Upcycling onion waste into pectin derived-compounds via continuous water-only extraction

Onion peel waste (OPW), a major by-product of the agri-food industry, represents an underutilized biomass rich in valuable polysaccharides such as pectin. Addressing the need for sustainable valorization strategies, this study explores the transformation of OPW into pectin-derived compounds (PDCs) through a continuous subcritical water (SubW) extraction process conducted without chemical additives. The method leverages rapid hydrolysis at 200 °C with a residence time of approximately 5 s, markedly outperforming conventional batch (150 min at 125 °C) and semicontinuous systems in terms of efficiency and selectivity. The severity factor was applied to compare different reactor configurations, demonstrating the enhanced performance of the continuous process. This system maintained PDC yields of up to 30 %, preserved polymer integrity, and yielded compounds with significantly higher molecular weight (108 kDa vs. 78 kDa in batch). Furthermore, it reduced the formation of degradation products such as formic acid and furfural, resulting in a purer extract and facilitating downstream purification. These findings underscore the importance of precise control over extraction time to enable higher processing temperatures without compromising product quality. Overall, continuous SubW extraction emerges as a scalable, energy-efficient, and environmentally friendly approach for producing high-purity PDCs with potential applications in the food, pharmaceutical, and cosmetic industries. This work contributes to the advancement of the circular bioeconomy by promoting the upcycling of agricultural residues into high-value bioproducts. • Continuous SubW extraction of OPW achieves 30 % PDC yield without chemical additives. • Reaction times drop to ∼5 s at 200 °C, compared to 150 min in batch processes. • Continuous method yields PDCs with higher molecular weight (108 kDa vs. 78 kDa). • No degradation products (formic acid, furfural) found in continuous extraction. • Scalable, eco-friendly method enables sustainable production of high-purity PDCs.