Processing Technology And Qualit Of Low-Fluoride Brick Tea (Pu’er Green Tea)

Pu-erh brick tea is a compressed tea belonging to the Yunnan Pu-erh tea.The Pu-erh brick tea(raw tea)in this study is made from lower grade raw materials with more coarser old leaves and woody stems.Puerh brick teas are made after preliminary processing,steamed and sterilized,and then pressed into brick-shaped molds.They are mainly supplied to Yunnan province itself and Ti-bet and Sichuan.Pu-erh brick tea is an important part of China’s border tea.Border tea is a necessity in the life of Tibetan people.However,because the fluorine content of brick tea is very high,and the local people drink a large amount of tea per capital daily,so it is easy to cause "drinking brick tea fluorosis".This situation has received widespread attention from the country and theregion.Therefore,how to reduce the fluoride content in brick tea has become a hot issue related to the livelihood of the people.At present,the main way to reduce the fluoride content are by changing the variety,cultivation method,raw material tenderness,processing and brewing methods,and adding fluoride reducing substances.In this study,we hope to reduce the fluorine content in brick tea by changing the processing method of brick tea,and explore a new processing process of low-fluoride Pu’er brick tea with low cost and high efficiency.By sampling the brick tea on sale in the market factories in Yunnan Province and analyzing them.Thus,the current status of the fluorine content of brick tea on sale in the market were understood.Through single-factor test,the best method of water-removing,drying and twisting in the preliminary processing of brick tea was selected.At the same time,the drying method was optimized for the refining process after autoclaved sterilization.The optimal parameters in each process were determined by orthogonal experiments and response surface experiments.Finally,the new process of low-fluoride pu-erh brick tea(raw tea)was determined through the detection of the substances contained in the old and new processes and sensory evaluation.The following conclusions were drawn:(1)Through sampling and analysis of commercially available brick tea in Yunnan Province,it was found that 6 out of 16 samples had fluorine content exceeding the standard,with a qualified rate of about62.5%.The content of tea polyphenols ranged from 7.06% to 21.35%,and the overall tea polyphenol content was low;the caffeine content ranged from 2.18% to 3.89%;amino acids ranged from 2.01% to 4.11%;water leachate content was24.56%～39.14%.All of them were within the qualified range.(2)Through the experiment of different killing methods,it was determined that the boiling method was the best killing method,because its fluorine content was the lowest(183.01mg/kg)compared with the other three,there was a significant difference,and the fluorine content(372.64mg/kg)was two times lower than the frying heat killing,and the inclusions were the lowest among the four killing methods,but they were all within the normal quality range;the fluorine content of the microwave killing sample was relatively lower,and the inclusions were within the normal quality range.The fluorine content of the microwave killed samples was relatively low and the inclusions were well maintained,so it was recommended to use microwave killing method when the samples were tender.(3)Through different drying experiments,it was determined that microwave drying was the best drying method,because the fluorine content was significantly the lowest compared with the other three drying methods,and the pattern of drying > freeze-drying > sun-drying > microwave drying was shown,and the inclusions were better retained,and all factors were optimal.(4)Through the experiment of kneading and non-kneading,it was found that the fluorine content of the tea samples after the kneading process was significantly higher than that of the non-kneading treatment,and the quality components were better,so this process was chosen to be discarded in the new process.(5)Through the selection of different drying methods after steam pressing,it was determined that microwave drying after steam pressing was the optimal drying method,which was the same as the optimal drying method of the primary process.Because its fluorine content is significantly lower than that of oven-dried tea samples,and the content of tea polyphenols,caffeine and amino acids are significantly higher than the other two drying methods.It not only keeps the low fluorine content,but also has a certain improvement on the tea quality.(6)The effect of the killing factors on the fluorine content was determined by single-factor test,and the results showed that the higher the leaf feeding amount and the killing temperature,the higher the fluorine content;the effect of water injection in the killing pot on the fluorine content had no significant pattern and the difference was not significant;the effect of longer killing time on the fluorine content might be negatively correlated.(7)The correlation rule of drying factors on fluorine content was determined by orthogonal and response surface experiments as drying time(p=0.08)> drying temperature(p=0.1)> leaf feeding amount(p=0.4).The optimal experimental method for drying was the combination of the shortest time(12 min),the highest temperature(p100),and the largest amount of leaf feeding(100 g).(8)The low-fluorine optimization study of the killing and drying factors was conducted by response surface experiments,and a quadratic multinomial regression model was obtained: Y fluorine content = 306.04-11.52* A-30.3* B-21.91*C+28.62* AB-6.04* AC-18.56* BC+37.08* A2+60.78* B2+43.56* C2,and according to the regression equation model,the predicted optimal conditions were obtained as follows: cooking time of 4.144 min,microwave time of 8.640 min,and microwave temperature of 86.321°C.