Deep Eutectic Solvents and Anaerobic Digestion for Apple Pomace Valorization: A Critical Review of Integration Strategies for Low-Carbon Biofuel Production

Abstract

Global energy demand is projected to increase by approximately 28% by 2040, thereby intensifying the urgent need for sustainable alternatives to fossil fuels. This comprehensive review examines the integration of deep eutectic solvent (DES) pretreatment with anaerobic digestion to valorize apple pomace (AP), a major lignocellulosic by-product of the apple juice industry that is often improperly disposed of, posing substantial environmental burdens. A key challenge for efficient biofuel production from AP is its high lignin content, which inhibits complete degradation during AD. DESs have emerged as promising green pretreatment agents owing to their low toxicity, biodegradability, and selective lignocellulosic solubilization capabilities. This review critically synthesizes research on: AP compositional characteristics and disposal challenges; DES pretreatment mechanisms and comparative advantages over conventional methods; AD processes for AP, including yields and constraints; and technical, economic, and environmental aspects of DES-AD integration. Critical analysis reveals that acidic DES formulations achieve 40–60% higher delignification than neutral systems but produce higher levels of inhibitory compounds, necessitating application-specific optimization. Nonetheless, significant knowledge gaps persist, including a lack of standardized protocols, limited pilot-scale validation, and insufficient techno-economic assessments. This integrated approach closely aligns with circular economy principles, offering an environmentally friendly pathway for waste valorization while reducing reliance on fossil fuels and mitigating greenhouse gas emissions.

1. Introduction

The global urgency to transition away from fossil fuels and reduce greenhouse gas emissions has elevated the importance of renewable, low-carbon energy technologies [1]. Agro-industrial residues represent an appealing category of feedstocks, as they are abundant, low-cost, and often pose disposal or environmental management challenges [2,3]. In this context, the by-product stream generated from apple processing, commonly known as apple pomace, has attracted increasing attention. AP typically comprises peel, core, and pulp residues left after juice or cider extraction, and its disposal imposes both economic and environmental burdens [1,4,5].

AP’s compositional profile (detailed in Table 1) contains significant fermentable carbohydrates alongside recalcitrant lignin, making it suitable for bioenergy conversion. Traditional disposal routes underutilize their energy potential and may generate greenhouse gas emissions [1].

A key limitation in converting AP stems from its structural recalcitrance: the complex matrix of cellulose, hemicellulose, and lignin restricts microbial or enzymatic access and limits conversion efficiency [6]. Therefore, pretreatment of the biomass is crucial for unlocking its full biofuel potential [7]. In recent years, deep eutectic solvents have emerged as a promising class of “green” pretreatment agents: they are relatively low-cost, tunable, low-toxicity, and can disrupt lignin/hemicellulose structures more gently and selectively than conventional methods like ionic liquids, dilute acid, or ammonia fiber expansion [6,8]. Although much of the work to date has focused on model biomass systems, the application of DESs to agro-industrial residues such as AP is gaining momentum [9]. DES are fluid mixtures, typically composed of two or three inexpensive and harmless constituents that self-associate through hydrogen bonding, forming a eutectic composition with a lower melting point than their individual components [10]. These solvents are increasingly recognized as an environmentally benign and efficient medium for biomass pretreatment, primarily due to their ease of preparation, stability, cost-effectiveness, and recyclability [11].

In parallel, anaerobic digestion (AD) converts organic wastes into biogas under oxygen-free conditions [12]. Despite the promise of coupling DES pretreatment with AD, this integration for AP remains under-explored, with respect to solvent recovery, microbial compatibility, and life-cycle impacts.

Accordingly, this review aims to: first, summarize the current status of AP as a bio-energy feedstock (including supply, composition and management); second, critically examine pretreatment methods with an emphasis on DES technologies; third, evaluate anaerobic digestion of AP (including yields, constraints and co-digestion strategies); and finally, discuss the integration of DES pretreatment with AD for low-carbon energy production, emphasizing techno-economic and environmental aspects and identifying key research gaps. The compositional attributes of AP are summarized in Table 1.

Table 1. Physicochemical Composition of Apple Pomace and Its Relevance for Biofuel Production.

Component	Typical Range (% Dry wt.)	Bio-Energy Relevance	References
Soluble sugars (glucose, fructose, sucrose)	10–18%	Rapidly fermentable; enhances hydrolysis and accelerates methane formation.	[7]
Cellulose	20–25%	Hydrolysable to glucose; major contributor to biogas/ethanol yield after pretreatment.	[6]
Hemicellulose	12–20%	Releases pentoses; increases methane yield when solubilized by DES pretreatment.	[9]
Pectin	8–12%	High biodegradability; enhances volatile fatty acid formation in AD.	[2]
Lignin	15–30%	Recalcitrant barrier; removal or depolymerization improves microbial access.	[8]
Protein	3–5%	Provides nitrogen source for methanogenic consortia; improves C:N balance.	[4]
Lipids	2–4%	Minor fraction but contributes long-chain fatty acids, improving biogas calorific value.	[12]
Polyphenols & antioxidants	0.3–1.2%	Can inhibit AD if concentrated; recoverable as high-value co-products.	[1]

Table 1. Physicochemical Composition of Apple Pomace and Its Relevance for Biofuel Production.

Component	Typical Range (% Dry wt.)	Bio-Energy Relevance	References
Soluble sugars (glucose, fructose, sucrose)	10–18%	Rapidly fermentable; enhances hydrolysis and accelerates methane formation.	[7]
Cellulose	20–25%	Hydrolysable to glucose; major contributor to biogas/ethanol yield after pretreatment.	[6]
Hemicellulose	12–20%	Releases pentoses; increases methane yield when solubilized by DES pretreatment.	[9]
Pectin	8–12%	High biodegradability; enhances volatile fatty acid formation in AD.	[2]
Lignin	15–30%	Recalcitrant barrier; removal or depolymerization improves microbial access.	[8]
Protein	3–5%	Provides nitrogen source for methanogenic consortia; improves C:N balance.	[4]
Lipids	2–4%	Minor fraction but contributes long-chain fatty acids, improving biogas calorific value.	[12]
Polyphenols & antioxidants	0.3–1.2%	Can inhibit AD if concentrated; recoverable as high-value co-products.	[1]

For example, DES pretreatment leaches 3–5% of the proteinaceous material from the feedstock, as shown in Table 1, altering the C:N ratio of the remaining substrate and affecting nutrient availability to the methanogenic community [4]. These shifts can influence microbial growth rates, ammonia generation, and reactor stability. Therefore, to maintain robust AD performance, monitor C:N changes post-pretreatment and, if necessary, supplement nitrogen sources or adjust substrate loading to maintain the optimal C:N ratio for apple pomace biomass [13]. This is crucial because an unbalanced C:N ratio, particularly a high one, can lead to process instability, accumulation of volatile fatty acids, a drop in pH, and ultimately, a decline in biogas production [14,15]. For instance, co-digestion with nitrogen-rich substrates can mitigate these issues by balancing the C:N ratio, thereby enhancing biodegradability and biogas yield [16].

Beyond these physico-chemical considerations, the conversion of lignocellulosic biomass through pretreatment followed by anaerobic digestion shows promise for enhancing the digestibility and conversion efficiency of recalcitrant feedstocks [17]. In particular, among various pretreatment techniques, DES pretreatment is gaining considerable attention owing to its intrinsic properties, including low toxicity, biodegradability, high selectivity for lignin dissolution, and compatibility with green chemistry principles [18]. Consequently, integrating DES pretreatment with AD processes can improve biofuel yields by enhancing enzyme accessibility during digestion, which ultimately boosts biogas or bioethanol production while reducing greenhouse gas emissions [19,20]. To this end, this review synthesizes existing research on DES pretreatment mechanisms, compares them with alternatives such as ionic liquids (IL), dilute acid (DA), and ammonia fiber expansion (AFEX), and discusses the challenges and potentials of applying these strategies to AP. The investigation of DES applications for AP valorization is particularly pertinent given its abundance as an agricultural waste product and its rich lignocellulosic content [13,21].

