| Hubei Qingzhuan tea is one of the three major border-selling brick teas in China,which has unique flavor characteristics such as "aged fragrance".However,owing to the long processing cycle and the difficulty in controlling the fermentation conditions,the flavor of the product is often unstable.The composition of its aroma components,the aroma characteristics and formation mechanism of key aroma components are still unclear,which seriously affects the development of the Qingzhuan tea and industrial level.In the present study,we took Qingzhuan tea and its main processing samples as the research object,and systematically analyzed the flavor characteristics and chemical substance basis,the change rules of main taste and aroma chemical components,the characteristic components of aged fragrance and its aroma characteristics,and then explored the formation mechanism of key olefinic aldehydes.The main findings were listed as follows:1.Based on quantitative descriptive analysis,the sensory flavor characteristics of Qingzhuan tea were studied.After deleting of the synonymous,combining with M-value and statistic analysis method,17 aroma attributes and 4 taste attributes were ultimately obtained.The flavor wheel of Qingzhuan tea was drawn from the aspects of basic flavor,characteristic flavor,defect flavor,basic flavor,concentration and taste.2.The good flavor characteristics of Qingzhuan tea was aged fragrance and mellow taste.The content of tea polyphenols,amino acids,tea pigments and other conventional chemical components of Qingzhuan tea from different manufacturers was significantly different.Aldehydes and ketones were the principal aroma types.Correlation analysis showed that the levels of(E,E)-2,4-heptadienal,(Z)-linalool oxide,camphor,1-methylnaphthalene and longicyclene were significantly correlated with the aroma score obtained from the sensory evaluation(P<0.01).The main representative components wereβ-cyclocitral,β-ionone,hexanal,(E)-2-pentenal,heptanal,naphthalene,limonene and 6-methyl-5-heptene-2-one,suggesting they were the key aroma constituents,which could basically reflect the flavor characteristics of Qingzhuan tea.Sensory evaluation revealed that there were no significant differences in the sensory quality of Qingzhuan tea produced by four main cultivars(Chibi community cultivars,Echa No.1,Zhongcha 108 and Zhu yeqi)and they all had good suitability.Significantly differences were observed in the chemical compositions of Qingzhuan teas made from four different cultivars.The water extracts,flavonoids,tea polyphenols,amino acids,caffeine,total catechins,alcohol compounds,etc.in Qingzhuan tea made from the Zhongcha 108 were higher.The gallic acid and ketones in Qingzhuan tea made by the Chibi community cultivars were higher;the content of soluble sugar,theabrownins and alkenes in Qingzhuan tea made by the Zhu yeqi variety was higher;while the content of theaflavins and aldehydes in Qingzhuan tea made by the Echa No.1 was higher.3.During the processing of Qingzhuan tea,the evolution trend of the taste characteristics was green and astringent,mellow and slightly astringent,mellow and smooth.Water extracts,tea polyphenols,amino acids,soluble sugars,flavonoids,etc.all showed a downward trend,while the content of caffeine was basically stable.Thearubigins increased and then decreased gradually after the first turn over,while theabrownins showed a steady increase.The gallic acid showed a trend of first increase and then decrease,and the turning point was the second turn over process.Most of the catechins and amino acids showed a downward trend,and the contents of valine and leucine increased sharply after first turn over,and then held relatively steady in the subsequent processing.OPLS-DA analysis screened out differential metabolites,and the metabolic pathways involved were mainly flavonoids,amino acids,organic acids and nucleotides,and flavonoids were the main metabolic pathways.Compounds such as caffeine,kaempferol-3-glucuronic acid,EGCG,EGC,pheophorbide A,kaempferol-3-rhamnoside,and valine were the main differential metabolites that distinguish finished Qingzhuan tea from raw tea.4.The evolution trend of aroma characteristics during the processing of Qingzhuan tea was chestnut with sun-drying flavor,pile smell,and aged fragrance.A total of 108 volatile flavor substances were identified,of which aldehydes accounted for the highest proportion.The content of aldehydes decreased significantly during the pile fermentation process,and increased significantly during the aging process.The content of ketones was relatively stable during processing,while the content of alcohols and esters gradually decreased.The concentration of methoxybenzene compounds increased significantly during aging and decreased significantly during steaming and pressing.OPLS-DA analysis screened out 34 differential aroma compounds,of which 19 volatiles were significantly up-regulated,including 1,2,3,4-tetrahydro-1,1,6-trimethylnaphthalene,1,2-dimethoxy benzene,cis-2-heptenal,etc.The other 15 volatiles were significantly downregulated,including linoleic acid,propionaldehyde,valeraldehyde,α-farnesene,etc.5.Thirty-three key aroma compounds were identified by GC-MS/O intensity method,and 10 substances had high aroma intensity(AI ≥ 2),such as β-ionone,α-ionone,trans,trans-2,4-nonadienal,trans,trans-2,4-heptadienal,etc.were the main aroma active compounds.Twenty-five key aroma compounds were screened by OAV value,among which β-ionone,trans,cis-2,6-nonadienal and 1-octen-3-one were important contributors to the characteristic aroma of Qingzhuan tea.The recombination model formed by substances with OAV ≥ 1 was close to the actual sample flavor.Omission tests have demonstrated that 10 components including trans,trans-2,4-decadienal,trans,trans-2,4-nonadienal,trans,trans-2,4-heptadienal,β-ionone and α-ionone were the key odor active compounds of Qingzhuan tea.Metabolic pathway results found that the biosynthesis of unsaturated fatty acids,α-linolenic acid,linoleic acid and sulfur-containing compounds,and pyruvic acid metabolism were related to the aromatic active components formed during the processing of Qingzhuan tea.6.The fatty acids of Qingzhuan tea increased first and then decreased during processing.The conversion of fatty acids from aging to finished tea was mainly degradation,while α-linolenic acid and cis-9,12-linoleic acid decreased significantly.PLS-DA analysis screened out 8 differential fatty acids,including α-linolenic acid,palmitic acid,cis-9,12-linoleic acid,etc.The total amount of oxidized lipids showed an overall increasing trend during processing,especially during the period from the end of pile fermentation to 6 months of aging,which increased by 3.8 times.ALA,LA and EPA lipids all increased significantly in the early stage of aging,and the increase trend was similar during processing.The content of ARA increased significantly during the pile fermentation,and did not change much during the aging period.PLS-DA analysis screened out 9 differentially oxidized lipids,including 13S-HOTr E,9S-HOTr E,±9(10)-Di HOME,etc.The results of the correlation between lipids and aroma components showed that the aroma components with positive correlation were mainly cis-4-heptenal,trans,cis-2,6-nonadienal,3-octen-2-one and so on.The results of pathway enrichment analysis of key fatty acids showed that the metabolic pathways involved mainly include unsaturated fatty acid biosynthesis,fatty acid biosynthesis,cutin,suberin and wax biosynthesis,linoleic acid metabolism,fatty acid elongation,α-linolenic acid metabolism and fatty acid degradation.The simulation test results of the target lipid precursors showed that some aldehydes and ketones could be generated by α-linolenic acid and linoleic acid lipids as precursors through enzymatic or non-enzymatic degradation reactions. |
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