Study On The Quality Formation Mechanism Of Processed White Tea During Withering Proces

Craft white tea is one of the six traditional teas in China,which has high economic value and nutritional value.The processing technology of white tea is simple,with only two processes of withering and drying.Withering is the key process to determine the quality of white tea,and the gradual change of the components during the long-term withering process forms its unique quality.At present,there have been some studies on the mechanism of the quality formation of processed white tea during the withering process,but the screening and confirmation of the withering environmental factors have not been considered in the study,and there have been no studies on the mechanism of quality formation in the optimal withering environment from the perspective of transcriptome combined with metabolomics.In this paper,we first studied the various withering environmental parameters of craft white tea.Under the selected optimal withering environmental conditions,the changes and relationships between differential metabolites and differential genes of craft white tea in different withering time periods were further studied from the perspective of transcriptome combined with metabolomics.The main results are as follows :(1)The sensory evaluation of white tea made by withering treatment of yellow light,blue light,red light,red-blue light and white light was studied.Then the aroma components of five withering treatments were analyzed by headspace solid-phase microextraction(HS-SPME)combined with gas chromatography and mass spectrometry(GC-MS),and the main quality components were detected.The results showed that the quality of white tea made by yellow light withering was the best,and the aroma,taste and weighted total score were significantly higher than the other four light qualities.There were significant differences in several aroma components among the five light quality treatments.The content of 9 main aroma components in the yellow light treatment group was significantly higher than that in the other treatments.The caffeine content of white tea made by yellow light withering treatment was significantly lower than that of other treatments,and the ratio of phenol to ammonia was also lower.(2)The sensory evaluation of process white tea made by yellow light withering treatment with different light intensities(1000 lx,5000 lx,10000 lx,15000 lx)was studied.Then,the aroma components of five light withering treatments were analyzed by HS-SPME combined with GC-MS,and the main quality components were detected.The results showed that the quality of processed white tea was the best when the withering light intensity was 5000 lx,and the comprehensive score of sensory evaluation was significantly higher than that of other treatments,followed by 1000 lx,10000 lx and 15000 lx.Among all the seven characteristic aroma components,three characteristic aroma components were significantly higher than the other treatment groups under 5000 lx withering treatment.The caffeine content under 5000 lx treatment was at a lower position in each treatment,and the ratio of phenol to ammonia was the lowest in each treatment.(3)The sensory evaluation of white tea made by withering treatment at different temperatures(20 °C,25 °C,30 °C,35 °C)was studied and the main quality components were detected.The results showed that the quality of white tea made by withering at 25 °C was the best,its comprehensive score was significantly higher than that of other treatments,and the ratio of phenol to ammonia was the lowest.The sensory evaluation of white tea made by withering treatment with different wind speeds(0.5 m / s,1 m / s,1.5 m / s,2 m / s)was studied and the main quality components were detected.The results showed that the scores of color,aroma and taste of white tea soup made by 1.5 m / s withering were significantly higher than those of other treatment groups,and the ratio of phenol to ammonia was the lowest.(4)Transcriptomics and non-targeted metabolomics techniques were used to study the relationship between differential metabolites and differentially expressed genes during withering.The results showed that the contents of 21 free amino acids changed significantly during the withering process,and showed a gradual upward trend.The contents of leucine and isoleucine in protein amino acids increased significantly,while the content of GABA in non-protein amino acids increased most significantly.The contents of 8 catechins decreased gradually during withering.There was no significant change in caffeine content;some flavonols(quercetin,myricetin and luteolin)in flavonoids showed a significant upward trend.Combined with transcriptome analysis,the expression levels of related genes in the leucine and isoleucine synthesis pathways showed an upward trend during the withering process.At the same time,the expression of GAD that synthesizes GABA was significantly increased during the withering process.In the catechin synthesis pathway,its related genes showed a downward trend during the withering process,while polyphenol oxidase and peroxidase that promoted catechin degradation were significantly upregulated at 24 h and 12 h.The F3’ 5’H gene for the synthesis of quercetin,myricetin and luteolin in the flavonoid synthesis pathway showed an upward trend.In summary,the increase of leucine,isoleucine and GABA during withering improved the freshness of tea taste,and also provided the precursor material for the formation of white tea ’ haoxiang ’;the decrease of catechin content and the increase of theaflavin content reduced the bitterness in tea soup and promoted the formation of white tea soup color.The improvement of some flavonols enhanced the antioxidant capacity of processed white tea.