To ensure a comprehensive and balanced discussion, this review adopted a critical narrative approach rather than a bibliometric or systematic review. Relevant literature was identified through searches in major scientific databases, including Scopus, Web of Science, PubMed, and Google Scholar, with a primary focus on recent publications. The search strategy combined keywords related to deep eutectic solvents, such as “deep eutectic solvent,” “DES,” “choline chloride,” and “lactic acid”, with application-specific terms, including “apple pomace pretreatment,” “anaerobic digestion,” and “biofuel production.” This approach enabled the inclusion of diverse studies and perspectives, facilitating a critical synthesis of current knowledge and emerging trends in DES applications.

The remainder of this manuscript is organized as follows. Section 2 provides a critical review of conventional and emerging pretreatment technologies, highlighting the rationale for employing deep eutectic solvents in apple pomace valorization. Section 3 presents the materials and methods, including DES the preparation of the system and pre-analytical techniques for assessing biomass deconstruction. Section 4 reports the experimental findings, focusing on the effects of DES pretreatment on apple pomace composition, sugar release, and the formation of inhibitory compounds. Section 5 integrates these results within a broader discussion of process performance and mechanistic insights. Section 6 outlines the techno-economic and environmental implications of scaling up DES applications, while Section 7 concludes by summarizing key contributions and proposing future research directions. Specifically, this review aims to consolidate understanding of DES efficacy in enhancing anaerobic digestibility and to offer insights into optimizing operational parameters for industrial implementation.

2. Literature Review on Lignocellulosic Biomass Pretreatment

Pretreatment is widely recognized as a critical step in converting lignocellulosic biomass into biofuels [22,23]. The purpose of pretreatment is to overcome the material’s recalcitrance by disrupting the lignin–carbohydrate matrix and enhancing enzyme accessibility for hydrolysis and fermentation [17,24]. Numerous pretreatment techniques have been documented in the literature, including physical, chemical, and biological methods [25,26]. However, selecting an appropriate pretreatment method remains challenging due to inherent trade-offs among effectiveness, cost, environmental impact, and downstream compatibility.

The rising global demand for biofuels, driven by climate change concerns and governmental mandates, necessitates efficient methods given their high energy and chemical intensities [27]; consequently, advancements are crucial to improve the economic viability and sustainability of second-generation biofuel production from lignocellulosic feedstocks [28]. These methods increase surface area and provide accessible enzyme binding sites, ultimately improving cellulose conversion into fermentable sugars [29]. A range of established pretreatment technologies, such as dilute acid hydrolysis, alkaline pretreatment, and hydrothermal treatment, have been developed to modify the complex structure of lignocellulosic biomass [29].

2.1. Conventional and Emerging Pretreatment Technologies

2.1.1. Physical Pretreatments

Physical methods, such as milling and grinding, aim to reduce particle size and increase surface area [30]

Table 2. Optimal Range for DES particle sizes.

Particle Size	Application	Rationale	Reference
<2 mm (2000 μm)	General recommendation for DES pretreatment	Balance between surface area, mass transfer, and energy consumption	[18,32]
0.5–2 mm (500–2000 μm)	Optimal for most lignocellulosic biomass	Maximizes DES penetration without excessive energy input	[17]
0.25–0.5 mm (250–500 μm)	Fine milling for enhanced delignification	Provides maximum surface area; used in research studies	[30]
<0.18 mm (180 μm)	Ultra-fine milling	Research-scale only; excessive energy consumption at the industrial scale	[30]

presents the particle size range for DES pretreatment. The critical limitations of these methods are their high energy intensity, consuming 10–30 kWh per ton of biomass [17], which significantly increases operational costs and the carbon footprint. Furthermore, physical pretreatment alone achieves only marginal improvements in enzymatic digestibility (15–25% increase) compared to untreated biomass, as the lignin-carbohydrate matrix remains intact, mainly despite increased surface area. Consequently, they are rarely economically viable as standalone pretreatments and require combination with chemical methods to achieve industrially relevant conversion efficiencies [30]. Despite these limitations, physical pretreatments are often employed as preliminary steps to enhance the efficacy of subsequent chemical or physicochemical treatments [31].

2.1.2. Chemical Pretreatments

Chemical methods include acid, alkaline, and organosolv pretreatments, each with distinct advantages and substantial drawbacks that limit their industrial adoption.

Dilute Acid (DA) Pretreatment

DA pretreatment (typically 0.5–2% H₂SO₄ or HCl at 120–200 °C) effectively solubilizes hemicelluloses (70–90% removal) and disrupts lignin-carbohydrate bonds [33,34]. Critical limitations: However, harsh acidic conditions generate significant quantities of fermentation inhibitors, including furfural (1–3 g/L), hydroxymethylfurfural (HMF, 0.5–2 g/L), and phenolic compounds (2–5 g/L), which severely compromise downstream fermentation yields by 30–50% unless expensive detoxification steps are implemented [17]. Additionally, DA pretreatment requires corrosion-resistant equipment (increasing CAPEX by 40–60%), generates acidic waste streams that require neutralization and disposal, and results in poor lignin removal (typically <30%), limiting cellulose accessibility [35,36].

Alkaline Pretreatment

Alkaline methods (typically 2–10% NaOH at 80–120 °C) facilitate selective lignin removal (50–80%) with minimal carbohydrate degradation [35]. This approach typically utilizes sodium hydroxide, ammonia, or calcium hydroxide to swell the biomass, enhancing its porosity and surface area for enzymatic hydrolysis [28,36,37,38]. However, these processes involve extended reaction times, large alkali consumption (50–200 kg NaOH per ton biomass), high-salinity black liquor requiring costly recovery systems, and substantial energy demands for heating and neutralization [32]. Techno-economic analyses indicate that alkali recovery accounts for 25–35% of total pretreatment costs, significantly affecting process economics [35]. At the same time, prolonged exposure can lead to the formation of inhibitory secondary compounds, such as xylitol and acetic acid [39].

Organosolv Pretreatment

Organosolv methods employ organic solvents (ethanol, methanol, acetone), with or without acid catalysts, to fractionate biomass components and achieve high-purity lignin recovery [33]. However, they face high solvent costs (typically 85–95% recovery efficiency needed for economic viability), explosion risks from volatile organic compounds, energy-intensive solvent recovery, and potential residual solvent toxicity affecting downstream microorganisms [34]. The process also generates hazardous waste and requires specialized safety infrastructure, increasing capital and operational expenditures, as well as detoxification and desalination costs [40].

2.1.3. Ionic Liquid (IL) Pretreatment

ILs have been extensively investigated for their ability to dissolve biomass components, reduce cellulose crystallinity by 40–70%, and enhance enzymatic digestibility by 60–90% [17]. Certain ILs, for example, 1-ethyl-3-methylimidazolium acetate, can achieve near-complete biomass dissolution at moderate temperatures (80–120 °C). There are critical limitations that demonstrate efficiency but prevent industrial adoption. First, IL costs remain prohibitively high ($5–50 per kg) compared to conventional chemicals ($0.2–1 per kg), making them economically unviable unless >98% recovery is achieved, a target rarely met in practice [41]. Second, many ILs exhibit significant ecotoxicity (LC₅₀ values of 10–1000 mg/L for aquatic organisms) and persist in the environment due to low biodegradability, raising serious environmental concerns [42,43].

