Polyphenols & Antioxidants – The Chemistry of Tea

After looking at the chemistry of coffee in the previous post, it seemed only fair to also consider the chemistry of tea, just so all the tea drinkers out there don’t feel left out. Much like coffee, tea contains a hugely wide variety of chemical compounds, but some of the most important in terms of its taste and colouration are the polyphenols.

Black tea is produced by the full oxidation of tea leaves, which are then dried. Compounds from the flavonoids family called catechins make up 27% of the composition of unoxidised green tea; due to the oxidation process, this is reduced in black tea to around 4%. The oxidation products of catechins in black tea are polyphenols, which influence both its colour and flavour.

The polyphenols in black tea can be split into a wide range of categories. One of the most important and well categorised of these are the theaflavins, whilst the thearubigens are a large group of polymeric polyphenols whose specific structures are largely unknown to this point. Both of these groups contribute towards the orange-red colouration of tea, as well as to the taste.

The tea polyphenols have been reported in various research to exhibit antioxidant properties. Antioxidants have been touted as having a range of health benefits, but the scientific evidence for these is still a little vague in parts. Studies have shown that antioxidants can protect cells from damage as a result of free radicals – molecules with an unpaired electron – but the results of some longer term trials have been inconclusive as to their efficacy, particularly in cancer treatments. Meanwhile, other studies have indicated an inverse relationship between tea consumption and cardiovascular disease.

Irrespective of their debated health benefits, there are also conflicting opinions as to the effect adding milk to tea has on the antioxidants contained therein. Studies have indicated that the casein proteins in milk can bind to the polyphenols in black tea and form complexes. Whilst some research has suggested that this could impact on the amounts of antioxidants gained by drinking tea, others have suggested that this does not actually affect the concentration of antioxidants in the blood after ingestion, as the protein-polyphenol complexes can be broken down during digestion.

There’s a wide range of literature out there on the variety of chemical compounds found in tea and their chemistry – if this has piqued your interest, you can read more via the links supplied below. To download a PDF version of the graphic itself, click here.

Edit: As an interesting side note, the stimulant effects of coffee and tea differ slightly due to the presence of a unique amino acid in tea, L-Theanine. Studies have shown that this compound can affect aspects of brain function in humans, relaxing the mind without bringing on drowsiness. Further research has indicated that the combination of L-Theanine and caffeine can improve speed, performance and accuracy in cognitively demanding tasks – put simply, L-Theanine ‘smooths out’ the stimulating effects of caffeine. Thanks to Andrew Welleford for the pointer on this one!

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References & Further Reading

• ‘Black tea: chemical analysis & stability’ – S. Li et al, Food & Function 2013.
• ‘Tea chemistry’ – M. Harbowy & D. Balentine, Critical Reviews in Plant Sciences 1997.