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

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.