Tea leaves are rich in a group of chemicals known as polyphenols that amazingly account for almost one-third of the weight of the dried leaf. Both the colour of the tea and much of its taste are due to these compounds.

One group of polyphenols, the thearubigins, are the red-brown pigments found in black tea and constitute between 7 per cent and 20 per cent of the weight of dried black tea.

The colour of black tea is also influenced by the concentration of hydrogen ions in the water. Thearubigins in tea are weakly ionising acids and the anions (negatively charged ions) they produce are highly coloured. If the water used to brew tea is alkaline, the colour of the tea will be deeper due to greater ionisation of the thearubigins.

If lemon juice, which is an acid, is added to the tea, the hydrogen ions suppress the ionisation of thearubigins, and that makes the tea lighter.

Interestingly, the theaflavins - the yellow-coloured polyphenols - in black tea are not involved in the change in colour that is associated with a change in acidity.

Johan Uys , Bellville, South Africa