Effect of various drying methods on the physicochemical characterizations, antioxidant activities and hypoglycemic activities of lychee (Litchi chinensis Sonn.) pulp polysaccharides

Suitable drying method is critical for the preservation of physicochemical and pharmacological quality of lychee pulp polysaccharides (LPPs). In current work, the effects of five drying methods, i.e. air drying (A), infrared drying (I), heat pump drying (H), vacuum freeze drying (F) and freeze vacuum drying combined with heat pump drying (FH) on the physicochemical characterizations, antioxidant activities and hypoglycemic activities of LPPs were explored. Results showed all five drying methods led to thermal aggregation of LPPs and the stronger the thermal effect induced by drying, the more serious the aggregations were. Additionally, the thermal aggregation significantly affected the composition, structure and biological activity of LPPs. Less thermal aggregation was observed in LPPF and LPPFH, which exhibited stronger oxygen, DPPH and ABTS radical scavenging activities, higher ferric-reducing power and better α-glycosidase and α-amylase inhibition activities, resulting from their higher contents of neutral sugar, protein and uronic acid and lower molecular weight than LPPA and LPPI. Besides, FH consumed about half drying time and one fifth energy of F. Therefore, from industrial perspective, FH is a promising alternative to F for producing LPPs by comprehensively considering physicochemical characterizations, bioactivity as well as energy consumption.