The Selection Of Raw Materials,Post-Fermentation Processing Technology And Quality Evaluation On Tibetan Tea

Tibetan tea,named after the sale of Sichuan to Tibet,is a typical dark tea product in China.Since dark tea products have anti-radiation,regulation of lipid metabolism,anti-oxidation and other health functions,in 2015,it has become the second largest tea category in the domestic market,with a production capacity of 29.71 million tons.As an important part of sichuan tea industry,Tibetan tea plays an important role in the development of sichuan tea industry.But for a long time,there are many problems in the production of Tibetan tea,such as chaos of raw materials,rough processing and poor sanitary conditions,which lead to a large gap with other dark tea producing areas between quality and standard,which seriously restricts the development of Tibetan tea industry.The experiment systematically studies the raw material tenderness standard,tea plant cultivar,the feasibility and process optimization of different kinds of raw tea,and then analyzes the differences between the sensory quality and the physical and chemical components compared with traditional Tibetan tea,and provides scientific basis for the formulation of the standard and process specifications of Tibetan tea.The main results are as follows:（1）The results of the raw material tenderness standard screening test showed that the comprehensive score of the sensory evaluation of one bud and three leaves Tibetan tea was the highest,which had the characteristics of orange and bright soup color,long stale flavour and mellow and thick sweet taste.With the increase or decrease of fresh leaf tenderness,the comprehensive score of sensory evaluation showed a tendency of decline,indicating that it is not suitable for the fresh leaf to pass high or too low to process Tibetan tea.The one bud and three leaves Tibetan tea has relatively high water extract,tea polysaccharide,theabrownin and soluble sugar content,and has relatively low caffeine content,respectively 41.02%,2.150%,10.479%,6.71%and 3.06%.At the same time,the fluorine content is 118.79mg/kg and the lead content is 2.08mg/kg,which meets the safety limit standard.There are 45 kinds of aroma components were detected of different tenderness Tibetan tea,including 16 kinds of alcohols,8 kinds of ketones,6 kinds of aldehydes,5 kinds of esters,4 kinds of acids,3 kinds of pyrrole and 3 other kinds,and all of them have the highest relative content of alcohols.Therefore,it is suggested that one bud and three leaves of tea plant should be used as the raw material for Tibetan tea.（2）The cultivar fitness experiment results showed that there are significant differences in the chemical composition of fresh leaves,sensory quality and the main flavor components of the four tea cultivar.Comprehensive analysis,it is recommended that FuYun 4 tea plant cultivar as processing optimization of Tibetan tea.Its Tibetan tea has the characteristics of orange red and bright soup color,rich stale flavour and mellow and thick sweet taste,at the same time has high content of water extract,tea pigment and tea polysaccharide,respectively 40.06%,14.240%,and 3.283%.（3）Post-fermentation experiments of five kinds of raw tea with same fresh tea leaves showed that black and white raw tea are not suitable for post-fermenting,but the post-fermented sensory quality of green,yellow and oolong tea meet the quality requirements of Tibetan tea.（4）The results of three types raw tea post-fermentation showed that the optimum process parameters of green tea,yellow tea and oolong tea were respectively A₃B₁C₃（80℃,20%,80h）,A₃B₁C₃（80℃,20%,70h）,A₁B₂C₂（60℃,30%,30h）;Temperature（A）,water（B）,and time（C）are three factors,which have the primary and secondary relationships of the influence of the sensory quality of the three types of raw tea are respectively A>C>B（green raw tea）,A>B>C（yellow raw tea）,A>C>B（oolong raw tea）;The extreme difference analysis can be obtained,and the temperature（A）is the most important factor affecting the sensory quality of the three types of raw tea,and it is proved that the temperature plays an important role in the post-fermentation process.（5）There are obvious differences in the sensory quality of the Tibetan tea produced with optimal post-fermentation process of three types of raw tea.The aroma is mainly composed of stale flavour and ripe fragrance,and the flavor is concentrated alcohol or thick,and the color is orange,orange red or red bright,but all meet the sensory quality requirements of Tibetan tea.In the main taste ingredients,the tea polyphenols and the catechins are the lowest of 7.51%and 0.44%of green raw tea post-fermented sample;The content of amino acid of oolong raw tea post-fermented sample was 5.59%,which was2.39 times that of traditional Tibetan tea sample;The water extract content of oolong raw tea post-fermented sample was 39.51%,significantly lower than the other three treatments.The soluble sugar content was the highest of 6.96%of yellow raw tea post-fermented sample;The total amount of tea pigment was the highest of 16.589%of traditional Tibetan tea sample;The content of tea polysaccharide was the highest of 3.33%of yellow raw tea post-fermented sample.In the catechins components,the content of EGCG in four treatments was:oolong raw tea post-fermented sample（1.972%）>traditional Tibetan tea sample（0.422%）>yellow raw tea post-fermented sample（0.098%）>green raw tea post-fermented sample（0.052%）,Catechin astringent index:traditional Tibetan tea sample（8.81）>yellow raw tea post-fermented sample（7.94）>oolong raw tea post-fermented sample（6.43）>green raw tea post-fermented sample（5.66）,it is indicated that the post-fermentation can promote the oxidation and transformation of catechins better with raw tea.In the amino acid components,there are 21 kinds of amino acid components detected in the four treated Tibetan tea,but there were significant differences in the total amount and composition of amino acids;Oolong raw tea post-fermented sample has higher levels of theanine（The）and gamma-aminobutyric acid（GABA）with content of24.717mg/g and 0.966mg/g;In the aroma components,45 aroma components were detected in 4 processed Tibetan tea,and the aroma substances of traditional Tibetan tea sample,yellow raw tea post-fermented sample and oolong raw tea post-fermented sample were the highest in alcohols,respectively 32.00%,28.67%,34.56%;However,the aromatic substance of green raw tea post-fermented sample has the highest proportion of ketone to25.82%;Moreover,the relative content of pyrrole was 22.39%of green raw tea post-fermented sample is much higher than that of the other three treatments.To sum up,the Tibetan tea produced after the post-fermentation of three types of raw tea not only conforms to the quality requirements of Tibetan tea,but also shows the diversity of taste and aroma,which has enriched the flavor types of Tibetan tea.