HPLC quantification of the dihydrochalcones, aspalathin and nothofagin in rooibos tea (Aspalathus linearis) as affected by processing

An HPLC method was developed for determination of the C-glucoside dihydrochalcones, aspalathin and nothofagin, in rooibos tea. Gradient separation of the phenolic fraction was achieved on a reversed-phase C18 column. The polyphenolic fraction was prepared by extraction of the phenolic compounds with hot water, followed by liquid-liquid extraction with ethyl acetate. The method was applied to unprocessed, partially oxidized (unfermented) and fermented rooibos tea as well as tea dried in the sun and under controlled conditions. Aspalathin and nothofagin oxidation occurred as soon as the tea leaves were comminuted which resulted in browning as indicated by CIELAB parameters. The drying method had no effect on the degree of oxidation of aspalathin and nothofagin.