Effect Of Aspergillus Niger On Southern-route Tea Quality During Fermentation

Southern-route tea,produced in Ya’an district of Sichuan province,is a daily necessity for ethnic minority compatriots in Tibetan areas.The post-fermentation process with the participation of microorganisms significantly affects the formation of Southern-route tea quality.Aspergillus niger is the dominant strain in the piled dark tea during post-fermentation,but whose role in the formation of characteristic flavor of Southern-route tea is not clear.In this study,Aspergillus niger was inoculated into the raw material of Southern-route tea to simulate post-fermentation,and the changes of main quality components,enzyme activities,metabolite composition and sensory quality of dark tea samples were comprehensively analyzed.Thus,the effect of Aspergillus niger on the quality formation of Southern-route tea in the process of fermentation would be clarified.The specific results were as follows:1.The single factor experiment was employed to investigate the effects of different Aspergillus niger suspension concentration,initial water content of tea,fermentation temperature and fermentation time on the biochemical composition and sensory quality of Southern-route tea.The results showed that,when the concentration of bacterial suspension was 10~3～10~7cfu/ml,with the increase of bacterial suspension concentration,the content of tea polyphenols,amino acids,soluble sugar,theaflavins,thearubigins,and theabrownins fluctuated,while catechin levels directly decreased.When the concentration of bacterial suspension was 10~5cfu/ml,the fermented tea quality was the best.When the initial water content of tea was 15%～35%,with the increase of which,tea polyphenols levels directly decreased,the contents of catechin,soluble sugar,and thearubigins decreased at first,then increased,while the change trend of theabrownins content was the opposite.When the initial water content of tea was 25%,the fermented tea quality was the best.Additionally,when the fermentation temperature was 35℃～55℃,The content of main biochemical components of tea fluctuated with the increase of temperature,and the fermented tea quality was the best when the temperature was 55℃.Moreover,when the fermentation time was 0～15 d,with the extension of time,the contents of tea polyphenols,catechins,caffeine,amino acid,soluble sugar,chlorophyll and tea pigments fluctuated and decreased.The best tea quality was obtained when fermented time was 15d.2.The fermentation condition was optimized utilizing four-factor quadratic regression orthogonal rotation combination design experiment.The results showed that,within the scope of the study,the effects of various factors on the sensory quality of tea were as follows:fermentation temperature>bacterial suspension concentration>fermentation time>initial water content of tea.The optimized fermentation condition was as follows:Aspergillus niger suspension concentration was 10~5cfu/ml,fermentation temperature was 60℃,initial water content of tea was25%,fermentation time was 20d.Southern-route tea processed under such condition exhibited brown dried tea color,pure aroma with a litter sweet,bright orange liquor color,and mellow and sweet taste.Moreover,the contents of tea polyphenols,amino acids,caffeine,soluble sugar,chlorophyll,catechins,theaflavins,thearubigins and theabrownins in the fermentation tea were 2.15%,0.29%,0.95%,5.49%,0.80mg/g,3.99mg/g,0.03%,0.69%and 3.56%.3.The raw materials of Southern-route tea were sterilized,inoculated with Aspergillus niger,and then fermented under the optimal conditions.The aseptic samples and naturally treated samples were set as control simultaneously.The possible effect of Aspergillus niger on tea quality was explored.The results showed that,compared with the other treated samples,the samples fermented by Aspergillus niger increased the richness of aroma and the material sense of taste.Additionally,compared to the aseptic samples,the contents of tea polyphenols,catechins,caffeine and chlorophyll of samples fermented with Aspergillus niger decreased by 8.69%,18.19%,14.80%and 9.19%respectively after fermentation,while the contents of amino acids,soluble sugar,theaflavins,thearubigins and theabrownins increased by33.39%,98.43%,62.26%,8.60%and 2.30%respectively,and the activities of cellulase,pectinase,polyphenol oxidase andβ-glucoglycerase increased by 9.21%,9.50%,15.91%and 10.25%,respectively.Furthermore,the values of L,a and b of drie tea color of Aspergillus niger treated samples decreased by 13.63%,28.57%and30.00%respectively,a and b values of tea liquor increased by 1140.00%and 20.49%respectively,b value of infused tea decreased by 125.00%,but a value increased by100.00%.Totally 655 metabolites including amino acids and their derivatives,phenolic acids,nucleotides and their derivatives,flavonoids,lignin and coumarins etc.were detected in fermented tea samples.Compared with the aseptic samples,the relative contents of phenolic acids,nucleotides and their derivatives,flavonoids,alkaloids and organic acid metabolites in Aspergillus niger fermented tea decreased by8.17%,7.92%,26.16%,17.74%and 25.79%respectively,while the relative contents of amino acids and their derivatives,lignans,coumarins,tannins,terpenes,steroids and lipids increased by 4.18%,28.73%,7.41%,53.26%,0.54%and 56.95%respectively,apart from that,the content of soluble sugar,amino acid,thearubigins and theabrownins in Aspergillus niger fermented tea were 138.34%,13.31%,94.33%and 14.26%higher than those in natural tea samples.the tea liquor extracted from Southern-route tea raw material was inoculated by Aspergillus niger(10~5cfu/ml×3ml),and fermented at 60℃for 20d,the speed of shaking table was set as 120 r/min.After fermentation,the contents of tea polyphenols,amino acids,soluble sugar and caffeine in tea soup fermented by Aspergillus niger were 5.41%,12.74%,34.11%and4.07%higher than those in tea soup of aseptic treatment respectively.To sum up,Aspergillus niger can change the composition of tea metabolites,affect the content of tea quality components,improve the aroma richness and substance sense and sweetness of taste of Southern-route tea during the fermentation.