Effects of Extraction Methods on the Physicochemical Properties and Bioactivities in Vitro of Raspberry (Rubus idaeus L.) Polysaccharides

To determine the effects of extraction on the physicochemical properties and bioactivities in vitro of raspberry polysaccharides (raspberry polysaccharides, RAPs), water extraction, alkali extraction, acid extraction and enzyme extraction methods were studied. The microstructure of the polysaccharides and function groups were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The rheological properties of the polysaccharide were determined by HR-1 Discovery mixed rheometer. Additionally, the antioxidant activities were evaluated by DPPH, ABTS⁺ and hydroxyl radical scavenging activities, and the hypoglycemic activities were analyzed by α-amylase and α-glucoside inhibition. Furthermore, their hypidemic activities were evaluated by their oil, cholesterol and sodium cholate binding activities. The results showed that extraction yields, total polysaccharides and their molecular weights, constituent monosaccharides, proteins and phenolic compounds, rheological properties and binding capacities in vitro of the RAPs varied depending on the extraction method employed. RAPs obtained by enzyme extraction exhibited the highest molecular weight (5.80×10⁵ Da) and apparent viscosity. Moreover, in terms of biological activity, they showed strong antioxidant activities and significant α-amylase and α-glucosidase inhibitory effects. And the raspberry polysaccharides obtained by enzyme extraction exhibited higher cholesterol binding capacity (22.92±0.41mg/g) and bile acid-binding capacity (83.55%±0.16%). In conclusion, compared with the other three methods, polysaccharides extracted by enzyme extraction showed stronger biological activity in vitro. The results from this study might provide a scientific basis for the isolation and application of RAPs in the pharmaceutical and functional food industries.