The science of tea

All tea is made from the leaves of the tea plant, Camellia sinensis. Six main types of tea are produced – white, yellow, green, oolong, black and post-fermented. Each type of tea has a unique aroma, taste and visual appearance. In Western countries, green and black tea are more commonly known. The distinctive flavours of these teas are created by their different chemical compositions.

Six types of tea

As the six main types of tea are from the leaves of the same plant, the key to the chemical and taste differences is found in the processing – in particular, in the steps of oxidation, stopping the oxidation, forming the tea and drying it.

Chemicals in tea

Tea contains lots of different chemicals, but the main ones associated with the taste, aroma and health effects of tea are polyphenols – mainly flavonoids. The plant produces these chemicals not for our benefit but to help the plant fight against predators and stress.

Polyphenols make up approximately 30% of the dry weight of the leaves of the tea plant. Other chemicals that contribute to the flavour and effects of tea (but to a lesser degree) include caffeine and amino acids, mainly theanine.

The chemical composition is different in each type of tea. This is because of chemical changes during processing of the fresh leaves, mainly caused by oxidation. Variations in the processing steps affect the degree of oxidation and create the chemical differences in each type of tea. The chemical composition of tea may also be affected, although to a lesser degree, by the environment, the time of harvesting and which leaves are used, farming practices and effects of pests and disease.

Oxidation changes chemical composition

The polyphenols in the tea leaves are stable as long as the leaves remain on the live plant. The process of oxidation starts naturally as soon as the leaves are picked (much like an apple starting to go brown once it has been cut). This reaction is caused by the release of the enzyme polyphenol oxidase in the leaves.

Oxidation converts the polyphenols into new compounds, mainly theaflavins and thearubigins. Applying heat during processing stops the oxidation process and the enzyme activity. By controlling the degree of oxidation, the tea-maker creates the distinctive flavour and chemical profile of their tea.

Green, white and yellow teas are subject to very little oxidation because they are heated soon after picking. The polyphenol content of these teas is therefore very similar to that of the fresh leaves, the brewed tea liquid is a light yellow or yellow-green colour and they have a mild flavour.

In contrast, for black tea, the leaves are cut and bruised. This disrupts the cell structure and allows all the leaf juices (containing polyphenols) and enzymes to mix together, allowing complete oxidation. As a result, black tea contains very few polyphenols as they are mostly converted to theaflavins and thearubigins. Thearubigins give black tea its distinct red-brown colour and stronger flavour.

Oolong tea falls in between green and black tea as it is semi-oxidised. During processing, it is allowed to oxidise for a short period before being exposed to heat, and therefore it retains a higher polyphenol content than black tea. Oolong generally has a more complex flavour than white and green tea but is not as strong a taste as black tea.

Post-fermented tea undergoes a period of ageing from several months to many years. During ageing, the tea is allowed to slowly oxidise and undergo microbial fermentation by controlling moisture and temperature. This process alters the smell of the tea and mellows the taste compared to the more astringent black tea. Post-fermented tea is usually sold in compressed shapes, such as spheres or bricks.

The health benefits of tea

Tea is one of the most popular beverages in the world, having been consumed for thousands of years. It has long been associated with health benefits, and studies suggest it may protect against heart disease, allergies, inflammatory diseases and some cancers. Many of the health benefits are linked to the high levels of polyphenols, which have antioxidant properties. However, further clinical research is needed to confirm the benefits.

The small amounts of caffeine in tea are associated with mental alertness. Theanine, an amino acid found only in tea leaves, is thought to enhance relaxation.

The perfect cup of tea

How to brew tea is a matter of personal choice – there is no one ‘right’ way. However, there are many customs and rituals associated with drinking tea that have developed in different cultures around the world.