Transforming Apple Pomace into Functional Oligosaccharides via Microwave Optimisation

Apple pomace is an abundant agro-industrial residue rich in oligosaccharides, whose efficient and sustainable extraction remains challenging. To overcome these limitations, this study delivers a novel biorefinery strategy by coupling ultrasound-assisted extraction pretreatment with optimised microwave-assisted hydrolysis. Through response surface methodology, the optimal microwave hydrolysis extraction conditions were achieved at 20 min and 120 °C. The obtained oligosaccharides presented a molecular-weight of 191,071 Da, a polydispersity index of 2.19 and a high content of galacturonyl groups (9.95 g/L) and arabino substituents (7.27 g/L). Furthermore, a biphasic system was applied at the optimal point obtained for the monophasic system in order to examine its potential for selective oligosaccharide fractionation. In this case, the galacturonic acid did not form part of the obtained oligosaccharides, and instead they were predominantly formed by galactose and arabinose (6.55 and 4.48 g/L respectively). Moreover, this biphasic setup yielded fewer polydisperse fractions with a lower content of degradation products. Overall, this work highlights the promise of optimised microwave-assisted hydrolysis as a sustainable strategy for oligosaccharide valorisation from apple pomace while demonstrating that the extension to a biphasic system represents a promising alternative for improving process selectivity.