Optimization Of Polysaccharide Extraction From Kiwifruit ‘hongshi No.2’ And Study Of Its Structural Characteristics And In Vitro Bioactivity During Post-ripening

Kiwifruits(Actinidia chinensis Planch)are rich in a variety of natural active ingredients with high nutritional and medicinal value.Polysaccharides are one of the main active ingredients in red-fleshed kiwifruit,with potential for exploitation as a natural health food.The main reason for the rapid softening of kiwifruit during storage is the change in the structure of the polysaccharides in the cell wall,which in turn leads to a change in their bioactivity.This study was carried out to investigate the softening mechanism of kiwifruit and to provide a reference for the development of kiwifruit polysaccharide-related health food.In this study,we firstly optimized and purified the hot water extraction method of kiwifruit polysaccharides to investigate the changes in structural characteristics and in vitro bioactivity of kiwifruit polysaccharides during the ripening process.The main findings were as follows:(1)Process optimization and purification of kiwi polysaccharides extracted by hot waterIn order to obtain the optimal process conditions for the extraction of kiwifruit polysaccharides by hot water,a single-factor and response surface experimental design were used for optimization.The optimal extraction conditions were: material-to-liquid ratio of18.00 m L/g,extraction time of 2.10 h,and extraction temperature of 41.00°C.The actual extraction rate of kiwifruit polysaccharides could reach 3.20% under these conditions.The polysaccharides from the five softening periods were separated and purified using 10 k Da and 100 k Da ultrafiltration membranes,respectively.The kiwi polysaccharide fractions with higher purity and better activity in each softening period were screened by analyzing the basic polysaccharide composition and in vitro antioxidant activity: KPS-A2,KPS-B2,KPSC2,KPS-D2,KPS-E2,all polysaccharide fractions were between 10 k Da and 100 k Da.The results of the study can promote the high value utilization of kiwifruit resources.(2)Changes of structural characteristics of polysaccharides in kiwi after ripeningBy analyzing the primary structure(monosaccharide composition,molecular weight,type of glycosidic bond)and higher structure(Congo red experiment and atomic force microscope)of kiwi polysaccharide,the changes of structural characteristics of kiwi polysaccharide during post-ripening were discussed.The results showed that the kiwi polysaccharide was composed of 9 monosaccharides.During storage,the contents of glucose and xylose decreased continuously,the contents of galactose,galacturonic acid and arabose increased continuously.The RG-Ⅰ domain is the main structure of the five kiwifruit polysaccharides,and the glucan content decreases during ripening,the xylose side chains in the HG and RG-I structures gradually decrease,and the galactose and arabinose side chains gradually increase.The molecular weight decreased,and the polydispersity increased slightly.At the storage A,KPS-A2 contains low ester pectins and is pyranose;subsequently,KPS-B2,KPS-C2 and KPS-D2 contain more β-pyranose and less α-pyranose and are high ester pectins;finally,KPS-E2 contains a small amount of α-pyranose and is low ester pectins.Methylation analysis showed that the main glycosidic bond types of kiwifruit polysaccharides changed during ripening.In the process of post-ripening softening,the polysaccharide of kiwi did not possess the three-strand helix structure.There was no single chain structure of polysaccharide in the five polysaccharides,and they were all the polymers with multiple sugar chains winding and crosslinking.The maximum vertical height of polysaccharide molecules first increased and then decreased,and gradually aggregated from cross-network structure(KPS-A2)to form slightly larger short chains and columnar polymers(KPS-B2),and then continued to aggregate to form long chains and larger columnar polymers(KPS-C2).The linear cluster columnar polymers(KPS-D2)were formed,and finally the columnar polymers(KPS-E2)with small distribution of fine fragments were formed.In conclusion,the structure of polysaccharides in kiwi fruit changes during postripening.The results of this study help to better understand the structural characteristics of polysaccharides in different post-ripening periods,and provide a theoretical basis for the development of functional food and health food.(3)Changes of bioactivity of polysaccharides in vitro during post-ripening of kiwi fruitThe antioxidant activities(ABTS,NO,hydroxyl radical scavenging activities),hypoglycemic activities(α-amylase inhibitory activities,α-glucosidase inhibitory activities)and lipid-lowering activities(cholesterol binding amount and cholate binding rate)of kiwi polysaccharide in vitro were evaluated to explore the changes in its post-ripening process.The results were as follows: The antioxidant activity of kiwi polysaccharide in vitro was dose-dependent,which increased first and then decreased during post-ripening,and reached the peak value at KPS-C2.The in vitro hypoglycemic activity of kiwifruit polysaccharides is concentration-dependent,and it increases first and then decreases during the whole storage process.The inhibitory activity of α-amylase of KPS-C2 is the highest,and the inhibitory activity of α-glucosidase of KPS-B2 and KPS-C2 is similar,which is higher than other postripening periods.The in vitro lipid lowering ability of kiwifruit polysaccharide showed a gradual and slow decline,which may be related to the molecular weight of polysaccharide.In summary,KPS-C2 showed superior biological activity in vitro,and the kiwi was half soft and half ripe at this time.The results of this study elucidates the changes of biological activity of polysaccharides in vitro during the post-rippling process of kiwifruit,which may have a certain relationship with its primary structure and higher structure,and provide theoretical basis for the development of functional food and health food.