Characterisation of polyphenols in green, oolong, and black teas, and in coffee, using cyclic voltammetry

Phenolic compounds are important for their astringent taste and as antioxidants in health. Green, oolong and black tea and an instant coffee were each diluted 50 times in a pH 7.0 phosphate buffer and, along with phenolic standards, were analysed by cyclic voltammetry at a carbon electrode. Standards with a pyrogallol group (gallocatechins) were strong reducing agents and produced a peak at 90–120 mV, while those with a catechol or gallate group peaked at 180–220 mV. The response of green and oolong teas was dominated by epigallocatechin gallate, and levels determined by HPLC were consistent with the electrochemical response. Black tea behaved like a theaflavin extract, and coffee like 5-O-caffeoylquinic acid, both with peaks at around 230 mV. The level of phenolics increased with water temperature from 20 to 100 °C, while lower levels were obtained for repeat infusions. The addition of milk (skim or full-fat alike) lowered the response more in the teas than in the coffee.