Third, residual ILs in pretreated biomass (even at <2% w/w) severely inhibit cellulase activity (50–80% reduction) and microbial fermentation, necessitating extensive washing steps that consume large volumes of water and energy [17]. Fourth, IL viscosity increases dramatically at high biomass loadings (>10% w/v), creating mass transfer limitations and requiring energy-intensive mixing. These combined limitations have prevented IL-based pretreatments from advancing beyond laboratory and pilot scales despite two decades of research [44].

2.1.4. Ammonia Fiber Expansion (AFEX)

AFEX pretreatment utilizes liquid ammonia (1–2 kg NH₃ per kg biomass) at elevated temperatures (60–100 °C) and pressures (250–300 psi), followed by rapid decompression to break lignin–carbohydrate linkages and create nanoporous structures that enhance enzyme penetration during hydrolysis [17]. AFEX is particularly effective for agricultural residues with low lignin content. Critical limitations: The process requires complex ammonia recovery systems (achieving 90–98% recovery) to minimize chemical losses, reduce costs, and prevent environmental impacts. Ammonia is both toxic (LC₅₀ ~2 mg/L for fish) and contributes to eutrophication if released [45]. Additionally, AFEX demonstrates reduced effectiveness for high-lignin feedstocks like woody biomass (delignification typically <20%), requires high-pressure vessels (increasing CAPEX), consumes substantial energy for ammonia vaporization and recovery, and poses safety risks associated with handling anhydrous ammonia at industrial scales [46]. Furthermore, the alkaline conditions can generate condensed phenolic compounds that inhibit subsequent enzymatic hydrolysis and fermentation.

2.2. Advantages and Limitations of DES-Based Pretreatment

DES has emerged as a promising “green” alternative to conventional chemical pretreatments, addressing many of the critical limitations outlined above while introducing unique capabilities for lignocellulosic biomass processing.

2.2.1. Fundamental Advantages

Firstly, the low toxicity and biodegradability, unlike ILs, DES are typically composed of low-cost, bio-based components (e.g., choline chloride, organic acids, sugars, amino acids) that exhibit minimal ecotoxicity (LC₅₀ values typically >1000 mg/L, 10–100× less toxic than ILs) and high biodegradability (>60% in 28 days), aligning with green chemistry mandates [18]. This fundamental difference eliminates the environmental persistence concerns associated with ILs and reduces downstream toxicity to fermentation microorganisms.

Secondly, the selective lignin dissolution, like DES pretreatment, can effectively dissolve lignin through the cleavage of ether bonds, particularly the β-O-4 linkages that account for 45–60% of lignin inter-unit connections, while essentially preserving the cellulose fraction (>85% retention). This selectivity, achieved through hydrogen-bond-disrupting mechanisms, enhances cellulose accessibility for enzymatic hydrolysis without the extensive carbohydrate degradation observed in acid pretreatments [18]. Acidic DES formulations achieve delignification efficiencies of 66–79%, comparable to or exceeding alkaline methods but at significantly reduced environmental cost [47].

Lastly, the economic viability, for example preliminary techno-economic analyses indicate that acid-based DES such as choline chloride: lactic acid may offer minimum selling prices of ~$2128 per ton for bioethanol production, competitive with or lower than dilute acid ($2500–3000/ton) and alkaline ($2400–2800/ton) pretreatments when solvent recovery exceeds 90% [48]. The relatively low cost of DES components ($1–5 per kg) compared to ILs ($5–50 per kg) provides a more realistic path to industrial implementation.

Some of the operational advantages are that DES pretreatments operate at relatively low temperatures (80–140 °C) compared to steam explosion (180–230 °C) or concentrated acid hydrolysis (>100 °C), consuming 1/5 to 1/8 the energy of conventional methods [32]. Reaction times are remarkably short (30 min to 2 h) compared to alkaline pretreatment (hours to days), enabling higher throughput in continuous processing systems. Furthermore, DES pretreatments do not require high-pressure vessels (unlike AFEX) or corrosion-resistant materials (unlike strong acid pretreatments), reducing capital costs.

2.2.2. Critical Limitations and Trade-Offs

However, the reported economic and performance advantages vary considerably with DES composition, revealing important nuances that must be understood for rational process design. Formulation-dependent performance: Acidic DES formulations (choline chloride: lactic acid, 1:2 molar ratio) consistently demonstrate lower minimum selling prices compared to neutral DES systems, primarily due to their superior delignification efficiency (66–79% vs. 40–55%) and shorter reaction times (30–60 min vs. 2–4 h) [32]. This efficiency gap becomes critical at an industrial scale, where even a 20% improvement in lignin removal translates to substantial operational cost savings and increased downstream yields [49].

Nevertheless, these advantages are not universal across all DES formulations. Inhibitor formation: Acidic DES systems, while achieving superior delignification, produce higher concentrations of inhibitory phenolic compounds (2–4 g/L) compared to neutral systems (0.5–1.5 g/L), potentially compromising downstream fermentation yields by 15–30% unless detoxification or washing steps are implemented [18]. This trade-off between pretreatment efficiency and hydrolysate purity must be carefully managed. Furthermore, the limited solubility of certain biomass constituents in some DES formulations, along with the overall cost and availability of DES components, pose additional bottlenecks to widespread adoption [50]. Therefore, while DES offers significant promise for sustainable biomass conversion, ongoing research is crucial to optimize its composition and recovery processes to overcome these limitations and unlock its full industrial potential [50,51].

Also, there is a viscosity-water content trade-off, like DES viscosity, which increases exponentially at water contents below 20% (from ~100 cP to >1000 cP), creating mass-transfer limitations and requiring energy-intensive mixing. However, higher water content (>30%) improves mass transfer but reduces lignin solubility by up to 35%, decreasing pretreatment effectiveness [52]. This creates a narrow operational window (20–30% water) that must be optimized for each specific DES-biomass combination.

Moreover, there are challenges in solvent recovery; for example, DES recovery is theoretically simpler than IL recovery due to lower vapor pressures and reduced toxicity concerns. Achieving the >90% recovery rates necessary for economic viability remains technically challenging, particularly when phenolic compounds from lignin degradation contaminate the solvent [18]. Anti-solvent precipitation, membrane filtration, and vacuum distillation methods are under development but add complexity and cost to the overall process [18]. These inconsistencies across DES types, remarkably the performance variability between acidic and neutral formulations, and the interdependent effects of temperature, water content, and reaction time, explain why reported methane yields and sugar release efficiencies vary so significantly in the literature (coefficient of variation 25–45%). Understanding these trade-offs is essential for rational DES selection and process optimization. Despite these hurdles, the inherent advantages of deep eutectic solvents, such as their biodegradability and simpler fabrication compared to ionic liquids, underscore their potential for sustainable biomass conversion, especially as ongoing research seeks to overcome limitations such as solubility and cost [8,50]. However, for the widespread adoption of DES technology, efficient, cost-effective recycling strategies are paramount, given the large solvent volumes typically required in pretreatment processes [49].

2.3. Comparative Analysis of Pretreatment Methods

A summary comparison of selected pretreatment techniques is provided in

Table 3. Comparative Analysis of Selected Biomass Pretreatment Methods.

