| Compared to other teas,dark teas can improve the quality,and promote healthy value by post-fermentation.For the purpose of keeping better quality and healthy aspect of dark tea,it is necessary to preserve them in a well-established storage environment under a certain period.Besides,the tea market is terribly disturbed by rule-breakers who using the inferior or old teas to replace the high or new ones.Therefore,it is imperative to develop efficient method to characterize aged dark tea scientifically for the industry development of the national dark tea,which can promote preserved dark teas into the consumer markets.The method will be benefit for rights and interests protection of consumers and market surveillance.As is known to all,the biggest preservation stock in China was located in the city of Dongguan,Guangdong province.The top two of preservation stock was kang brick teas in Dongguan.Kang brick teas（KBT,as below）divided into seven different storage periods used as experimental materials,including 1-year of period,6-year of period,10-year of period,15-year of period,22-year of period,25-year of period and 30-year of period.In this study,we evaluate the potential regularity of post-fermentation of KBT by taste components,volatile,fungal community,antioxidant activity and near infrared spectrum information.By chemometrics and multivariate statistics,deep correlation between storage period and the quality had been analyzed to integrative differentiate from storage periods of KBT,which explored for objectively distinguishing post-fermentation quality and controlling the quality standards.The main results have been reviewed as follows.（1）The taste and aroma of KBT in different storage periods were assessed.The ingredients including tea polyphenols,catechins,gallic acid,caffeine,total soluble sugar and glucose,decreased during the first 10-year storage.The contents of maltose and fructose had no significantly difference among the storage periods,but theabrownin contents kept a stable level,during the first 10-year storage.During the next storage period of 20 years,the chemical compositions increased,including,non-ester catechins,gallic acid,theabrownin,caffeine,total soluble sugar,glucose,maltose and fructose,but the contents of free amino acids decreased.The sucrose content had changed significantly during the storage period of10 years,and then not positively decreased during the next 20 years.In the entire storage period of 30 years,the number of main volatile composed of the aroma gradually decreased.Esters and heterocyclic compounds substitute for aldehydes,alcohols and ketones were constituted the main volatile,starting from 10-year of period.According to the storage periods of KBT,the taste characteristics were divided into 3 stages,including the period of1-5 years,the period of 10-15 years and the period of 22-30 years.The stage of the aroma profile included the period of 1-5 years,the period of 10-22 years and the period of 25-30years.The storage period of 10 years was critical point for the conversion of taste characteristics and aroma profile.The storage period of 10-22 years was the key period to form the stale flavour.The aroma and taste identified as nuty and sweety or herbal flavour by sensory evaluation had formed during the period of 25-30 years.Based on the integratively discrimination,the content-differential volatile were the critical elements to discriminate the period of 25-30 years from that of 1-22 years,including 5-ethyl-2-furaldehyde,5-hepten-2-ol,6-methyl-,5,9-undecadien-2-one,6,10-dimethyl-,（Z）-and alpha-ionone.3 of the rates of chemical components as the variables,were the most important elements to distinguish the period of 1-6 years from that of 15-22 years,including non-ester/ester catechins,GA/EGCG and GA/ester catechins.（2）Flavone glycosides and flavonol glycosides were the main differential metabolites in four storage periods of KBT,including 1-year of period,10-year of period,22-year of period and 30-year of period.Apigenin,luteolin and chrysoeriol were the main aglycone in kang brick teas,forming C-glycosides through a C-C bond or O-glycosides through a C-O bond.Galactose and rhamnoside were bound to the aglycone with kaempferol and myricetin.Glycosides of quercetin were determined with glucose and rutinose as glycone.The differential flavonoid metabolites gradually decreased in storage period of 1-10 years,storage period of 10-22 years and storage period of 22-30 years.During the storage period of1-10 years,the metabolites of flavanones,flavanones and flavanols significantly reduced,and flavanone glycosides and flavanol glycosides were also significantly reduced.The flavone glycosides and flavanol glycosides with rhamnosus,arabinose and glucuronic acid as glycone increased significantly during the storage period of 10-22 years.During the storage period of 22-30 years,the flavone glycosides with rhamnoside as glycone decreased significantly,and the baicalein-glucuronic acid increased significantly.Tectorigenin were the characteristic metabolites in 30-year of period.Tricin O-glucuronic acid just existed in the storage period of 1-6 years.The relative contents of tectochrysin and baicalein-7-O-glucuronide increased with the prolong storage period.（3）The abundance and diversity of the fungal community decreased with the prolong of the storage period,but the abundance and diversity of the fungal community increased in period of 30-year.The dominant fungus population evolves in differential periods.Aspergillus asperoides was the dominant fungus in 1-year of period and 6-year of period.Xeromyces bisporus turned to the dominant fungus during the storage period of 10-30 years.Malic acid played an important role in the succession of fungal community between the storage period of 1-10 years and the storage period of 10-30 years,causing p H value to drop.（4）KBT had the ability of scavenging free radicals and reducing iron ion.The antioxidant activities had identified as followed,DPPH·>ABTS·>FRAP.With prolong of the storage period,the reduction capacity and the ability to scavenge DPPH·had showed the given regularity,but the ability to scavenge ABTS·had no correlation with the storage years.The ability to reduce iron and scavenge DPPH·during the storage period of 15-22 years were significantly higher than that during the storage period of 1-10 years.And the ability of scavenging DPPH·for the 25-years of period was closely near to that of 10 years.The reduction capacity and the ability to scavenge DPPH·of 30-years of period was near to that of 15 years.Gallic acid and theabrownin were the main reasons for the differential antioxidant activity of KBT.（5）The near infrared spectrum information of KBT in 1-year of period,6-year of period and storage period of 25-30 years were obviously separated from that in other storage periods.The prediction models of DPPH,ABTS and FRAP EC50based on PLS algorithm were established.For ABTS prediction model,R and RMSECV was 0.98 and 0.03,respectively.R and RMSECV of prediction model for DPPH and FRAP were above 0.92 and0.15,respectively.The prediction models for DPPH and ABTS EC50were more effectively than that of FRAP.The quantitative prediction model of theabrownin and total soluble sugar was established based on full-spectrum PLS algorithm.When the number of principle components was 5 or 4,R of the prediction model for total soluble sugar and theabrownin was 0.93 and 0.78,respectively.RMSECV was 0.35 and 1.58,respectively. |
