| Tea quality is significantly influenced by manufacturing process and tea varieties.The standard manufacturing process of green tea includes few basic steps like fixing,rolling and drying.Among these steps,the key step is the fixing process.During the fixing process,all kinds of oxidases and hydrolases in the fresh tea leaves are inactivated,and thus the catechins,caffeine and vitamins of the tea leaves are maintained.Meanwhile,many non-volatile compounds convert into volatile compounds,and some volatile compounds will also be converted and degraded.So,the fixing process plays an important role for the formation of the flavor of green tea.Aroma and taste is an important indicator of the quality of tea,and consumer’s preference is also dependent on this two aspects to evaluate the quality of tea.A variety of volatile compounds are the determinants of the aroma of tea.While the catechins and caffeine are the determinants of the taste of tea.Flavor determinants in tea infusions and their changes during manufacturing processes were investigated in this study using two Camellia sinensis cultivars:(i)‘Bai-Sang Cha’(‘BAS’),possessing significant floral scents and(ii)‘Fuding-Dabai Cha’(‘FUD’),with common green tea odor.Metabolite profiling revealed that ‘BAS’ contained higher levels of the active odorants b-ionone,linalool and its two oxides,geraniol,epoxylinalool and decanal than‘FUD’(P<0.05),including those that have non-OAV or OAV less than 1,such as trans-nerolidol,nerol,α-cubebene,copaene,methyl salicylate,cis-3-hexenyl butyrate,trans-2-hexenyl butaonate,cis-3-hexen-1-ol.Taste determinants catechins was also higher in ‘BAS’ than in ‘FUD’,but the content of caffeine had no significant difference between this two cultivars.Transcript level of the genes related to terpenoid biosynthesis showed that some upstream genes including HMGS,CMK,DXR and GPPS were highly expressed in ‘BAS’than in ‘FUD’(P<0.05).Moreover,transcript level of the genes encoding GES and NES were also higher in ‘BAS’ than in ‘FUD’(P<0.05),those are crucial for the biosynthesis of geraniol and nerolidol,respectively.Expression levels of the flavonoid biosynthesis genes F3 H,LAR,ANS and ANR were also higher in ‘BAS’ than in ‘FUD’,which were consistent with the results of the chemical analyses.Enhanced transcription of some terpenoid and catechin biosynthetic genes in ‘BAS’ suggested genetically enhanced production of those flavor compounds in this cultivar.Due to manufacturing processes,the levels of linalool,geraniol,trans-nerolidol,methylsalicylate and cis-3-hexenyl acetate decreased whereas those of linalool oxides,indole and geranyl acetone increased.In addition,b-ionone,cis-jasmone,methyl jasmonate and nonanal were only present in the processed tea samples.b-Ionone and cis-jasmone had the highest OAV and were the most odor-active compound due to their low odor threshold among the detected compounds with available threshold data.Compared with pan-fired tea,steamed tea had reduced proportion of linalool and its derivatives.However,the content of most volatile compounds such as epoxylinalol,α-cubebene,indole and cis-jasmone were higher in steaming tea than in pan-firing tea.Methyl jasmonate,nonanal and α-cadinol were detected in the steamed tea only,whereas hotrienol was detected in the pan-fired tea only.Steamed tea also possessed reduced levels of catechins compared with pan-fired tea,but the content of caffeine was not influenced.Consequently,the content of activeodorant(OAV>1)profiles,like linalool,geraniol and b-ionone related to floral scent of tea,reducing catechins related to astringency of tea.Our study suggests that flavor determinant targeted modulation could be made through genotype and manufacturing improvements. |
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