWs for large-scale biogas production [101]. Integrating AI with digital twins and IoT systems in biogas plants could lead to adaptive digestion control, adjusting parameters such as pH, temperature, and HRT in real-time for maximum output [102].

## Biorefinery approaches for integrated product recovery

Transitioning from single-product biofuel systems to integrated biorefineries is vital for economic and environmental sustainability [103]. In this approach, FPWs serve as multipurpose feedstocks, yielding not only biogas or bioethanol but also high-value co-products such as organic acids, enzymes, pectins, pigments, and biofertilizers [104]. Citrus peels can be used for limonene extraction before AD, thereby mitigating inhibitory effects while generating valuable essential oils [105]. Similarly, pectin and polyphenols from mango and apple peels can be recovered via enzymatic or solvent extraction, adding economic value and reducing waste load [106]. The integration of thermochemical platforms like pyrolysis and hydrothermal liquefaction with biological routes enables the conversion of lignin-rich residues, otherwise recalcitrant to digestion, into bio-oil and biochar [107–110]. Biochar, in turn, can be applied to digestate to enhance soil amendment properties or used as an adsorbent within the
digestion system itself, promoting microbial stability and gas yield [111,112]. Such circular resource loops embedded within biorefineries enhance both energy recovery and environmental performance, promoting the establishment of decentralized bioenergy hubs tailored to regional fruit waste profiles.

**Genetic and microbial engineering for enhanced conversion**

The engineering of microbial consortia and synthetic microbial strains represents a cutting-edge frontier for improving FPW-to-biofuel conversion [113,114]. Lignin-degrading and hemicellulose-hydrolyzing microbes, such as *Clostridium thermocellum*, *Phanerochaete chrysosporium*, and *Trichoderma reesei*, have been genetically modified to enhance enzyme secretion and substrate affinity [113–114]. CRISPR-based genome editing allows precise modification of metabolic pathways in anaerobic bacteria to improve their tolerance to inhibitors like limonene or to boost methane and hydrogen production under fluctuating conditions [115]. Furthermore, metagenomic and metatranscriptomic analyses have enabled the identification of novel plastic- and lignin-degrading enzymes from the microbiome of fruit-waste-rich environments, which can be harnessed to tailor microbial consortia with desirable biodegradation profiles [116]. Engineered co-cultures that combine cellulolytic, proteolytic, and methanogenic functionalities can be used to optimize co-digestion processes with CD, improving substrate utilization rates and system resilience [117,118].

**Conclusion**

The employment of FPWs for the production of biofuels signifies a promising and environmentally prudent pathway for the generation of sustainable energy. This comprehensive review highlights the biochemical abundance and seasonal availability of diverse fruit wastes, including banana peels, mango peels, apple pomace, avocado seeds, as well as citrus and melon residues, which render them appropriate substrates for the production of biogas, bioethanol, biodiesel, and biohydrogen via various biochemical and thermochemical conversion methodologies. Notwithstanding their compositional merits, FPWs frequently manifest elevated C/N ratios, diminished nitrogen concentrations, and the presence of recalcitrant or inhibitory substances such as lignin and limonene. These impediments can substantially influence the efficiency of AD and

the overall stability of the process. Nevertheless, pretreatment strategies encompassing mechanical, thermal, chemical, and biological methodologies can augment substrate bioavailability and mitigate inhibitory impacts. Particularly, co-digestion with CD is emerging as a robust approach to rectify nutrient imbalances, enhance buffering capacities, and stimulate synergistic microbial activities. A multitude of empirical investigations have substantiated significant enhancements in methane yield and process efficiency through the optimization of mixing ratios and co-digestion protocols. Moreover, the environmental and economic advantages associated with the valorization of fruit waste are quite compelling. These benefits encompass reduced landfill dependency, the alleviation of greenhouse gas emissions, the production of clean energy, and the creation of digestate as a biofertilizer. In conjunction with the advent of AI-based predictive models, integrated biorefinery frameworks, and advancements in microbial and genetic engineering, the prospective trajectory of FPW-based biofuel systems appears increasingly feasible and scalable. In summary, fruit wastes are not merely byproducts of agro-industrial processes but rather valuable resources capable of making significant contributions to global renewable energy objectives. Through scientific innovation, supportive policies, and decentralized implementation models, the conversion of FPWs into bioenergy can yield both ecological and economic transformations.

**Conflict of interest**

There is no conflict of interest to be stated.

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