Pretreatment Method	Lignin Removal Efficiency	Energy Consumption	Process Time	Notable Advantages	Critical Limitations	Economic Viability	References
DA	Moderate to High (30–50%)	Moderate (15–25 MJ/kg)	>1–2 h	Solubilizes hemicellulose effectively; proven at an industrial scale	High inhibitor formation (furfural 1–3 g/L, HMF 0.5–2 g/L); requires corrosion-resistant equipment (+40–60% CAPEX); poor lignin removal; acidic waste disposal	Moderate ($2500–3000/ton product)	[17]
IL	High (60–90%)	High (25–40 MJ/kg including recovery)	Variable (1–24 h)	Decreases cellulose crystallinity dramatically; near-complete dissolution is possible	Prohibitive cost ($5–50/kg); significant ecotoxicity (LC50 10–1000 mg/L); residual IL inhibits enzymes/microbes; requires >98% recovery for viability	Poor (>$5000/ton product unless >98% recovery)	[17,41]
AFEX	Moderate (10–30%)	Moderate (20–30 MJ/kg)	Variable (5–60 min)	Creates nanoporous structures; no inhibitor formation; effective for low-lignin biomass	Complex NH3 recovery (90–98% needed); limited effectiveness for high-lignin feedstocks; safety risks with anhydrous NH3; high-pressure equipment required	Moderate ($2400–2800/ton product)	[17,46]
Alkaline	High (50–80%)	High (30–45 MJ/kg)	Long (hours to days)	Selective lignin removal; minimal carbohydrate degradation	Extended processing times; high chemical consumption (50–200 kg NaOH/ton); costly alkali recovery (25–35% of costs); high-salinity waste	Moderate to Poor ($2400–3200/ton product)	[32,35]
DES (acidic)	High (66–79% delignification)	Low (5–10 MJ/kg, 1/5–1/8 of other methods)	Short (<30–60 min)	Low toxicity (LC50 > 1000 mg/L); biodegradable; recyclable (>90% potential); low equipment costs; selective delignification	Moderate inhibitor formation (2–4 g/L phenolics); viscosity management required; solvent recovery validation needed; formulation-dependent performance	Promising ($2100–2500/ton with >90% recovery)	[32,48]
DES (neutral)	Moderate (40–55%)	Low (5–10 MJ/kg)	Longer (2–4 h)	Minimal inhibitor formation (0.5–1.5 g/L); cleaner hydrolysates; better enzyme compatibility	Lower delignification efficiency; longer processing times; requires optimization for high-lignin feedstocks	Moderate (requires further validation)	[18,52]

, highlighting not only operational parameters and observed outcomes but also critical economic and environmental limitations that influence industrial viability.

A critical analysis of Table 2 reveals that DES pretreatment addresses several limitations of conventional methods while introducing a more favorable trade-off balance.

While both DA and acidic DES achieve moderate-to-high delignification, DES pretreatment operates at significantly lower temperatures (80–120 °C vs. 160–200 °C), reducing energy consumption by 40–60%. More importantly, DES-treated biomass contains 50–70% fewer furan inhibitors (furfural, HMF) than DA-treated biomass due to milder reaction conditions and selective lignin targeting, substantially improving downstream fermentation performance without expensive detoxification steps [17]. Additionally, DES eliminates the need for corrosion-resistant equipment and neutralization/disposal of acidic waste streams, reducing both CAPEX and environmental burden.

DES achieves delignification comparable to ILs (66–79% vs. 60–90%), but at dramatically lower cost ($1–5/kg vs. $5–50/kg) and with substantially reduced ecotoxicity (10–100× lower). Critically, residual DES in pretreated biomass causes minimal enzyme inhibition (<15% reduction in cellulase activity at 2% w/w DES) compared to residual ILs (50–80% inhibition at similar concentrations), eliminating the need for extensive washing steps [18]. This fundamental difference in biological compatibility, combined with easier recovery due to the non-volatile nature and hydrogen-bonding properties of DES components, makes DES substantially more viable for integration into biological conversion processes such as anaerobic digestion.

For low-lignin agricultural residues, AFEX demonstrates effectiveness without the formation of inhibitors. However, for high-lignin feedstocks like apple pomace (15–30% lignin), DES achieves 2–3× higher delignification (66–79% vs. 10–30%) at comparable or lower energy input, without the safety risks, high-pressure requirements, or complex ammonia recovery infrastructure required for AFEX [45]. This makes DES more versatile across different feedstock types.

While alkaline methods achieve excellent lignin removal (50–80%), they require significantly longer processing times (hours to days vs. 30–60 min), consume large quantities of chemicals that require costly recovery systems, and generate high-salinity waste streams. DES pretreatment achieves comparable or higher delignification in a fraction of the time, with simpler recovery processes and substantially lower environmental impact [32].

The most critical insight from the comparative analysis is that acidic DES formulations achieve the highest lignin removal (66–79%) and consume significantly less energy (1/5–1/8 that of conventional methods). Still, they face greater downstream challenges due to phenolic contamination, which can affect both solvent recovery and potentially fermentation. Conversely, neutral DES systems produce cleaner hydrolysates with minimal inhibitor formation but require 2–3 times longer processing times and achieve only moderate delignification (45–60%), data not commonly highlighted in promotional literature [52].

This performance variability explains why direct comparisons between studies are problematic: researchers often optimize for different endpoints (lignin removal vs. sugar yield vs. inhibitor minimization vs. downstream process compatibility), making “superior performance” claims context-dependent rather than absolute. For apple pomace specifically, with its moderate lignin content (15–30%) and acid-labile pectin and sugar components, the optimal DES formulation may differ substantially from that optimized for woody biomass (25–40% lignin, minimal pectin/free sugars). Furthermore, DES facilitates a more sustainable approach to lignocellulose fractionation, reducing reliance on hazardous chemicals and high temperatures typically associated with traditional methods [53,54]. Their efficacy in dissolving lignin by inducing β-O-4 cleavage and partial carbon-carbon bond cleavage, while largely retaining cellulose and hydrolyzing hemicellulose, underscores their distinct advantage in biomass valorization [52].

3. DES Pretreatment for Apple Pomace

DES pretreatment uses eutectic mixtures of hydrogen bond donors and acceptors to disrupt lignocellulosic structures. Mechanistic details are provided below.

3.1. Mechanistic Insights of DES Action

The mechanism by which DESs function involves several key processes:

• Disruption of Lignin–Carbohydrate Complexes: The solvent mixture penetrates the biomass matrix, selectively breaking the ester and ether bonds, especially the crucial β-O-4 linkages, that connect lignin to hemicellulose or cellulose [18]. This disruption is essential for liberating the polysaccharide components.
• Hydrogen Bonding Interference: The abundant hydrogen-bond network in DES formulations competes with native intermolecular bonds in biomass. For instance, DESs based on choline chloride and lactic acid can form extensive hydrogen bonding, weakening the internal structure of lignocellulose, thereby reducing its crystallinity and enhancing enzyme accessibility [18].
• Selective Lignin Solubilization: DESs are particularly effective in preserving the cellulose fraction while solubilizing lignin and, to some extent, hemicellulose. This selectivity is vital for maintaining the quality of the cellulose residue, which is essential for subsequent biofuel production pathways [18,32].

The mechanistic distinction between acidic and neutral DES systems is critical to understanding performance variability. Acidic DES (pKa < 4.5) donates protons that protonate the ether oxygen in β-O-4 linkages, facilitating heterolytic cleavage and yielding lower molecular weight lignin fragments (800–2000 Da) compared to neutral systems (2000–5000 Da) [18]. This explains the 40–60% higher delignification efficiency observed with lactic acid-based DES versus glycerol-based systems. However, this proton-mediated mechanism also generates furfural and hydroxymethylfurfural (HMF) from sugar degradation at concentrations that can inhibit anaerobic digestion when exceeding 1 g/L. Consequently, acidic DES formulations require careful pH neutralization and washing steps that neutral systems do not, partially offsetting their kinetic advantages.

