Invesigation On The Structural And Bioactive Changes Of Longan Pulp Polysaccharides By Different Maillard Reaction Systems

Longan is a dual-use fruit for both medicine and food.It has not only desirable flavor and abundant nutrients,but also a variety of physiological functions.Polysaccharide is the main active ingredient in longan pulp.Fresh longan fruit is prone to browning and decay during storage and transportation.Drying is the most common method for the industry to process fresh longan fruits.But it is not clear whether the structure and activity of polysaccharide from longan pulp has changes during heat processing.Maillard reaction is common in food processing.In recent years,there are few reports on the modification of structure and the improvement of function of polysaccharides by Maillard reaction.Early studies suggest that the polysaccharide of dried longan pulp contains more binding proteins,and its activity increased,which may be related to the Maillard reaction between polysaccharides and proteins/ amino acids.In order to further reveal the relationship between the structural and bioactive changes of the polysaccharides from longan pulp in the traditional drying process,the structural and bioactive changes of longan pulp polysaccharides by the different Maillard reaction systems was investigated.The main research contents and results are as follows:(1)Changes of polysaccharides properties and activity in longan pulp during drying.Polysaccharides from longan pulp at different drying stages were isolated.Their basic compositions,infrared spectrum characteristics and molecular weight distributions were analyzed,and their antioxidant,antitumor and immunostimulatory activitieswere then evaluated in vitro.With the increase of drying time,the physicochemical characteristics of longan pulp polysaccharides(LP)showed significant changes mainly including: mass ratio of total sugar to protein,molar ratio of glucose to mannose in polysaccharide and free amino group content in protein were all decreasing,typical amide bands in FTIR spectrum were diverse,new fractions with relative high molecule weight formed at the advanced stage of drying according to the HPSEC chromatogram.The evaluations of in vitro activities indicated that various activities of LP,involving in DPPH radical scavenging,hydroxyl radical scavenging,FRAP total antioxidant capacity,inhibitory effects against tumor cells(HepG2 and SGC7901)and immunostimulating effects on the NO production and TNF-? secretion of macrophages,would increase after 12–24 h drying of longan pulp.But the activities would decrease after more than 60 h drying.The above changes in physicochemical characteristics and activities might be related to the Maillardreaction between polysaccharide and protein in longan pulp during drying.(2)Investigation of the Maillard reaction between polysaccharides and proteins from longan pulp and the improvement in activities.The polysaccharide-protein mixtures of fresh longan pulp(LPPMs)were co-prepared by an alkali extraction–acid precipitation method.They were then dry-heated under controlled conditions for monitoring the characterization of the Maillard reaction by the measurement of the free amino group content,ultraviolet-visible spectrum,Fourier transform infrared spectrum and molecular weight distribution.All the physicochemical analyses indicated the development of the Maillard reaction between polysaccharides and proteins.The in vitro activity evaluation indicated that the Maillard reaction could effectively enhance the antioxidant,antitumor and immunostimulating activities of LPPMs.The enhancement of DPPH radical scavenging activity and ferric reducing antioxidant power displayed both a positive correlation with the reaction time(p<0.05).LPPMs dry-heated for three days obtained relatively strong inhibitory activity against HepG2 cells and SGC7901 cells,as well as strong immunostimulating effects on NO production and TNF-? secretion of macrophages.The characteristic structure of LPPMs in dry heat system changes with the change of maillard reaction time,and then affects its activity.(3)Structural characteristics and activity evaluation of Maillard reaction products of longan pulp polysaccharides fraction(LPI)and lysine.An aqueous extract of polysaccharides from longan pulp was chromatographed on a Sephacryl S-100 column to yield a fraction(LPI).The characterization of the Maillard reaction of LPI and the different amino acids was investaged by the measurement of the free amino group content,browning degree and molecular weight distribution.The Maillard reaction products of LPI and lysine(LPI-Lys)were prepared under optimal conditions.The structural features of LPI and LPI-Lys was analyzed by UV/FTIR,HPLC,HPSEC and other methods.The results showed that the content of polysaccharides,proteins and free amino acids in LPI-Lys were all lower than LPI.The mole ratio of monosaccharide in LPI and LPI-Lys was different significantly.Compared with LPI,there were more amide groups,methyl and methylene substituted groups in the FTIR of LPI-Lys,some characteristic absorption peaks of polysaccharide was weaken or disappear.The weight average molecular weight of LPI-Lys was obviously greater than LPI.Both LPI and LPI-Lys contained 1?6 glycosides,and there were more 1?6 glycosides and branches in LPI-Lys.The in vitro activity measurement indicated that LPI and LPI-Lys could effectively scavenge DPPH and hydroxyl radicals in theexperimental concentration range,and the enhancement of DPPH and hydroxyl radicals scavenging activity displayed a positive correlation with the sample concentration.At the same concentration,The scavenging rate of LPI-Lys for DPPH and hydroxyl radicals was obviously higher than LPI.LPI and LPI-Lys had inhibitory activity against HepG2 and SGC7901 cells cultured in vitro.The inhibitory activity of LPI displayed a positive correlation with the sample concentration(p<0.05).At the concentration of 400 ?g/mL,the inhibition rate against HepG2 cells of LPI-Lys increased significantly(p<0.05).At the concentration of 100 ?g/ mL,the inhibition rate against SGC7901 cells of LPI-Lys increased significantly(p<0.05).However,the inhibitory activity against the two cells of LPI-Lys was weaker than LPI.Both LPI and LPI-Lys had strong immunostimulating effects on the NO production and TNF-?secretion of macrophages(p<0.05).At the same concentration,the immunostimulating activity of LPI-Lys was higher than LPI.Based on the different reaction systems and reaction substrates,the changes of the physical and chemical properties and biological activity of longan pulp polysaccharide during the process of Maillard reaction were studied in this work,the potential relationship of the structure and activity of polysaccharide was analysised preliminarily.Maillard-type intermacromolecular interaction is suggested to be an effective and controllable method for improving the functional activities of polysaccharides and proteins from longan pulp.The results of this research provide new ideas for the regulation of the functional quality of dried longan pulp.