Effects Of Different Withering Methods On Components Metabolism Related To Color,Aroma And Taste Quality In Green Tea

In actual green tea manufacture,one problem throughout the pick season is the oversenescence or metamorphosis of a large amount of postharvest tea leaves as a result of the limited manufacture capacity from shortage of workers and machines.The ideal preservation method for postharvest tea leaves is that which can prolong the withering period of fresh leaves,and ensure or even improve the sensory quality of green tea.In this study,fresh leaves were withered in four different treatments,including indoor natural withering?CK?,low temperature plus darkness?LTD?,low temperature plus yellow light?LTY?,and low temperature plus CO₂?LTCD?.We aimed to investigate the variations of physiological characteristics,chlorophyll metabolism,nonvolatile metabolism and volatile biosynthesis in postharvest tea leaves under different low-temperature withering treatments,using indoor natural withering as a control.This knowledge will provide a theoretical basis for raw tea material preservation in green tea manufacture.The main research contents and results are as follows:1. Effects of different withering treatments on the physiological characteristics of postharvest tea leavesWith increasing withering time,the moisture content in fresh tea leaves decreased gradually,the low-temperature condition could effectively reduce the water-loss rate of postharvest tea leaves,the withering periods of different treatments were 7 h?CK?,14 h?LTD?,18 h?LTY?and 48 h?LTCD?,respectively.Leaf lightness?L*?showed a decreasing trend,leaf greenness?-a*?and yellowness?b*?both showed an upward trend first,followed by a downward trend in all of the withering treatments.The low-temperature condition?especially LTCD?could effectively?P<0.05?delay the drop of color values in postharvest tea leaves.The newly picked tea leaves had the highest intensity of breathing strength and ethylene release,with increasing withering time,the two indexes and the stomatal opening rate of tea leaves gradually dropped to a low level,indicating that postharvest tea leaves did not follow a leap respiration mode like some fruits.The stomatas on the back of tea leaves were almost closed,and the release of CO₂and ethylene were also significantly?P<0.05?suppressed under LTCD treatment.As the water-losing degree of postharvest tea leaves increased,the production of H₂O₂ and O₂^·-increased and caused the synthesis and accumulation of MDA,resulting in an increase in the relative conductivity of withered leaves.Compared with CK,the low-temperature condition?especially LTCD?showed an obvious promotion effect on the activities of antioxidant enzymes?SOD,POD,CAT?,thus could effectively alleviate the damage by active oxygen on postharvest tea leaves.2. Effects of different withering treatments on chlorophyll metabolism in postharvest tea leaves and color quality of green tea productsAt the initial stage of withering,a significant increase was observed in the chlorophyll content,expression of chlorophyll-synthesis-related enzymes?such as glutamyl-t RNA synthetase,etc.?,and chlorophyll synthase activity in newly picked tea leaves.Moreover,an obvious decrease was found in the content of L-glutamate as the foremost precursor substance of chlorophyll synthesis.Hence,the findings revealed that the chlorophyll synthesis reaction was induced by the light-dehydration-stress in the initial withering of tea leaves.Chlorophyll decomposition was found closely related with chlorophyllide,pheophorbide,and pheophytin.The content ratio of?Chl+Cd?/?Po+Py?decreased from5.35?0 h?to 1.28?CK-7 h?,2.71?LTD-14 h?,1.62?LTY-18 h?and 4.70?LTCD-48 h?,indicating that green is the main leaf color in the withering process,and low-temperature treatments help to maintain the leaf greenness just as they help to delay the increase of the dark degree.Mg-dechelatase?Cs NYE1?,chlorophyllase?Cs CLH1?,and pheophorbide a oxygenase?Cs PAO?performed functions in promoting the degradation of chlorophyll in withering tea leaves,low-temperature withering?especially LTD and LTCD treatments?played a significant?p<0.05?inhibition effect on the enzyme activity and gene expression of chlorophyll-degradation-related enzymes.The senory evalutaion of green tea