Extraction,Isolation,Structural Characterization,Chemical Modification,and Biological Activities Of Qingke (Hordeum Vulgare L.)β-Glucans

As the staple food of the Tibetans and the important livestock feed of the Qinghai-Tibet Plateau,Qingke has high economic and nutritional value.Studies have shown thatβ-glucan is the main bioactive ingredient in Qingke,and appropriate intake ofβ-glucans can effectively reduce the risk of diabetes,hypertension,and hyperlipidemia.In order to further promote the applications of Qingkeβ-glucans in the food/functional food industry and to provide a scientific theoretical basis for the development of functional foods and health products based on Qingkeβ-glucans,the physicochemical properties and biological activities of Qingkeβ-glucans extracted by different extraction methods were determined and compared.Subsequently,the Qingkeβ-glucans were purified,and their structure-function relationships were discussed.Finally,effects of chemical modifications(sulfation,carboxymethylation and phosphorylation)on the physicochemical properties and biological activities of Qingkeβ-glucans were further discussed.The main results of this study were as follows:1.Effects of different extraction methods on the physicochemical properties and biological activities ofβ-glucans extracted from QingkeFour extraction methods,including ultrasonic-assisted extraction,traditional hot water extraction,microwave-enzyme-assisted extraction,and pressurized hot water extraction,were used to extract Qingkeβ-glucans,and their physicochemical properties,in vitro antioxidant activities,and in vitro hypolipidemic activities were measured and compared.Results showed that the extraction yields of Qingkeβ-glucans ranged from 2.93%to3.62%,and the pressurized hot water extraction method(3.56%)and microwave-enzyme-assisted extraction method(3.62%)had higher extraction yileds than that of other methods.Different extraction methods significantly affected the apparent viscosity,molecular weights,and molar ratios of constituent monosaccharides of Qingkeβ-glucans.Besides,Qingkeβ-glucans extracted by pressurized hot water extraction had a higher molecular weight(1.86×10~5 Da),higher apparent viscosity,and higher purity(90.61%)than those of other methods.Compared with other extraction methods,Qingkeβ-glucans extracted by pressurized hot water extraction exhibited higher in vitro antioxidant activities,in vitro binding capacities,and in vitro inhibitory effects on the pancreatic lipase,which may be due to their higher molecular weight and apparent viscosity.Results showed that the pressurized hot water extraction method is a simple and efficient extraction method,which is expected to be used for further extraction of Qingkeβ-glucans with relatively high bioactivities for industrial applications.2.Purification,structure characterization and structure-function relationships of Qingkeβ-glucansIn this section,crudeβ-glucans were obtained by pressurized hot water extraction,and the impurities(such as starch and protein)were removed by addingα-amylase and pancreatin.Furthermore,β-glucans were further purified by fractionated alcohol precipitation and centrifugal ultrafiltration.Subsequently,the structural characteristics,biological activities,and the effects of molecular weight and viscosity on the biological activities of purified Qingkeβ-glucans were studied.Results showed that the purity,the molecular weight,and the intrinsic viscosity of purifiedβ-glucans were 93.8%,1.962×10~5Da,and 1.86 d L/g,respectively.Two kinds of low molecular weight Qingkeβ-glucans were prepared by partial acid hydrolysis.Their molecular weights and intrinsic viscosities were determined to be 0.638×10~5 Da and 0.358×10~5 Da,and 0.98 d L/g and 0.58 d L/g,respectively.Furthermore,results showed that the antioxidant activities,in vitro binding capacities,and in vitro inhibitory effects on the pancreatic lipase ofβ-glucans were positively correlated to their molecular weights and intrinsic viscositys.Results showed that Qingkeβ-glucan with high molecular weight could be further developed and used as functional ingredients,and the rapid and efficient purification method established in this study was expected to be used for industrial large-scale extraction and preparation of Qingkeβ-glucans.3.Effects of chemical modifications on the physicochemical properties and biological activities of Qingkeβ-glucansIn this section,the structure-activity relationships of Qingkeβ-glucans were further investigated according to the introduction of bioactive groups by chemical modifications.Effects of three chemical modifications(sulfation,carboxymethylation,and phosphorylation)with different degrees of substitution(high,medium,and low)on the physicochemical properties,in vitro antioxidant activities,and in vitro hypolipidemic activities of Qingkeβ-glucans were compared.Results showed that all chemical modifications had no effects on the types of constituent monosaccharides of Qingkeβ-glucans,but changed their molar ratios.All chemical modifications significantly increased the solubility of Qingkeβ-glucans and significantly reduced their apparent viscosity and molecular weight.The solubility increased with the increasing degree of substitution,while molecular weight and apparent viscosity decreased with the increasing degree of substitution.Furthermore,results showed that both sulfated and carboxymethylated modifications could significantly improve the antioxidant activities,in vitro binding capacities,in vitro inhibitory effects on the pancreatic lipase of Qingkeβ-glucans,and the biological activities increased with the increasing degree of substitution.However,the high degree of phosphorylated substitution significantly reduced the in vitro antioxidant activities and hypolipidemic activities of Qingkeβ-glucans,which might be attributed to the destroy of key active structures of Qingkeβ-glucans under the severe reaction conditions.In particular,sulfated modification of Qingkeβ-glucans with a high degree of substitution exhbited the highest in vitro antioxidant activities and in vitro hypolipidemic activities.Results showed that all chemical modifications significantly affected the physicochemical properties and biological activities of Qingkeβ-glucans.By choosing the appropriate modification method and the appropriate degree of substitution could significantly improve the in vitro antioxidant activities and in vitro hypolipidemic activities of Qingkeβ-glucans.In summary,this article clarifies the best extraction method of Qingkeβ-glucans and establishes a fast and efficient purification method,which will help to extract and prepare Qingkeβ-glucans on a large scale in the industry.In addition,this article explores the structure-activities relationship of Qingkeβ-glucans,especially the relationship between molecular weight and chemical modification and in vitro antioxidant and hypolipidemic activities of Qingkeβ-glucans;This will help the research and development of functional foods and health products related to Qingkeβ-glucans.