Effect Of Low Temperature Treatment Of Fresh Leaves On The Quality Of Yellow Tea

Yellow tea is one of the six kinds of tea in China.It is processed by tedding fresh leaves,de-enzyming,heaping for yellowing and drying processes,among which ‘heaping for yellowing' is the key process to form the quality characteristics of ‘yellow soup and yellow leaves'.At present,the processing of yellow tea mostly adopts the way of ‘heating and suffocating yellow' with high temperature and high humidity,which not only consumes large energy,but also has low yellowing efficiency.Therefore,to explore the efficient degradation of chlorophyll and controllable oxidation of tea polyphenols in the process of yellow tea processing,it is the key to the innovation of yellow tea processing technology.This study takes ‘Jin Xuan' one bud and one leaf as raw material,at room temperature(25?)natural stands for comparison,set up three low temperature(10,5,0?)and five duration treatment(8,16,24,32 and 40 h)a combination of processing.The relative conductivity,lipid soluble pigment content and volatile components were determined.At the same time,sensory evaluation,physical and chemical analysis and detection of the main quality biochemical components of the processed yellow tea were carried out to study the influence of low temperature treatment of fresh leaves on the quality of yellow tea,determine the best low temperature treatment scheme of yellow tea processing,and explore the internal mechanism of the influence of low temperature treatment of fresh leaves on the taste and aroma of yellow tea from the molecular level.The main research results are as follows:(1)Compared with the control fresh leaves,the relative electrical conductivity of leaves treated at low temperature increased by 44.5%?186.3%,the contents of chlorophyll and carotenoids decreased by 0.82?1.306 times and 0.23?0.80 times respectively,and the proportion of aromatic components,benzene and alkane compounds in volatile substances detected by electronic nose increased.These results indicate that low temperature treatment can increase the damage of leaf membrane,promote the yellowing and the change of flavor.(2)The yellow tea processed after fresh leaves were treated at 10? for 8?32h,5? for8?24h or 0? for 8?24h conforms to the quality characteristics of ‘yellow soup and yellow leaves',with mellow and smooth taste and lasting fragrance of flowers.Among them,the fresh leaves treated at 5?10? for 16?24h had the best quality.(3)The b*(yellowness)and L*(brightness)values of yellow tea soup under low temperature treatment significantly increased,and the average particle size showed a decreasing trend.The contents of tea water extract,soluble sugar,soluble protein and simple catechin were significantly increased,among which the soluble sugar content increased by44.78%?66.05%.The results showed that low temperature treatment of fresh leaves could improve the yellowing degree of yellow tea,the brightness of tea soup,and the mellow degree of tea soup.(4)The aroma of yellow tea under low temperature treatment was more intense and fragrant,and the relative proportion of heterocyclic,aldehydes,ketones,esters and unsaturated hydrocarbons was increased,and 39 volatile components with floral characteristics were added,such as lauryl alcohol,jasmonolactone and ?-rhodoxone.(5)The expression levels of SPS,SUS,INV and PFK genes related to glucose metabolism were up-regulated in fresh leaves treated with low temperature.Meanwhile,GPS,GGPS and LOX6 genes related to aroma metabolism were also up-regulated.The above results were consistent with the increase and change of soluble sugar content and main aroma components in tea after fresh leaves were treated at low temperature,which to some extent clarified the internal reasons for the influence of low temperature treatment of fresh leaves on the taste and aroma of yellow tea.