Studies on the measurement of ascorbic acid and dehydroascorbic acid in tea plants

A spectrophotometer, O-2 electrode, and HPLC were used to measure ascorbic acid in green tea cultivated by hydroponics. In the spectrophotometry, ascorbic acid was analyzed by absorbance changes at 265 nm upon addition of ascorbate oxidase, and using an Oz electrode, ascorbic acid was estimated by O, uptake upon addition of ascorbate oxidase. The apparatus for HPLC was a Hitachi L-6000 model liquid chromatograph with a UV detector set at 244 nm, and separation was done on a Shimpack-CLC-NH2 column. The accuracy of these three methods gave 98.6% similar to 104.5% recoveries and 0.2% similar to 1.76% coefficients of variation. The measurement of ascorbic acid in green tea extracts was done using four methods including hydrazine, and equal values were obtained. Application of these three methods to analysis of ascorbic acid were rapid, simple, and equal to or more sensitive than the hydrazine method. After the treatment of dehydroascorbic acid with dithiothreitol, dehydroascorbic acid was estimated by HPLC. Dehydroascorbic acid was completely reduced with a ten-fold excess of dithiothreitol within 10 min at room temperature. This simple procedure was used in the analysis of dehydroascorbic acid in green tea extracts.