The Characteristic Aroma Components In White Tea And Their Formation Metabolic Mechanism During Withering

Aroma is one of the crucial indicators for evaluating the quality of white tea,and its stylistic characteristics are primarily determined by various endogenous flavor compounds formed during the withering process.Currently,research on the aroma quality of white tea mainly focuses on the differences in volatile components among different samples and their variations during the processing,while paying little attention to the direct correlation between flavor compounds and aroma attributes,as well as the molecular regulatory mechanisms of endogenous flavor compound metabolism during the withering process.Investigating these issues contributes to a deeper understanding of the chemical basis of tea aroma quality,clarifying the regulatory mechanisms of tea processing on aroma quality formation,refining the metabolic regulatory network of volatile compounds in tea,and providing a scientific basis for precise control of tea aroma quality and the development of corresponding processing control technologies.This study focuses on white tea as the research subject and comprehensively applies research methods such as sensory analysis,chemometrics,sensoryomics,bioinformatics,and transcriptomics.The study systematically investigates the correlation between characteristic aroma compounds and aroma attributes of white tea,as well as the molecular regulatory mechanisms of temperature and relative humidity on its metabolic pathways during the withering process.The main research findings are as follows:1.A novel apparatus and extraction method called Simultaneous Nitrogen-blowing Distillation Extraction(SNDE)for white tea aroma extraction has been invented.Compared to the traditional Simultaneous Distillation Extraction(SDE)method,the SNDE method yields essential oil with sensory profiles that are closer to the national standard evaluation results.It can effectively differentiate the sensory differences among different types of white tea.The number and relative content of volatile compounds obtained by SNDE are lower than those obtained by SDE.However,certain substances’ relative content can still reflect significant differences between samples.The optimal extraction conditions were determined using response surface methodology,which resulted in a liquid-to-material ratio of 5,an extraction time of 80 min,and a nitrogen flow rate of 400 m L/min.2.Four attributes,namely Qingxiang(green aroma),Haoxiang(pekoe aroma),Tianxiang(sweet aroma),and Huaxiang(floral aroma),can summarize and differentiate the aroma characteristics of 10 representative white tea essential oils obtained through the SNDE method.The composition and relative content of volatile compounds,including alkanes,alkenes,alcohols,aldehydes,ketones,esters,oxygenated compounds,pyrrole and its derivatives,vividly reflect the information regarding the origin,variety,and maturity of the samples.Weighted Gene Co-expression Network Analysis(WGCNA)identified seven compound modules significantly correlated with the aroma attributes of white tea.From these modules,the key volatile compounds,including 2-pentylfuran,phenylacetaldehyde,N-ethylsuccinimide,β-cyclocitral,methyl ethyl maleimide,4-(2,4,4-trimethyl-1,5-cyclohexadien-1-yl)-3-buten-2-one,β-ionone,dihydroactinidiolide,farnesylacetone,indole,dibutyl phthalate,cedarol,2-methoxy-4-vinylphenol,2-hexenal,trans-linalool oxide(furanoid),benzonitrile,ocoumaric acid,linalool,trans-linalool-3,7-oxide,phenanthrene,vinyl hexanoate,benzaldehyde,phenethyl alcohol,δ-octalactone,cis-hexahydro-8a-methyl-1,8(2H,5H)-Naphthalenedione,and 3,4-dehydro-β-ionone,were screened,resulting in a total of 27 key volatile compounds that contribute to the aroma characteristics of white tea.The formation pathways of these compounds can be divided into two major categories:enzyme-catalyzed pathways,including carotenoid metabolism,lipid metabolism,amino acid metabolism,and glycoside hydrolysis,as well as non-enzyme-catalyzed pathways,such as Maillard reactions and flavonoid alcohol oxidation-driven Strecker degradation.Among these pathways,enzyme-catalyzed pathways are the main routes for the formation of key volatile compounds in white tea.3.During the withering process of white tea under conditions of darkness and no wind,most of the genes involved in withering showed a downregulation trend,while some genes were upregulated.Transcriptional regulation in response to environmental factors mainly occurred during the stage when leaf moisture content was above 60%.Environmental temperature and relative humidity significantly influenced the expression patterns of differentially expressed genes during withering.Hightemperature withering treatment inhibited the differential expression of most genes during withering but selectively activated the transcriptional response of certain genes.The differentially expressed genes under high-temperature treatment were mainly associated with gene ontology(GO)categories such as membrane,membrane components,membrane composition,DNA binding in regulatory regions,DNA binding in transcription regulatory regions,phenylalanine ammonia-lyase activity,and flavonoid metabolism processes.They were also associated with Kyoto Encyclopedia of Genes and Genomes(KEGG)pathways,including plant-pathogen interaction,plant hormone signal transduction,plant MAPK signaling pathway,zeatin biosynthesis,α-linolenic acid metabolism,and glucosinolate biosynthesis.On the other hand,the differentially expressed genes under high relative humidity treatment were primarily associated with GO categories such as cytoplasm,catechol oxidase activity,peroxidase activity using catechols as donors,coumarin catabolic process,and lignin catabolic process.They were also related to KEGG pathways,including selenium compound metabolism,monoterpenoid biosynthesis,sesquiterpenoid biosynthesis,photosynthesis,galactose metabolism,carbon metabolism,starch and sucrose metabolism,mismatch repair,protein processing in the endoplasmic reticulum,and m RNA surveillance pathway.Furthermore,environmental temperature and relative humidity affected the expression of key rate-limiting enzyme genes such as PLA,LOX,HPL,ADH,AOC,4CL,ACX,AIM,TCS,JMT,SAMT,AOS,OPR,PED,AAT,HMGS,HMGR,MVK,PMK,MVD,DXS,ISPD,ISPE,ISPH,GGPS,AO,AST,TAT,HPAT,HPD,PAL,C4 H,and BGL in pathways related to α-linolenic acid metabolism,terpenoid backbone biosynthesis,phenylalanine metabolism and phenylpropanoid biosynthesis pathway.This regulation influences the metabolic flux and subsequently affects the synthesis and accumulation of key aroma compounds during the withering process of white tea.