3.2. Optimization Parameters for DES Pretreatment

For application to AP, several parameters (as presented in

Table 4. Comparative Performance of Pretreatment Methods: Lignin Removal and Methane Yield Improvement.

Pretreatment Method	Lignin Removal (%)	Experimental Conditions	Methane Yield (mL CH4/g VS or NL CH4/kg VS)	Improvement vs. Untreated	References
Untreated Apple Pomace	Not Available (N/A)	Baseline	~230 NL CH4 kg−1 VS	Baseline (0%)	[55]
Acidic DES (ChCl:Lactic Acid 1:2)	66–79%	120 °C, 30 min	310–360 NL CH4 kg−1 VS	+35–55%	[9,17,32]
Neutral DES (ChCl:Glycerol)	40–55%	100–120 °C, 1–2 h	265–290 NL CH4 kg−1 VS	+15–25%	[17,18]
DES (various mild formulations)	45–60%	80–120 °C, variable	>120 mL g−1 VS	~30–50%	[9,52]
Dilute Acid (H2SO4 1–2%)	30–50%	160–200 °C, 1–2 h	250–290 NL CH4 kg−1 VS	+10–25%	[17,56]
Alkaline (NaOH 2–10%)	50–80%	80–120 °C, hours to days	280–320 NL CH4 kg−1 VS	+20–40%	[35,56]
Ionic Liquids	60–90%	80–120 °C, 1–24 h	300–350 NL CH4 kg−1 VS	+30–50%	[17]
AFEX (Liquid NH3)	10–30%	60–100 °C, 5–60 min	260–300 NL CH4 kg−1 VS	+15–30%	[17,45]
Thermal Pretreatment	15–35%	150–200 °C, 30–60 min	250–280 NL CH4 kg−1 VS	+10–20%	[56]
Enzymatic Pretreatment	20–40%	50 °C, 24–48 h	270–300 NL CH4 kg−1 VS	+15–30%	[56]
Organosolv (Ethanol/acid)	65–95%	170–200 °C, 1–3 h	290–340 NL CH4 kg−1 VS	+25–45%	[33]

) must be optimized to ensure effective pretreatment, such as temperature and reaction time. Studies have shown that effective DES pretreatment can be achieved within 30 min at temperatures around 120 °C [32]. Short process times reduce energy consumption and limit potential degradation of sugars. On the other hand, high solid loading enhances lignin dissolution while minimizing solvent consumption. However, maintaining an ideal biomass-to-solvent ratio is critical, as excessive loading may impede mass transfer and reduce effective delignification [18,32]. Also, incorporating a controlled amount of water into DES formulations can lower viscosity and improve penetration into the biomass.

Nevertheless, excessive water may compromise the solvent’s capacity to solubilize lignin effectively [18,52]. Moreover, the molar ratio of HBD to HBA (e.g., choline chloride to lactic acid) must be carefully calibrated to maximize hydrogen bonding and solvent efficiency. Acidic DES formulations have been reported to achieve delignification yields of 66–79% in certain wood waste studies, indicating their potential applicability to AP [32,52].

However, these optimization parameters are interdependent rather than independent, complicating comparisons across studies. For instance, increasing temperature from 80 °C to 120 °C enhances delignification rate 3-fold but also increases sugar degradation by 40–60%, particularly in acidic DES systems [32]. Similarly, while a high solid loading (1:5 biomass: DES ratio) is economically attractive, it increases viscosity by 200–500%, requiring either higher temperatures (which increase inhibitor formation) or longer processing times (which negate the energy advantage). These competing effects explain why reported optimal conditions vary dramatically across studies: researchers working with different biomass types, reactor configurations, and downstream processes arrive at different optima even when using identical DES formulations.

3.3. Application to Apple Pomace

AP, while compositionally distinct from woody biomass, shares a typical lignocellulosic architecture that poses similar challenges for biomass conversion. Its high content of pectin, residual sugars, and lignin-bound cell wall components implies that a robust pretreatment method is necessary to achieve:

• Enhanced Cellulose Accessibility: By effectively removing lignin, DES pretreatment can expose cellulose fibers, rendering them more amenable to microbial or enzymatic attack during anaerobic digestion [18].
• Improved Sugar Yield: The cleavage of lignin and carbohydrate bonds facilitates the release of fermentable sugars, which can subsequently be converted into biofuels. Preliminary studies on other feedstocks suggest that DES pretreatment conditions can be tuned to optimize sugar conversion rates [18].
• Minimized Inhibitor Formation: DES pretreatment allows operation under relatively mild conditions, reducing the formation of degradation products often associated with harsher chemical treatments.

The capacity to tailor DES formulations specifically for AP, given its relatively lower lignin content compared to woody biomass, presents a promising avenue for enhancing subsequent anaerobic digestion performance. Critically, the lack of head-to-head DES comparisons specifically for apple pomace limits definitive conclusions. The three studies that report AP-DES pretreatment use different DES formulations (ChCl: lactic acid 1:2, ChCl: glycerol 1:2, and ChCl: urea 1:2), different temperatures (80–140 °C), and measure different outcomes (sugar release vs. methane yield vs. lignin removal), making direct performance comparisons impossible. This methodological inconsistency is a critical gap in the current literature. Preliminary data suggest that AP’s lower lignin content (15–30% vs. 25–40% in woody biomass) may reduce the relative advantage of aggressive acidic DES systems, potentially making milder neutral DES formulations more economically attractive for this specific feedstock—a hypothesis requiring systematic validation.

4. Anaerobic Digestion of Apple Pomace

Anaerobic digestion (AD) is a well-established biological process that converts organic waste into biogas and digestate, which can serve as renewable energy sources and fertilizers, respectively [57,58,59]. In the context of AP, AD represents a viable pathway to produce methane-rich biogas while contributing to waste minimization and environmental sustainability [60,61].

4.1. Process Overview of Anaerobic Digestion

The AD process involves a series of biological reactions that occur in the absence of oxygen [62,63,64]. Typically, the process is subdivided into four key stages, as shown in Figure 1.

Each stage of AD is sensitive to feedstock composition. In AP, the presence of easily digestible sugars and pectin can accelerate initial hydrolysis; however, lignin and recalcitrant fibers may inhibit microbial access and enzyme efficiency [56,65].

4.2. Enhancing AD Efficiency Through Pretreatment

DES pretreatment enhances AD by improving biomass digestibility [18]. Specifically, it can reduce particle size and crystallinity, lowering lignin content and increasing the surface area available for enzyme action.

Then, a release of fermentable sugars and soluble compounds facilitates the growth and activity of hydrolytic bacteria. Enhanced substrate digestibility is directly correlated with higher and more rapid methane yields.

Table 5. Analysis of Selected Biomass Pretreatment Option Focusing on Reactor Setup.

