Changes of Polysaccharides in Longan Pulp during Drying: Physicochemical Properties and Biological Activities

This work aimed to investigate the changes of polysaccharides in terms of physicochemical properties and biological activities during the drying of longan pulp. Polysaccharides from longan pulp at different drying stages were isolated. The proximate composition, Fourier transform infrared spectroscopic characteristics and molecular weight distribution of polysaccharides were analyzed, and their antioxidant, antitumor and immunostimulatory activities were evaluated in vitro. With the increase of drying time, the physicochemical characteristics of longan pulp polysaccharides (LP) showed significant changes. The mass ratio of neutral sugar to protein, the molar ratio of glucose to mannose and free amino group content were decreased, the typical amide bands in the infrared spectrum were diverse, and new fractions with relative high molecule weight were formed at the advanced stage of drying process according to the high-performance size-exclusion chromatogram. The evaluation of in vitro biological activities indicated that various activities of LP such as 1,1-diphenyl-2-picrylhydrazyl radical scavenging, hydroxyl radical scavenging, ferric reducing antioxidant power increased after 12-60 h drying of longan pulp, inhibitory effects against tumor cells (HepG2 and SGC7901) and immunostimulating effects on nitric oxide production and tumor necrosis factor-alpha secretion in macrophages increased after 12-24 h drying of longan pulp. But the activities decreased after more than 60 h drying. These changes in activities might be related to the Maillard reaction between polysaccharides and proteins in longan pulp during drying. The results can be useful for the processing of dried longan pulp with high quality. © 2017, China Food Publishing Company. All right reserved.