| Congou black tea is a traditional products unique to China.It has been two hundred years since its born in the mid-eighteenth century.Keemun black tea,Sichuan black tea,and Yunnan black tea are the main representative products.During the process of rolling and fermentation of Congou black tea,polyphenols are oxidized by polyphenol oxidase and peroxidase,forming the quality characteristics of “red soup and red leaves”.In this study,we used Jiukeng cultivar which have a bud and two leaves as the raw materials.With reference to the Qimen black tea processing technology,rolling temperature was set at every other 3 ℃ from 10 ℃ to 40 ℃,producing 11 different tea samples.Sensory evaluation and other related quality indicators were used to analyze the changes of quality characteristics of tea samples.In the process of rolling,the total amount of catechins decreased,and the higher the rolling temperature was,the faster the catechins reduced.From 10 ℃ to 40 ℃ of rolling temperature,catechin retention was from 64.8% to 38.4%.Seven catechin monomers were detected,among which EGCG had the highest content,GCG,CG,C,EGC and EGCG had the fastest reducing speed while the decreasing speed of EC and ECG were the lowest.Because GA tastes bitter,the accumulating of GA during processing is not conducive to the quality of black tea.But the content of GA was found increasing along with the rolling temperature,indicating a higher rolling temperature will be detrimental to the quality of black tea.PPO activity analysis showed that at a rolling temperature lower than 16 ℃,PPO activity maintained at the initial level of the rolling process and even slightly increased.While at a temperature higher than 19 ℃PPO activity had a small increase before 30 min,and then rapidly decreased to the initial level.This trend was even more significant with increasing temperature.Below 16 ℃,POD activity gradually increased up to about 20% during the whole rolling process.When the temperature is higher than 28 ℃,the changing trend of POD activity was similar to PPO’s,which had a 10% decrease.The sensory evaluation of tea color found that along with the increasing of rolling temperature,tea soup color score gradually increased.Especially when the temperature reached above 31 ℃,the tea soup color got the highest score,exhibiting a red and bright appearance.The brightness,redness,yellowness,hue and saturation were measured using colorimeter,described by L*,a*,b*,c*,and h* values.Through the analysis of the relationship between the five indicators and the sensory evaluation of tea color,it was found that there was a significant positive correlation between a* value and sensory review,and its correlation coefficient reached 0.683,and there was a quadratic relationship between the color of the soup and the a* value: y = 0.199x2-2.482 x + 94.092(R2 = 0.752).The sensory evaluation of taste found that the rolling temperature should be lower than 25 ℃,and 16 ℃ was best rolling temperature.A low temperature can retain a mild and sweet taste of congou black tea while a higher rolling temperature will enhance the fermentation of tea,leading to an unpleasant sourness.Taste component analysis found that tea polyphenols,theaflavins,thearubigins and caffeine decreased with the increase of rolling temperature,and the theabrownin increased with the increase of the rubbing temperature.Using electronic tongue to analyze the sourness,sweetness,bitterness,saltness and umami,the correlation analysis found that SRS and SWS values respectively had significant negative correlation and positive correlation with the sensory scores of tea soup,and cluster analysis can divide all tea samples into three categories,namely,low temperature zone(10 ℃,13 ℃,16 ℃),middle temperature zone(19 ℃,22 ℃,25 ℃)and high temperature zone(28 ℃,31 ℃,34 ℃,37 ℃,40 ℃).The correlation between biochemical components and SRS,SWS values found that there was a highly significant negative correlation between SRS values and amino acids,theaflavin and thearubigins while a highly significant positive correlation with theabrownin.There was a highly significant positive correlation between SWS value and soluble sugar,amino acid and thearubigins while a highly significant negative correlation with theabrownin.Sensory evaluation of aroma found that between 19 ℃ ~ 25 ℃,tea samples exhibited a sweet and flowery aroma,having the highest score.When the temperature is higher than 25 ℃,the aroma quality gradually transformed from fruity,flowery and sweet flavor to weak sweetness and sourness.When the rolling temperature was lower than 19 ℃,the fruity and flowery aroma of congou could not be fully revealed.GC-MS detected 70 kinds of volatile aroma substances,including 16 kinds of alcohols,accounting for more than 60% of the total amount of volatile aroma compounds.Besides,there were 14 kinds of aldehydes,5 kinds of ketones,12 kinds of esters,4 kinds of acids,14 kinds of hydrocarbons and other 5 species.19 kinds of volatile aroma compounds had more than 1% content of the total,among which 14 species showed sweet fruity and flowery aroma,namely geraniol,phenylethyl alcohol,linalool,linalool oxide I,linalool oxide II,linalool oxide IV,phenylacetaldehyde,benzyl alcohol,dehydro-linalool,citral,nerolid terephthalate,beta-ionone,(Z)-orolene and(E)-basilene,and the total contents of these 14 aroma compounds was highest in the middle temperature zone.The other five volatile aroma compounds exhibited irritating and unpleasant odors,and the total amounts of these five aroma compounds in the middle temperature zone was significantly lower than those in the low and high temperature zones.This result was in good agreement with the results of sensory evaluation.Considering factors of infusion color,taste and aroma combined with sensory evaluation standard,the results showed that the quality of infusion color declined significantly below 16℃;once the temperature exceeded 25℃,the quality of taste and aroma became worse comparing with tea samples in middle and low temperature zone;therefore,congou processed between 16~25 ℃ had superior quality in infusion color,taste and aroma,which can satisfied the demands of most customers. |
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