Study On The Quality Of Laoqing Tea Fermented By Yeasts

Qingzhuan tea(QZT)is a kind of tightly pressed dark tea,which is a post-fermented tea,and the fermentation process is the key to its processing.Microorganisms are involved in this process by secreting extracellular enzymes,releasing microbial heat,and their metabolism in the transformation of tea components,which are strongly associated with the formation of the special flavor of fermented tea.Yeast has a rich enzyme system and produces a wide range of nutrients to improve fermented foods’ flavor.Using yeast to ferment Laoqing tea(LQT)improves its quality and flavor.It also increases the content and variety of bioactive compounds in the tea.In this study,LQT was inoculated with yeast for solid state fermentation.With the aim of improving the quality of fermented tea,suitable strains were screened and fermentation conditions optimized.Subsequently,the transformative effect of yeast fermentation on the quality components of tea leaves was studied.The main findings are as follows:(1)Eight strains of yeast from fermented food and QZT were inoculated into LQT for fermentation.The chemical composition,antioxidant capacity and volatile components of the fermented tea were determined and combined with sensory evaluation to screen the superior strains.The results showed that the brewer’s yeast fermented tea had the best sensory qualities.It had an ester-flavor,mellow and slightly sweet taste.Compared to the control samples,water extract,soluble sugars and free amino acids were significantly lower(p<0.05).Theaflavins and thearubigins were slightly higher,whereas theabrownines changed slightly.After yeast fermentation,the antioxidant capacity of fermented tea decreased.Ketones,alcohols,aldehydes and esters were the main types of volatile compounds.Among the 43 differential volatile compounds screened,those with higher contents were geranyl acetone,α-ionone,phenylethyl alcohol,sulcatone,geranyl formate,γ-nonanolactone and 4-(2,6,6-trimethyl-1,3-cyclohexadien-1-yl)-3-buten-2-one,presumably characteristic of yeast fermented tea.The phenylethyl alcohol content was significantly higher than the control sample,which had a pleasant aroma.The brewer’s yeast fermented tea had a higher content of differential volatile compounds with floral and fruity,grassy and creamy aromas,while the components with unpleasant aromas were significantly lower than the control sample.In conclusion,the brewer’s yeast fermented tea had the best quality,so brewer’s yeast was selected as the better strain for fermenting LQT.(2)Through single-factor and orthogonal tests,the fermentation conditions of the brewer’s yeast fermented tea were optimized to 10% inoculum,45% water content of tea leaves,28 ℃ fermentation temperature and 7 d fermentation time,and the tea samples under these fermentation conditions had better sensory quality than others.Fermentation conditions influenced sensory evaluation in the following order: inoculum level >fermentation temperature > water content > fermentation time.Compared to other fermentation conditions,the sensory quality of the brewer’s yeast fermented tea is significantly more excellent under the better fermentation conditions,showing a strong ester-flavor with wine aroma and sweet taste.There was also a decrease in water extract,free amino acids,soluble sugars,theaflavins,thearubigins and some catechins and a slight increase in theabrownins.Therefore,brewer’s yeast fermentation has a greater potential to improve the quality of LQT.(3)Brewer’s yeast fermentation of LQT was carried out under optimum fermentation conditions,with a fermentation period of 21 d.Samples were taken every 2 d.During the fermentation of brewer’s yeast tea,the sensory quality increased and then slightly decreased.The ester-flavor gradually stabilized in the middle stages of fermentation,with better sensory quality at the 13 th and 19 th of fermentation.During fermentation,the contents of water extract,soluble sugars and free amino acids in brewer’s yeast fermented tea decreased significantly in the pre-mid fermentation and stabilized later,while tea polyphenols increased and then decreased.CAF and ester-type catechins were relatively stable,and GA and tea pigments showed volatility.The main volatile components of brewer’s yeast fermented tea were ketones,hydrocarbons,alcohols and esters.The STEM trend analysis revealed 2 significant patterns of change in the volatile components of brewer’s yeast fermented tea,a linear decline and a fluctuating decline.Among the 42 differential volatile compounds screened,most of the volatile compounds with spicy,camphoraceous,grassy,fatty and waxy odors were significantly reduced by fermentation,producing globulol,cascarilladiene,2,5-di-tert-butylphenol,3,5,5-trimethyl-2-hexene,cis-α,α-5-trimethyl-5-vinyltetrahydrofuran-2-methanol,phenethyl alcohol and γ-nonanolactone with rose,sweet and creamy aromas.Quasi-targeted metabolomics results showed that brewer’s yeast fermentation had a significant effect on the non-volatile compounds.Flavonoids,amino acids and their derivatives,organic acids and their derivatives and sugars and their derivatives were the main metabolites of the fermentation samples.And they were also the main differential metabolites with significant changes in the pre-mid fermentation period.Using STEM trend analysis,the non-volatile components of brewer’s yeast fermented tea showed four significant patterns of change: decreasing,increasing,decreasing followed by rebound,and increasing in medium-term.Flavonoids,amino acids and their derivatives,sugars and their derivatives were mainly down-regulated during fermentation.Organic acids and their derivatives were up-regulated.It is clear that,especially in the pre-mid fermentation,brewer’s yeast has a significant effect on the quality of LQT.