| Green tea has recently received much attention due to its high content of flavanols (flavan-3-ols, catechins) which are recognized for their beneficial pharmaceutical functions. In order to increase the consumption of green tea and to meet the consumers' demand, food manufacturers are now packaging green tea extracts in cans or bottles. However, the commercial canned green tea beverage being produced presently does not maintain the sensory qualities of fresh extracts, and this obstructs the success of canned green tea products in the soft drink market. Therefore, new information for the production of high-quality canned green tea beverage is needed. The objective of this research is to investigate the effects of processing and storage on the qualities of green tea extracts.; Fresh tea leaves, immediately after harvest, were processed into commercial steamed and roasted green teas. After being extracted with 80°C distilled water for 4 minutes (a 160-fold dilution, w/w), green tea extracts were filtered and processed at 121°C for 1 minute, and then stored at 50°C for 12 days. Samples of fresh, processed, and stored tea extracts were measured for color, evaluated for sensory quality, and analyzed for chemical compositions.; Steamed green tea was found to be more suitable for the production of canned green tea beverage than roasted tea, because steamed tea extract was more stable to color and taste changes during processing and storage. Green tea extracts are sensitive to the heat treatment. During processing and storage, their color gradually turned dark brown, and the taste lost the characteristic bitterness and astringency. Chemical analyses showed that flavanols, especially gallo-flavanols, played a key role in the quality changes of green tea extracts. Flavanol epimerization caused by high heat treatment resulted in the increase of catechins which are structurally more stable than their epimers, epicatechins. Three compounds in the ethyl-acetate insoluble fraction resolved by HPLC were found to be oxidation products with two of them having similar chemical structures, according to the HPLC/photodiodearray assay. The colored oxidation products were bound strongly to the support material of HPLC column and produced convex broad bands on the baseline of HPLC chromatograms. Among the five antibrowning agents tested, citric acid performed well in both antibrowning and antioxidant effects on the green tea extracts, and its commercial application in the production of canned green tea beverage will be further evaluated. |