Reference	Reactor Type	Temperature	Substrate Strategy	Pretreatment (If Any)	Methane Yield/Biogas Rate	Stability Indicators (pH, VFA, OLR, C:N)
[55]	Batch BMP	Mesophilic (~38 °C)	Apple pomace (mono-digestion)	None	≈232 NL CH4 kg−1 VS (SMY)	Lag ≈ 4 d; T95 ≈ 20 d; low NH3/H2S; C:N > ~24:1
[12]	Batch; nutrient recovery (GPM)	Mesophilic	Co-digestion: swine manure + AP (0–30% VS)	None	AP 7.5–15%: yields comparable to manure alone; 30% AP reduced stability	Stable at ≤15% AP; nutrient (NH4+) recovered as (NH4)2SO4
[66]	Continuous co-digestion (CSTR)	Mesophilic	Acidic fruit-processing waste + WAS (AP-analogous)	None	≈350 mL CH4 g−1 VS (typical)	pH 6.8–7.3; VFA/TA < 0.16; buffered with sludge
[4]	Scenario/LCA (AD + compost)	—	Apple pomace management (AD vs. composting)	—	AD scenario preferred on GHG footprint (contextual)	Benefits when nutrient/energy recovery is included
[9]	Review + experimental notes	—	AP biorefinery (energy + pectin)	Hydrolysis	>120 mL g−1 VS for mild pretreatment cases; severe conditions lower	Emphasizes inhibitor control; staged product recovery
[67]	Review (biochar in AD)	—	Lignocellulosic residues (incl. fruit wastes)	Biochar amendment (process aid)	Up to ~10–30% improvements reported across cases	Enhanced buffering, DIET facilitation, VFA mitigation

synthesizes recent evidence on apple pomace AD across batch and continuous regimes. Untreated AP in mono-digestion shows specific methane yields around ~230 NL CH₄ kg⁻¹ vs. under mesophilic BMP conditions [55], while co-digestion with manure or sludge consistently improves buffering and process robustness, maintaining near-baseline yields at ≤ 15–20% AP (vs.) and avoiding VFA accumulation [12,66]. Integrated biorefinery strategies and pretreatments (e.g., selective hydrolysis before digestion) can increase yields, provided inhibitor formation and DES carryover (if used) are controlled [9]. Life-cycle assessments reinforce that AD-centered pathways outperform disposal when paired with energy and nutrient recovery [4].

4.3. Potential Performance of Apple Pomace in AD

Although specific studies on AP subjected to DES pretreatment and subsequent AD are still emerging, the magnitude of these improvements varies dramatically depending on DES type and severity. That is, methane yield improvements from acidic DES pretreatment increase cumulative methane yield by 35–55% compared to untreated AP, while neutral DES systems show more modest improvements of 15–25% [17]. This difference reflects the superior delignification achieved by acidic systems, but reported yields show high variability (25–40%) even within acidic DES studies, suggesting that operational parameters (temperature, time, solid loading) have effects comparable in magnitude to the choice of DES itself. Secondly, process stability trade-offs, while enhanced hydrolysis accelerates initial biogas production, severe DES pretreatment (>120 °C, >2 h) can reduce process stability by introducing inhibitory phenolics that accumulate during the acetogenesis phase, manifesting as VFA/TA ratios exceeding 0.3 (compared to <0.16 for optimally pretreated substrates). This creates a narrow operational window that is not clearly defined in the current literature. Lastly, digestate quality variables, digestate nutrient profiles differ significantly between acidic and neutral DES-treated AP: acidic systems produce digestate with 15–30% lower NH₄⁺, N due to nitrogen incorporation into condensed phenolic products, potentially reducing its value as fertilizer, an economic consideration absent from most techno-economic analyses.

Figure 2 frames five interacting factors that determine methane productivity in AP systems. The literature indicates that aligning these levers enables stable operation at moderate AP fractions with competitive methane yields while preserving the option to extract high-value co-products upstream.

5. Integration of DES Pretreatment with Anaerobic Digestion

The integration of DES pretreatment with anaerobic digestion offers a synergistic pathway for the valorization of AP. The enhanced accessibility of cellulose and hemicellulose resulting from effective DES delignification can substantially improve the efficiency of the subsequent AD process.

5.1. Proposed Process Flow for Integrated System

This section details the stepwise integration of DES pretreatment with AD for AP valorization. Apple pomace is preprocessed, and fresh AP undergoes initial size reduction (e.g., milling) to increase its surface area and facilitate solvent penetration. Thereafter, milled AP is mixed with an optimized DES formulation (i.e., a choline chloride/lactic acid system in a 2:1 molar ratio) under controlled conditions (approximately 120 °C for 30 min). During this step, DES disrupts the lignocellulosic matrix, selectively dissolves lignin, and modestly hydrolyzes hemicellulose while preserving cellulose integrity. Following pretreatment, the biomass is separated into a solid fraction (rich in cellulose and residual hemicellulose) and a liquid fraction containing solubilized lignin and degradation products. Solvent recovery is performed, possibly using techniques such as anti-solvent addition or dialysis, to recycle DES for further cycles. Then, the pretreated (and optionally, detoxified) solid fraction is directed into the anaerobic digester. Ideally, the improved digestibility of the pretreated biomass results in enhanced hydrolysis, acidogenesis, acetogenesis, and methanogenesis stages. Lastly, the AD process yields methane-rich biogas, which can be purified and used directly for heat and power generation. The digestate is further processed and used as a biofertilizer, closing the nutrient recycling loop.

The flowchart in Figure 3 illustrates the integrated process for converting apple pomace into renewable energy and value-added products using DES pretreatment and anaerobic digestion. The process begins with the collection of raw apple pomace, which is first milled to reduce particle size and enhance downstream processing. The milled material undergoes deep eutectic solvent (DES) pretreatment using a choline chloride–lactic acid mixture to improve biomass accessibility. Following pretreatment, a separation and solvent recovery step isolates the solid fraction, which is subsequently fed into the anaerobic digestion unit. Within the digester, the organic matter is biologically converted into biogas, which can be upgraded for electricity generation, heat production, or as a fuel for engines. The remaining digestate is recovered and used as a nutrient-rich fertilizer, closing the loop in a circular bioeconomy. Overall, the figure depicts a streamlined biorefinery process that transforms a low-value agro-industrial residue into multiple useful products through DES-enhanced anaerobic digestion.

5.2. Critical Comparison: DES Performance for Apple Pomace vs. Woody Biomass

Direct application of DES conditions optimized for woody biomass to AP may be suboptimal due to compositional differences. AP contains 8–12% pectin and 10–18% soluble sugars, components largely absent in wood, which are acid-labile and degrade rapidly above 100 °C in acidic DES. Consequently, while choline chloride/lactic acid (1:2) achieves 78% delignification of oak wood at 120 °C/30 min [32], preliminary trials suggest AP requires milder conditions (100 °C/20 min) to prevent sugar losses exceeding 30%.

Furthermore, AP’s lower lignin content (15–30% vs. 25–40% in wood) reduces the absolute benefit of aggressive delignification. A 70% reduction of 25% lignin (wood) yields greater improvement in digestibility than a 70% reduction of 20% lignin (AP), suggesting that for AP, moderate neutral DES systems may offer better cost–benefit ratios than aggressive acidic systems, a hypothesis requiring systematic validation that is currently absent from the literature [48].

6. Economic and Environmental Assessment

Integrating DES pretreatment with anaerobic digestion for AP valorization not only improves technical efficiency but also has significant economic and environmental implications. The assessment of these factors is critical in determining the potential for industrial adoption and scalability.

6.1. Economic Considerations

Similarly, in studies focused on bioethanol production, acidic DES systems, particularly choline chloride–lactic acid mixtures, have demonstrated competitive techno-economic performance. For example, Peng et al. [48] reported minimum ethanol-selling prices (MESPs) as low as 2128.1 USD per ton; however, this value was achieved under specific modeling assumptions, including high solvent recovery rates, optimized heat integration, and near-ideal fermentation efficiencies. The reported MESP therefore reflects a best-case scenario rather than a universally applicable cost estimate, and actual performance is likely to vary depending on feedstock characteristics, process configuration, and solvent recycling efficiency. Nevertheless, the study highlights the potential of acidic DES pretreatment systems to reduce overall production costs when process conditions are tightly controlled. Although the product in the current process is biogas rather than ethanol, similar economic benefits can be anticipated when converted on a methane energy-equivalent basis.

Furthermore, the cost-effectiveness of DES pretreatment is closely linked to effective solvent recovery methods. Advances in recycling techniques, such as anti-solvent recovery and membrane filtration, are essential to minimizing recurring solvent costs [18,52]. Higher reusability reduces the net cost and contributes to circular resource management.

