Comparative Effects of Drying Methods on Waste Seed Melon Pulp By-Product Quality and Polysaccharide Characteristics: Structural Features, Bioactivity, and Gastrointestinal Digestion Stability

Waste seed melon processing by-products are rich in bioactive polysaccharides. Nevertheless, their high moisture content and perishability restrict the high-value utilization of these polysaccharides. In this study, four pretreatment drying methods, namely sun drying (SD), 55 °C hot-air drying (HAD), weak microwave-assisted hot-air drying (WMHAD), and vacuum freeze drying (VFD), were adopted to investigate the effects of drying methods on the quality characteristics of dried seed melon, as well as the structural characteristics (micromorphology, monosaccharide composition, molecular weight distribution, and functional groups), biological activities (antioxidant capacity, α-glucosidase inhibitory activity, and pancreatic lipase inhibitory activity), and gastrointestinal stability of seed melon polysaccharides (SMPs). The structure–activity relationship of SMPs was further elucidated. The results showed that none of the four drying methods disrupted the glycosidic backbone of polysaccharides, only altering the molar ratios of monosaccharides. VFD preserved the loose structure of pulp; VFD-SMP possessed the highest weight-average molecular weight and galacturonic acid content and exhibited optimal α-glucosidase inhibitory activity and gastrointestinal stability. HAD induced polysaccharide degradation, and HAD-SMP displayed the strongest antioxidant capacity. WMHAD-SMP exerted prominent pancreatic lipase inhibitory activity. The SD-treated samples exhibited the lowest polysaccharide extraction yield as well as the poorest overall biological activities. This work may provide a preliminary theoretical basis and data reference for the targeted preparation of functional polysaccharides from seed melon processing by-products.