| Tea is a long-lasting beverage worldwide only after water.According to the manufacturing methods and flavor of products,the types of tea could be classified as green tea,white tea,yellow tea,oolong tea,black tea,and dark tea.The characteristic flavor and bioactivity of tea could be affected by many factors,including cultivar,growing conditions,processing methods,etc.In the present work,to compare the effect of processing methods on the flavor and biological activity of teas,fresh leaves of the single Camellia sinensis cultivar——Meizhan,was employed to make the six categories of teas.Their chemical composition,in vitro antioxidant activity,inhibition against?-glucosidase and three lipid metabolism enzymes of these teas were also investigated using the methods of metabolomics and in vitro activity evaluation,the relationships among them were analyzed further.This work aimed to provide a foundation for tea manufacturing and improve processing methods to strike a balance between flavor and bioactivity.The main research contents and achievements are as follows:1.Metabolomics was used to analyze and compare the main non-volatile compounds in teas.The test results showed that: As fresh leaves were processed into six categories of teas,the content of total polyphenols has changed significantly,which ranged from 133 mg/g to 238 mg/g,the content of total catechins has decreased in varying degrees while theaflavins has increased.Catechins were preserved as monomeric forms in green tea,and epigallocatechin gallate was the most abundant catechin in all the tested teas,especially in green tea and yellow tea.The content of epigallocatechin was second only to epigallocatechin gallate while it was not detected in black tea.Meanwhile,black tea had the highest content of theaflavins,white tea had the highest content of theaflavins in partially fermented teas(yellow tea,oolong tea,and white tea)and dark tea showed the lowest of all.There was no significant difference in the caffeine level of all tested teas.The content of amino acids in white tea was significantly higher than that of other teas.All six teas possess a low content of flavonols,while the content in white tea,oolong tea,and dark tea was slightly higher than that of other teas.Black tea has the highest total soluble sugar,followed by white tea.Its contents in oolong tea and dark tea was the lowest,and there was no significant difference in other teas.Oxalic acid,tartaric acid,malic acid,and acetic acid were the most abundant organic acids in teas,and significant differences were observed among the six teas,especially for acetic acid.A total of 9 variables,including the total content of organic acids,acetic acid,epigallocatechin,etc.,were screened out through orthogonal partial least squares discriminant analysis,and the changes of their content play a major role in the formation of differential tea taste.2.Combined with the calculation of taste activity value(TAV),the main compounds that contribute to the taste of different categories of teas were figured out,so as to illustrate the differences in taste formation of teas.The test results showed that:The TAV value of caffeine exhibited no significant difference in different teas,the concentration of bitter taste amino acid was below the threshold value,moreover,all teas had high TAV value of catechin components,so the differences in bitter taste may be attributed to the variation of catechin,and the contribution of bitter taste compounds to taste in fermentation or partially fermented was higher than fermented tea.Theaflavins with astringent taste were the main oxidation products of catechins and exhibited the highest TAV value in black tea.Thus,we can speculate that theaflavins may contribute much more to the taste of fermented black tea than other teas.Flavonols with soft and astringent taste possess high TAV values in all kinds of teas.Amino acids show a variety of taste properties,but only a small part of amino acids have TAV values greater than 1 in white tea and black tea,which contributes to the formation of fresh and sweet taste in white tea and black tea.Different processing methods regulate the formation of tea flavor by affecting the concentration of flavor compounds,which makes different tea varieties have distinct taste characteristics.3.GC-MS was employed to detect the volatile compounds in six categories of teas processed from fresh leaves of the single tea cultivar —— Camellia sinensis var.Meizhan in this study.Moreover,to compare the differences among the six teas on the aroma formation,the characteristic components and key variables of teas were screened by statistical analysis,and the contribution of each volatile compound to the aroma was evaluated by calculating the odor activity value(OAV)and aroma character impact value(ACI).The results showed that in addition to the characteristic components of each tea,a total of 10 compounds,including(E)-nerolidol,benzyl alcohol,1-pentadecanol,etc.,were screened out,and the changes of their content play a major role in the formation of differential tea aroma.Green tea and white tea possessed high proportions of compounds with fragrant properties in their aroma composition.Yellow tea had a relatively balanced proportion of green,floral,and fruity aroma compounds.Oolong tea had a high proportion of floral aroma compounds.Floral and fruity aroma compounds were dominant in black tea,and the proportion of fruity aroma compounds in dark tea was significantly higher than in other teas.Different processing methods could regulate the aroma formation of the six categories of teas by influencing the composition and content of volatile compounds.We have also confirmed the important contribution of the common components in the six teas to the aroma,and the composition ratios of dominant compounds could be deemed as the critical factor that influences the aroma type of each tea.4.The in vitro antioxidant activity,inhibition against ?-glucosidase and three lipid metabolism enzymes of these teas,and the relationships among them were also investigated.The results showed that all categories of teas exhibited strong antioxidant activity which was positively related to the contents of polyphenols and total catechins.The antioxidant capacity composite index from high to low was green tea,yellow tea,oolong tea,white tea,dark tea,and black tea.Furthermore,all categories of teas possessed an inhibition effect on the pancreatic lipase(PL),3-hydroxy-3-methylglutaryl coenzyme A reductase(HMG-COA reductase),lecithin cholesterol acyltransferase(LCAT),and ?-glucosidase.The inhibition rate of PL and ?-glucosidase appears to be positively influenced by theaflavins content(r =0.863,r =0.857,p < 0.05)while that of LCAT showed significant positive correlations with polyphenols content(r = 0.902,p <0.01).White tea demonstrated the strongest inhibition against the ?-glucosidase and green tea performed the highest inhibition rate on HMG-COA reductase. |
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