6.2. Environmental Impact and Life Cycle Assessment

A life cycle assessment (LCA) of the integrated process examines the environmental burdens and benefits associated with converting AP into biofuel, as given in

Table 6. Comparative Economic and Environmental Metrics of Integrated DES + AD Process Versus Conventional Pretreatments.

Metric	DES Pretreatment + AD	Conventional Pretreatment	Comments	References
Energy Consumption	Low (1/5–1/8 relative)	Moderate to High	DES operates at a lower energy input	[32]
Greenhouse Gas Emissions (GWP)	Lower (approx. 0.025–0.026 kg CO2-eq/MJ)	Higher (e.g., 0.04786 kg CO2-eq/MJ for alkali) [68]	Lower emissions due to reduced energy and solvent reuse	[68]
Minimum Energy Selling Price	Potentially competitive (~$2128.1/ton)	Higher due to extended process time and recovery issues	DES shows promising upfront economic feasibility	[48]
Waste Valorization	High (digestate reuse)	Moderate	Integration with AD converts waste to biogas and fertilizer	[48]

. The significant environmental benefits include reduced greenhouse gas (GHG) emissions, as valorizing AP through AD reduces reliance on fossil fuels and prevents the open dumping or landfilling of organic waste, a significant source of methane emissions. Furthermore, DES-based pretreatment has been shown to lower the process’s GWP compared to more conventional approaches [52,68]. Then, there is the challenge of lower energy consumption due to the inherent efficiency of DES pretreatment; the integrated system consumes less energy during pretreatment, thereby reducing overall carbon intensity. The recycling of DES further curtails the environmental footprint by minimizing the requirement for continuous fresh solvent production. Finally, there is a possibility of circular economy benefits, as holistic integration of waste valorization (i.e., converting AP into biogas and fertilizer) supports circular economy objectives by closing resource loops. The recovery of digestate for use as fertilizer reduces demand for synthetic fertilizers and supports nutrient recycling.

The combination of DES pretreatment with anaerobic digestion is expected to yield significant improvements in both cost efficiency and environmental sustainability. Energy savings, lower GHG emissions, and a robust circular economy approach collectively underscore the potential of this integrated process for low-carbon energy production.

6.3. Sensitivity and Risk Analysis

Sensitivity analysis indicates that key variables, such as enzyme conversion rates, solvent recovery efficiency, and variations in feedstock composition, can significantly impact process economics. Uncertainties associated with scaling up DES pretreatment, particularly regarding solvent recycling and energy demand, must be addressed in future studies to validate laboratory-scale findings at an industrial level. Overall, the integrated process aligns well with sustainability objectives and offers a promising avenue for the biodigestion of AP.

7. Scaling Up and Industrial Implementation

The transition from laboratory-scale optimization of AP valorization to industrial deployment requires a shift from yield-focused experimentation toward system-level integration, techno-economics, solvent recovery, supply chain logistics, and regulatory compliance. Although multiple studies have demonstrated improved methane yields after pretreatment or co-digestion at bench scale, commercial feasibility depends on whether DES pretreatment and AD can be embedded into industrial fruit-processing infrastructure at acceptable cost and with positive life-cycle performance outcomes [1,4].

7.1. Feedstock Logistics and Supply Chain Considerations

From a scale-up perspective, AP represents a strategically attractive substrate because it is generated in large volumes at single geographic nodes (juice, cider, and puree plants), avoiding the dispersed-collection challenge typical of agricultural residues like straw. However, production remains seasonally concentrated, creating a mismatch with the year-round substrate demand of continuous AD plants [69]. Therefore, commercial digestion systems require one of three buffering strategies: storage/ensiling of wet pomace, drying, or co-feeding with complementary residues, such as manure, sludge, or other fruit wastes, to stabilize organic loading throughout the year [4]. At an industrial scale, this logistics challenge often accounts for more of operating expenditure (OPEX) than reactor performance itself.

7.2. Integration with Existing Industrial Infrastructure

Studies modeling AP biorefineries show that co-location lowers CAPEX by reducing utility duplication and enabling process heat exchange, especially when biogas is converted to steam or electricity for the host processing plant [4]. To guide the integration,

Table 7. Three Configurations Dominate Current Feasibility Assessments.

Model	Opportunity	Limitation	References
On-site bolt-on biorefinery	Heat, utilities and wastewater can be shared with the apple-processing plant	Requires space and CAPEX approval from the food producer	[4]
Regional hub	Higher economies of scale via multi-supplier aggregation	Requires logistics contracts and feedstock guarantees	[4]
Third-party energy operator	Processor avoids energy-sector risk; income via gate fee	Revenue from energy shared; dependence on policy incentives	[4]

presents a comparative overview of three configurations under active feasibility evaluation, detailing their potential benefits and associated challenges.

7.3. Process Intensification and Scale-Up of DES Pretreatment

DES pretreatment is mechanically simple at the laboratory scale but becomes challenging in continuous operation. Industrial implementation requires DES to be recovered and recycled at ≥90% to remain cost-competitive [28]. Many published pretreatment trials use high solvent-to-biomass ratios, long reaction durations, and multistage washing, all of which inflate CAPEX and water demand when scaled. Cost and engineering analyses from the lignocellulosic biofuel sector indicate that pretreatment reactors, solid–liquid separation units, and solvent recovery loops account for >40% of the total capital cost in first-generation designs [70].

In addition, scaling up AD of partially pretreated AP requires validation of pumpability, phenolic carryover, foaming, viscosity changes, and the achievable OLR without acidification. Industrial AD plants handling fruit waste typically operate at 2–4 kg VS m⁻³ d⁻¹, but DES-treated substrates may support higher loads if inhibitor risks are managed [69].

7.4. Regulatory and Permitting Considerations

Commercialization is tightly coupled to regulatory frameworks governing waste classification, digestate land application, solvent toxicity, and renewable-energy certification. DES formulations for industrial use must demonstrate non-hazardous classification or include full solvent recovery to prevent regulatory downgrade of the residual solids [28]. Digestate must comply with pathogen and heavy-metal thresholds to qualify as fertilizer under EU and US soil regulations, while biomethane upgrading requires compliance with gas-grid purity specifications [69].

8. Key Challenges

While integrating DES pretreatment with anaerobic digestion holds great promise, a few challenges must be addressed to realize this technology’s potential for AP valorization fully. Figure 4 shows some of the technical and operational challenges that need to be mitigated to ensure successful biofuel production from apple pomace.

By addressing these challenges and pursuing targeted research, the integration of DES pretreatment with anaerobic digestion could emerge as a viable technology for converting AP into renewable, low-carbon biofuel. Continued advancements in both the chemical and biological aspects of the process will be critical for scaling up and commercializing this promising approach.

9. Future Research and Directions

Although the technical feasibility of integrating DES pretreatment with AD of AP has been demonstrated at laboratory scale, multiple knowledge gaps remain at the interface of chemistry, process engineering, microbial ecology, and techno-economics. The next phase of research must therefore move beyond single-parameter optimization and toward a systems-scale understanding that enables commercial deployment.

9.1. DES Chemistry, Solvent Recovery and Environmental Safety

Most published DES studies focus on delignification efficiency and sugar release but do not report full mass-balance solvent recoveries, solvent degradation kinetics or life-cycle impacts. Industrial uptake will depend on demonstrating ≥90% solvent recyclability, no long-term accumulation of DES residues in digestate, and validated microbial toxicity thresholds for common hydrogen-bond donors/acceptors (e.g., choline chloride, lactic acid, betaine) [28,70]. In addition, the environmental profile of DES must be benchmarked not only against ionic liquids but also against no-pretreatment baselines, to test whether the added energy and chemical demand truly lead to a net reduction in carbon intensity [4].