products showed that LTCD exhibited the best color and appearance quality,followed by LTD,and there was no significan between LTY and CK.3. Effects of different withering treatments on nonvolatile metabolism in postharvest tea leaves and taste quality of green tea productsThe influence mechanism of different withering methods on non-volatile compounds in postharvest tea leaves was investigated by UHPLC-Q-TOF/MS-based non-targeted metabolomic and transcriptomic analyses.A total of 66 differential metabolites?VIP>1?were identified and mainly classified into 7 types?13 amino acids,10 catechins and dimeric catechins,25 flavanol/flavone glycosides,3 alkaloids,4 theaflavins,4 phenolic acids,and 7 glycosidically bound volatiles?.At the end of withering,the total amount of amino acids increased and the theanine content decreased,the total amounts of catechins and flavonoid glycosides decreased,the contents of theaflavins and phenolic acids increased,the caffeine content increased and the theobromine content decreased.Compared with CK,low-temperature withering could slow down polyphenol oxidation by inhibiting polyphenol oxidase activity and keeping the expression of genes for flavanol synthesis.After withering,the proteinaceous amino acid content increased significantly,especially for LTCD and LTY,mainly due to the increased peptidase activity and up-regulation of genes involved in the biosynthesis of valine,leucine,aspartic acid,glutamic acid,phenylalanine,and proline.Moreover,LTCD and LTY enhanced the synthesis of?-aminobutyric acid and metabolism of phenylalanine-methyl salicylate and tryptophan-indole,respectively.Meanwhile,the transformation of theobromine to caffeine was accelerated under low-temperature withering.The taste quality of green tea product was shown by the score in the order of LTY?86.2?>LTCD?82.1?>CK?81.1??LTD?80.5?.LTY had the highest score for its fresh,brisk,and mellow taste,while the other three samples had a lower score due to their bitter and astringent taste.4. Effects of different withering treatments on volatile metabolism in postharvest tea leaves and aroma quality of green tea productsThe influence mechanism of different withering methods on volatile compounds in postharvest tea leaves was investigated by HS-SPME/GC-MS metabolomic and transcriptomic analyses.The relative total-amounts of VTs,AADVs and FADVs in postharvest tea leaves all obviously?p<0.01?added at the end of withering.Compared with CK,the content increments of VTs and AADVs were both larger in low-temeperature condition with the largest increment was observed in LTY,while the largest content increment of FADVs was observed in CK.At the initial stage of withering,a significant increase was observed in the expression of terpenoid-synthesis-related enzymes?MEP and MVA pathways?,Linlool synthtase?Cs LIS2,Cs LIS3,Cs LIS4?and Farnesene synthtase?Cs FS5?Cs FS10?in newly picked tea leaves.Compared with CK,the low-temperature condition showed stronger promotion effect on the reaction of terpemoid synthesis with the strongest effect was observed in LTY.The degradation reaction of?-Linolenic acid was also enhanced in the withering process,compared with the low-temperature condition,the expression of genes related to?-Linolenic acid catabolism showed higher level in CK.However,LTY treatment showd a distinct promotion effect on the biosynthesis of methyl jasmonate and the metabolism of phenylalanine-phenethyl alcohol/phenylacetaldehyde in withering leaves.During the withering process,only a few number of?-glucosidase?Cs BGLU?gene family were up-regulated,the expression of?-primeverosidase?Cs BPRIM?was down-regulated.The activities of?-glucosidase and?-primeverosidase both followed a trend of first increasing and then decreasing in all of the treatments.While the relative total-amount of GBVs significantly?p<0.05?added after withering,indicating that changes in the gene expression and enzyme activity of glycosidase in withering process may have a limited effect on the aroma formation of green tea product.The aroma quality of green tea product was shown by the score in the order of LTY?89.6?>LTCD?86.6?>CK?85.1??LTD?84.9?,LTY had the highest score for its high fragrance,with a strong fruity floral aroma,while the other three samples had a lower score due to their lower fragrance.