| Summary: | 碩士 === 國立臺灣大學 === 園藝學研究所 === 99 === The chemical composition of tea leaves may be effected by cultivars, geographical and climatic conditions, agronomic practice, and manufacture process. Ultraviolet radiation, a high energy radiation of light rays, is one of many environmental factors. When plants are put under ultraviolet stress, they are subject to UV radiation response, increasing plants’ UV-absorption secondary metabolites, which act as a filter, protect against UV damage.
Polyphenols are the most soluble substances in tea, accounting for 30% of dry weight. Catechins are the most abundant group of polyphenols. Since health foods have become popular in recent years, scholars are interestied in the biological effects of catechin. There are many papers proving that catechins have antioxidants, and prevent cardiovascular disease and cancer. Therefore, this study used ultraviolet radiation treatment on tea, to increase the catechins content in tea.
In the first part, ‘TTES No. 12’ and ‘Ching Shin Oolong’ tea trees were used as material. Samples of each were placed in three separate temperature-controlled chambers. The first chamber was treated with UV-A, the second with UV-B, and the last was the control. The treatment time was one day, five days, one week and two weeks. The results found that the UV treatment affected the catechins content of tea leaves. After one day of UV-A treatment, epicatechin content increased both in ‘TTES No. 12’ and ‘Ching Shin Oolong’ tea leaves. However, the trend of other catechins content was not consistent. In the UV-B treatment, the epicatechin (EC) content increased in only the ‘Ching Shin Oolong’ tea leaves after one day. Regardless of the length of treatment time, the catechins content has a decreasing trend in both varieties of tea leaves after UV-B treatment.
Using UV radiation in the field is inconvenient. In the second part of this study, fresh tea leaves were used as treatment material, to see whether the catechins content of tea leaves will change by UV radiation after harvest. Three different wavelengths of UV radiation, UV-A, UV-B and UV-C, were used in this postharvest treatment. Fresh ‘TTES No. 12’ leaves’ catechins content was lower than control after 10, 20, 30, 60, 90 and 120 minutes of treatment by UV-B. But in the UV-A and UV-C experiment, the catechins content was reduced in the beginning, and then increased after a longer treatment time. This result may occur because of the absorption peak of catechins, so the response to UV-B was more obvious than to UV-A and UV-C.
The result was not as expected. After UV treatment, catechins content of tea was reduced. However, the content of catechins in fresh tea leaves was significantly reduced by treating them with UV-B after harvest. This may be used in the green tea industry. Green tea is unfermented tea, so catechins will increase the bitter and astringent taste in tea. Before blanching, treating fresh tea leaves with UV-B may reduce the content of catechins in tea leaves and improve its quality. If this strategy works, it should be developed into a green tea process.
|
|---|