9.2. Technical and Operational Challenges

Research into advanced recovery methods (e.g., membrane filtration, crystallization, and anti-solvent recovery) is crucial. Moreover, the interdependence of DES challenges complicates optimization: reducing viscosity by adding water (>30%) improves mass transfer but decreases lignin solubility by 25–40% and increases solvent volume, requiring recovery. Lowering the temperature to minimize inhibitor formation (from 120 °C to 80 °C) reduces phenolic generation by 60% but extends processing time 3-fold, offsetting energy savings. These trade-offs, inadequately quantified in current AP literature, explain why “optimal” DES conditions vary dramatically between studies and why AP-specific systematic optimization remains a critical research gap.

9.3. Microbial Response to Pretreated Substrates

Studies show that mild pretreatments improve methane yield while severe pretreatments increase soluble phenolics and VFAs that inhibit methanogenesis [9]. However, research is still primarily descriptive with few metagenomic or proteomic datasets describing how DES-treated substrates select for (or suppress) hydrolytic, syntrophic, and methanogenic guilds. Future work should include multi-omics mapping of microbial communities exposed to DES residues, in both batch BMP assays and long-term continuous reactors. This is especially important because full-scale digesters operate under dynamic, multi-substrate loading, unlike controlled bench-scale studies [69].

9.4. Reaction Engineering and Solid Handling

Most DES–biomass studies still rely on batch flasks or stirred bench reactors. However, scalable industrial reactors are likely to be screw-reactors, plug-flow pretreatment vessels, or counter-current systems. The rheology and pumpability of DES-treated AP, particularly after partial dewatering, are poorly characterized. Process-scale modeling is needed to determine the shear, temperature, residence time, and mixing energy required to sustain continuous operation. Likewise, little is known about how DES pretreatment affects digestion hydrodynamics, gas–liquid mass transfer, and foaming propensity in large reactors.

9.5. Co-Product Valorisation and Biorefinery Design

Techno-economic assessments show that energy yield alone is insufficient to support plant-level profitability unless paired with co-extraction of pectin, polyphenols, and seed oil [4]. However, the sequencing of extraction, pretreatment, and digestion is not yet optimized. For instance, adding DES before polyphenol extraction may complicate downstream purification, while applying DES after extraction may reduce lignin crystallinity but leave pectin unrecovered.

Furthermore, detailed LCA studies are needed to quantify the environmental benefits, including reduced GHG emissions and resource conservation, thereby bolstering the case for regulatory approval and industrial adoption. Future work should include a comprehensive LCA that accounts for DES synthesis, pretreatment energy, solvent recycling, anaerobic digestion with digestate management, and byproduct valorization. Scenario analyses for solvent recovery rates, energy sources, and storage/drying assumptions would also be essential.

9.6. Scale-Up, CAPEX Prediction, and Investment Risk

The transition from laboratory to industrial scale requires significant capital investment in equipment capable of handling high solid loadings and efficient solvent recovery. Moreover, retrofitting existing biogas plants to accommodate DES pretreatment modules would require thorough process engineering assessments. Such studies should not only focus on DES pretreatment efficacy but also on the integration with continuous anaerobic digestion systems.

Most techno-economic models assume solvent recovery, biogas utilization, co-product markets, and digestate certification. Yet, few are validated against empirical data from pilot (>100 L DES, >10 m³ AD) or demonstration (>100 m³ AD) systems. Research needs to move toward hybrid TEA–LCA–risk models that incorporate carbon-credit price volatility, DES market maturity, and national feed-in tariffs, waste-to-energy subsidies, or biomethane upgrade premiums [71]. Without this evidence, investors will continue to classify DES–AD biorefineries as TRL 5–6, delaying industrial deployment. Moreover, updating and refining economic models must include sensitivity analyses of key parameters, such as feedstock variability, solvent recovery rates, and process scale, thereby helping identify the most critical cost drivers and process bottlenecks.

9.7. Policy, Certification, and Cross-Sector Alignment

DES-based biorefineries sit at the intersection of waste regulation, renewable-energy regulation, fertilizer certification, and food-grade solvent policy. Little work has modeled how different regulatory frames (EU RED II [72], US RFS [73], SA Biofuel Mandate [74], ISO 14067 [75]) interact with the process economics. The lack of standardization for fruit-waste digestate further prevents circular nutrient loops from returning to orchards. Future research must include policy-linked modeling, not only biochemical optimization.

9.8. Standardization and Systematic Comparison Gaps

A critical impediment to industrial adoption is the lack of standardized comparison protocols. Current literature reports DES performance using inconsistent metrics (% delignification, sugar yield, methane potential), different biomass: solvent ratios (1:5 to 1:20), varying temperatures (60–160 °C), and different analytical methods for lignin quantification (Klason vs. acetyl bromide methods, which can differ by 20–35%). This methodological heterogeneity makes it impossible to definitively rank DES formulations or identify optimal systems for specific feedstocks.

This review recommends that future research adopt standardized protocols like those established for ionic liquids (NREL/TP-510-42618), including a fixed biomass: solvent ratio (1:10 w/w), standard temperature-time profiles (80 °C/2 h, 100 °C/1 h, 120 °C/0.5 h), mandatory reporting of both delignification efficiency AND inhibitor formation, a unified lignin quantification method, and required solvent recovery efficiency measurement.

Until such standardization occurs, claims of DES “superiority” remain difficult to validate, and industrial adoption will remain tentative.

10. Conclusions

In summary, integrating DES pretreatment with AD holds significant promise for efficiently converting AP into renewable biofuels. Key findings of this review are that, first, DES pretreatment selectively delignifies AP by breaking down lignin–carbohydrate bonds and lowering cellulose crystallinity. This improves enzyme accessibility and accelerates the AD process, resulting in higher methane yields and a faster biogas production cycle. However, it is critical to note that these benefits are not universal across all DES formulations. Acidic DES systems (choline chloride: organic acid) achieve 40–60% higher delignification than neutral systems, but at the cost of increased inhibitor formation, whereas neutral DES formulations offer cleaner hydrolysates but require longer processing times and achieve lower lignin removal. This fundamental trade-off, inadequately addressed in the current literature, necessitates application-specific optimization rather than a “one-size-fits-all” approach. Secondly, the proposed integrated process, from preprocessing and DES pretreatment to anaerobic digestion and digestate recovery, demonstrates apparent synergistic effects that not only optimize biomass conversion efficiencies but also promote resource recycling and circular economy practices. Furthermore, critical challenges remain in solvent recovery, process optimization for AP’s unique composition, and scaling up laboratory successes to pilot and industrial scales. Future investigations should focus on advanced recycling technologies, refined DES formulations, and comprehensive life cycle and economic assessments.

In conclusion, DES pretreatment has emerged as a highly effective strategy for biomass processing, offering superior delignification performance with reduced energy consumption and shorter reaction times when compared to conventional methods such as DA, IL, and AFEX. Its integration with AD further enhances biogas production by improving substrate digestibility and contributing to greater process stability. Comprehensive economic modeling and life cycle assessment underscore the integrated system’s potential to deliver both environmental sustainability and cost competitiveness. However, for successful industrial deployment and market viability, critical challenges related to process scalability and solvent recovery must be systematically addressed. DES-based pretreatment integrated with anaerobic digestion offers a viable, sustainable, and economically attractive method for valorizing the insignificant by-products of the apple industry. This process not only contributes to renewable low-carbon energy production but also supports waste minimization and resource circularity, aligning with contemporary environmental and economic imperatives.