Effects Of UV-B On Formation Of Volatiles In Tea [Camellia Sinensis (L.) O. Kuntze.]

Tea plant is one of the most important crops for beverage. Tea aroma is one of the most important factors influencing the flavour and sensory quality of made tea. Tea aroma is getting more and more attention in the market. Quality and famous teas are more and more popular due to their good flavour. Alcoholic tea volatile compounds are a group of complex compounds among which more than 600 were identified, including some of alcohols, aldehydes, ketones, esters, acids and heterocyclic compounds substances etc. and they are considered to be great contributors to the floral aroma of tea. Owing to the difference in volatile composition and concentration, characteristics of tea flavour are diversed. The obtained studies revealed that the major volatile constituents of green tea were benzylalcohol, geraniol, pyridine, linalool etc., and oolong tea was rich in nerolidol, methyljasmante, indole, linalool, benzylalcohol, phenylethyl alchol, methyl, geraniol etc. with floral aroma.There are about 80 kinds of the alcoholic tea volatiles in the fresh tea leaves. Many of volatile compounds were formed during tea manufacturing process. Glycosides were considered to be major contributors to the formation of volatiles in made tea and the hydrolysis of glycosides were concerned by researchers. Large numbers of studies confirm that trpenoid and terpene alcoholic compounds were main volatiles in fresh tea leaf and they were in the non-volatile forms as glucoside bound with carbohydrates such as glucose. Further research showed that primeverosidase was the most important form of volatile precurdors and glucose was the next. There were two enzymes involved in the hydrolysis of volatile precursors ofβ-primeverosidase andβ-glucosidase into volatiles during tea processing. The genes encodingβ-primeverosidase andβ-glucosidase were isolated from tea plant and their expression profiles were investigated during the development of tea shoots.UV-B is a part of the solar radiation with wavelength of 280-320 nm. The UV-B has been attracted more attention than UV-A (320-380 nm) and UV-C (<280 nm) because it has great impact on macro molecules in organisms. UV-B induced the expression of genes related to accumulation of secondary metabolites such as carotenoids and flavonoids. The processes are considered to be actions of plant responses to UV-B stress. Our previous study showed that UV-B stimulated expression of genes related to biosynthesis of tea catechins. There have no been studies about the effects of UV-B on formation of tea volatiles and the expression of related genes. The present study aimed at revealing the characteristic volatile compounds of various teas and the effects of UV-B on formation of volatiles and the expression of related genes. The study will be interesting for developing new techniques to improve flavour of teas.1. Characteristic volatile compounds of various teas were investigated. Volatiles of samples of green tea, black tea and oolong tea were extracted by SDE method and analyzed by GC/MS. The study showed that there are marked differences in volatiles between different kinds of tea. Total concentration of volatiles was the highest in oolong tea (87.08-102.69μg / g), followed by black tea (50.41-65.70μg / g) and the least in green tea (22.24-22.51μg / g). The difference in tea volatiles between various kinds of tea was bigger than that between various origins of a same kind of tea. It suggested that tea processing had great influence on formation of tea volatiles and it is possible to improve tea flavour by improved processing techniques.2. Composition of volatiles in two tea cultivars, i.e., Wuniuzao and Xiangshanzao, were investigated. Although cultivar Xiangshanzao had lower total content of volatile compounds than Wuniuzao, the number of volatile compounds was more than Wuniuzao and various volatile components were more balanced in cultivar Xiangshanzao. Sensory evaluation showed that cultivar Xiangshanzao had better preference than cultivar Wuniuzao. Tea sample made from Xiangshanzao had no "over-fired" flavour. This shows that the balance of volatile compounds is important for the tea quality of the sensory response. There was no direct relationship between volatile level and sensory score. In studies of chemical evaluation, the level of volatiles, volatile composition and individual volatile threshold value should be considered so as to make an objective evaluation.3. Effect of UV-B on volatile formation was studied. Appropriate dosage of UV-B stimulated the release of volatile compounds from their processors during which the expressions of TBG and TBP genes were promoted. However, excessive irradiation of UV-B inhibited the expression of these two genes, resulting in decrease in the concentration of a volatiles and the number of volatile species. It was considered that during UV-B irradiation, the release of new volatiles and degradation of volatiles existed in the leaf took place coinstantaneously. At early stage of UV-B irradiation. the rates of release of new volatiles were quicker than the degradation of volatiles, which resulting the increase in total concentration of voltiles and the number of volatile species. But in the late stage of UV-B irradiation, the release of new volatiles were less than the degradation of volatiles, resulting the decrease in total concentration of voltiles and the number of volatile species. The study revealed that UV-B can be used to improve tea flavour if irradiation dosage is appropriate.