Sequential Ultrasound and Microwave-Assisted Autohydrolysis for Efficient Production of Oligosaccharides from Apple Pomace

This study investigates the valorisation of apple pomace (AP) by developing a sequential extraction process to obtain oligosaccharide-enriched fractions using intensified hydrothermal treatments. The research focuses on optimising a hydrothermal ultrasound-assisted extraction (UAE) and in the study of its effect on a subsequent microwave-assisted autohydrolysis (MAA). The UAE was optimised by an experimental design in which temperature, time and amplitude were the selected variables for the maximisation of the obtaining of these compounds. The optimal conditions were 66 °C, 16 min and 80% amplitude. Results indicated that glucooligosaccharides (GOS) were the predominant fractions obtained in both UAE and MAA, achieving a 37% glucan conversion during UAE. Furthermore, UAE effectively solubilised medium molecular weight carbohydrates and removed free soluble sugars, leading to the production of oligosaccharide-enriched fractions in the subsequent MAA. Moreover, different oligosaccharides were isolated at each step of the sequential extraction process, allowing a preliminary evaluation of the prebiotic activity of the precipitated extracts. Metagenomic analysis revealed significant shifts in microbial composition and relative abundance after 24 h. Substrates promoted the enrichment of Lactobacillus ruminis , Bifidobacterium longum and Bifidobacterium adolescentis among others, demonstrating the preliminary potential of these precipitated extracts to positively modulate gut microbiota.