Effects Of Exogenous Amino Acids On The Fermentation Process And Quality Of Pu-erh Tea

Effects of exogenous amino acids on the variation of pile temperature, microorganismnumber, and the activity of polyphenol oxidase have been explored. The changes of importantflavour components such as polyphenols, free amino acids, water-soluble sugars, caffeine, andthe water extracts during the fermentation process have been dynamic measured and analyzed.Simultaneous distillation and extraction method (SDE) and gas chromatography-massspectrometry (GC-MS) were used to determine the volatile aroma components. The aim ofthis study was to improve the microbial growth environment, shorten the fermentation timeand improve tea quality, as well as to provide fundamental basis for high quality of Pu-erh tea.In the early stage of the fermentation, exogenous amino acids could significantlypromote the pile temperature, and the highest temperature would reach48-55℃. The additionof Alanine, Leucine, and Phenylalanine at0.4%-0.6%could not only maintain thepile-temperature at higher level, but also could promote the growth and reproduction of mouldand yeast. Results showed that during the fermentation of Pu-erh tea, the activity ofpolyphenol oxidase presented downward trends, and was significantly positive correlated withthe number of mould（P<0.01）with a correlation coefficient of0.531.Exogenous amino acids could accelerate the oxidation and transformation of teapolyphenol and catechins, and the accumulation of theabrownins. The content oftheabrownins reached a maximum at the third turning (18thday of the fermentation) with avalue between6.315%-8.108%, which was the optimal standard content of the best liquorcolor. During the fermentation, theabrownins was significantly negative correlated with teapolyphenols, tea catechins and thearubigins, and the correlation coefficients were-0.658,-0.921and-0.670respectively. Teas fermented with addition of Alanine, Leucine andPhenylalanine at0.6%had higher content of tea polyphenol and tea catechin, and was morefavorable to the accumulation of theabrownins.The teas fermented with addition of Alanine, Leucine and Phenylalanine at0.4%or0.6%had the higher content of water extracts (36.69%-38.81%), free amino acids (3.30%-3.33%),water-soluble sugars (3.64%-3.80%) and caffeine (5.19%-5.49%) by the end of thefermentation, indicating that the exogenous amino acids had little effect on the content of caffeine, but it could decrease the loss of water extract and maintain the content of free aminoacids, which was favorable to the high quality of puer teas with fresh and pure taste and fill-bodied fragrance.The main volatile aroma components of teas fermented with addition of different aminoacids at0.6%were: Linalool (4.36%-5.89%),5-methylfurfural (3.28%-5.18%),5-methyl-2-Isopropyl-Cyclohexenone (3.13%-5.77%), et al. The main volatile aromacomponents of those at0.4%were: β-Elemene (5.55%-6.34%),6,10,14-trimethyl-2-Pentadecanone (3.50%-4.46%), Hexaldehyde (3.33%-3.63%), et al. Andthose fermented with addition of compounded amino acids were:6,10,14-trimethyl-2-Pentadecanone (6.46%-8.19%), Hexaldehyde (5.33%-6.61%),1,2,3-Trimethoxybenzene (3.29%-3.92%), et al, indicating that the flavor of teas fermenteddeponded on the sort and concentration of amino acids. Results of sensory evaluationshowed that the teas fermented with exogenous amino acids could form the mellow test andstale flavour, in addition, teas fermented with different amino acid had its owncorresponding characteristic flavor, such as the teas fermented with phenylalanine possessedobvious fragrance of orchid aroma, and those fermented with alanine and leucine could formstale and ligneous aroma. This may be related to the microbial action and the flavor of aminoacid itself.The pile-fermentaion time could be shortened to19days fermentation (the day after thirdturning) upon adding phenylalanine, alanine or leucine at0.4%and0.6%, and that of withadding of compounded phenylalanine and alanine (the adding amount was0.3%respectively)could be further shortened to only15days, the teas fermented under these contions possessedthe property of aged Pu-erh tea, red and glow liquor color, pure and mellow taste. And theliquor color, fragrance and taste were all the best.