Effect Of Low Temperature Treatment Of Fresh Leaves On The Quality Of Jinxuan White Tea

White tea is one of the six major tea categories in China.It is a slightly fermented tea.The traditional white tea is processed by withering and drying process.It has the quality characteristics of light yellow and bright soup color,fresh aroma and fresh taste.The phenomenon of "late spring cold" is easy to appear in early spring in Shandong Province.The sprouted new shoots of tea tree are susceptible to freezing and discoloration and are discarded,resulting in huge economic losses to the tea industry.It is found that fresh leaves treated by artificial low temperature can improve the quality of green or black tea.In order to enrich white tea products,optimize and improve the processing technology and quality of Jinxuan white tea,explore new ways to reduce losses caused by natural disasters,and boost the quality and efficiency of the tea industry,this topic takes Jinxuan tea variety one bud and two leaves tea as raw materials,and takes traditional white tea as the control.Fresh leaves were treated by different low temperature（5 ℃,10 ℃）and different duration（10,20,30,40,50h）stress treatments,and then white tea is made by indoor natural withering and drying,The effects of different low-temperature treatments on the quality of white tea were studied,the optimal lowtemperature treatment scheme of fresh leaves was screened and determined,and the differences of metabolites between the optimal low-temperature treatment and traditional white tea were explored by LC-MS / MS high-throughput and widely targeted metabonomics.The main results are as follows:1.Compared with the traditional white tea,the 5 ℃ and 10 ℃ treatment of fresh leaves improved the persistence of white tea aroma and the intensity of sweet and floral aroma.The sensory evaluation score of white tea treated for 40 h was the highest（floral aroma,rich and lasting,95 points）.Low temperature treatment promoted the increase of the types and contents of fragrant volatile substances in white tea.Compared with traditional white tea,white tea treated at 5 ℃ for 40 h increased 22 kinds of fragrant volatile substances（6 alcohols,7 aldehydes,5 esters and 4 ketones）.The total content of fragrant volatile substances in white tea treated at5 ℃ for 30 h was the highest（327.83 μg / g）,about 5 times that of traditional white tea;The white tea treated at 10 ℃ has a fragrant aroma,and the total content of volatile substances is higher than that of the traditional white tea,but lower than that of the white tea treated at 5 ℃.Only the white tea treated at 10 ℃ for 10,20 and 30 hours has a higher content of heterooxygenates than the white tea treated at 5 ℃ for 50 hours.Based on the changes of aroma sensory quality,types and contents of aroma volatile substances,it was considered that the aroma of white tea treated at 5 ℃ for 30 hours in the early withering stage was the best.2.Compared with traditional white tea,5 ℃ and 10 ℃ treatment of fresh leaves can increase the alcohol thickness of white tea taste,reduce bitterness and astringency,and slightly increase the freshness.The sensory evaluation score of white tea treated for 30 h was the highest（sweet and mellow,sweet and rhyme,96 points）.The white tea treated at 5 ℃ for 50 h had the highest sweet taste,the white tea treated at 10 ℃ for 30 h had the highest fresh taste,and the white tea treated at 5 ℃ for 30 h and 10 ℃ for 20 h had the lowest bitter and astringent taste.Low temperature treatment reduced the content of water extract and caffeine of white tea.The content of free amino acids in white tea treated at 5 ℃ for 10 h and 10 ℃,20,30 and 40 H increased.Except for white tea treated at 5 ℃ for 10 h,the content of tea polyphenols increased.The content of soluble sugar in white tea treated at 5 ℃ and 10 ℃ increased first and then decreased.The content of white tea treated at 10 ℃ for 30 h increased significantly by 56.28%compared with traditional white tea.3.The sensory quality,volatile and non-volatile components of white tea treated at different low temperatures were compared and analyzed.The aroma of white tea treated at 5 ℃ for 30 h was pleasant sweet flower aroma,with high aroma and strong persistence,and the total content of aroma volatile substances was the highest;The taste sensory quality was the best,the score was the highest,and the bitterness,astringency and astringent aftertaste were the lowest.Therefore,it is considered that the comprehensive quality of white tea treated at 5 ℃ for 30 h in the early stage of withering is the best,that is,the best low-temperature treatment process of white tea is treated at 5 ℃ for 30 h in the early stage of fresh leaf withering.4.Metabonomics study showed that 1161 metabolites were identified in traditional white tea and white tea treated at 5 ℃ for 30 hours,1152 and 1149 metabolites were detected respectively,and 132 differential metabolites were detected.The top 10 metabolites were scutellarin,3-hydroxyoctadecanoic acid,quercetin-4 ’-o-glucuronide and so on.Differential metabolites are mainly concentrated in linoleic acid pathway α-Linolenic acid metabolism pathway,linoleic acid has the functions of increasing cold resistance and regulating membrane fluidity in plants under low temperature stress,α-Linolenic acid has the function of regulating lipid metabolism,indicating that low temperature treatment affects the formation of aroma and taste substances of white tea by changing the metabolic pathway of fresh leaves.