Effects Of Roasting Process On The Quality-related Components Of Steamed Green Tea

Roasted green tea with unique roasty aroma and mellow taste,made from steamed green tea through roasting process,is becoming popular these days,which also can be used as the ingredient of tea drinks and various foods with the great market potential.Since the taste substances and aroma compounds of roasted green teas are affected by roasting process,there is still lack of relevant studies.Steamed green tea were used as raw materials for making different types of roasted green tea samples based on different roasting methods(far-infrared irradiation and drum roasting)and different roasting degrees(light,medium and heavy).The color,the contents of major flavor components as well as volatile compounds were measured.The aroma of roasted green tea samples were evaluated by quantitative descriptive analysis.A bioinformatic statistic method called Weighted Gene Co-Expression Network Analysis(WGCNA)was employed to explore the relationship between volatile compounds and aroma attributes.This study gives the basic information on the impacts of roasting treatments on the major quality-related substances of roasted green teas and the formation of characteristic aroma.This study provides a new idea for tea flavor study that establishing the correlations between chemical compounds and sensory evaluation by statistic method.The achieved major results are as follows:(1)The impacts of different roasting treatments on the color of dry tea and tea infusions of roasted green teas were investigated by color difference analysis.As the roasting degree increased,the dry tea samples turned from dark green to yellowish-brown under roasting treatments,and the value of total color difference ?E increased,suggesting that greater difference was observed between roasted green tea samples and steamed green tea(control).At the same roasting degree,the color of drum roasted samples was darker than that of far-infrared irradiated samples.Both far-infrared irradiation and drum roasting treatment reduced the brightness value L of tea infusion and increased the a value representing redness.Besides,the darkening effect of drum roasting treatment on tea infusion is stronger than that of far-infrared irradiation treatment.(2)The impacts of different roasting treatments on the contents of major flavor substances were investigated.The results showed that the contents of total free amino acids were greatly reduced due to roasting treatments,and the maintaining rates of total free amino acids were 61.9% and 45.9% for far-infrared irradiated and drum roasted samples at heavy degree.Drum roasting treatment significantly reduced the content of polyphenols.The maintaining rate of polyphenols was 77.5% for drum roasted sample at heavy degree.However,far-infrared irradiation treatment barely impacted the content of polyphenols in tea samples.Caffeine was stable during roasting treatment.Far-infrared irradiation at heavy degree and all the drum roasting treatments significantly reduced the contents of catechins and flavonol glycosides of roasted tea samples,corresponding to 57.9-165.8 mg/g of total catechins and2548.6-5798.3 ?g/g of total flavonol glycosides,which were significantly lower than those of control(186.1 mg/g of total catechins and 6077.4 ?g/g of total flavonol glycosides).Roasting treatment greatly increased the content of non-epi types of catechins but reduced the contents of other types of catechins.All types of flavonol glycosides declined during roasting treatments.The principal component analysis results showed that the different roasted tea samples were clearly distinguished based on the content of catechins and flavonol glycosides.(3)The impacts of roasting treatments on the contents of volatiles in roasted green teas were analyzed by GC-MS.A total of 105 volatile compounds were determined,and alcohols,aldehydes and ketones were the main volatile compounds for steamed green tea.As the roasting degree increased,the content of total volatiles increased and the contents and proportions of furans,pyrroles and pyrazines largely increased.Besides,far-infrared irradiation was more effective to preserve or produce alcohols meanwhile reduced the generation of furans compared with drum roasting.(4)A quantitative descriptive analysis was carried out to evaluate the aroma profiles of different roasted tea samples,including eight aroma attributes of fresh and brisk,tend,clean and lasting,sweet,sufficient fired,burnt,grassy and fried rice.The results showed that far-infrared irradiation endowed the roasted tea with distinct nutty aroma,while drum roasted teas were characterized with sufficient fired and burnt aroma features.(5)WGCNA was used to analyze the relationship between volatile compounds and aroma attributes.A total of 56 volatile compounds were screened from 105 volatiles,which had positive correlations with the aroma attributes of fried rice,sufficient fired,burnt while had negative correlations with the aroma attributes of grassy,fresh and brisk,as well as tend.Far-infrared irradiated sample at heavy degree had the highest contents of benzyl alcohol,2,5-dimethyl-pyrazine and hotrienol,while drum roasted teas contained relatively higher levels of(E,E)-2,4-heptadienal,3,5-octadien-2-one,furfural,1-ethyl-1H-pyrrole,methyl salicylate,1-ethyl-1H-pyrrole-2-carboxaldehyde and(E,E)-2,4-octadienal.This might be related with different characteristic aroma features of roasted green tea samples.