The oxidation process in tea and its effect on flavor

When one hears the word “oxidation,” it may call to mind rot, degradation, and that russet-colored incrustation on the fenders of cars. However, you might be surprised to know that this same process of oxidation plays an important role in the production of tea—in fact, it is arguably the most important determinant of its overall flavour.

Tea contains substances called polyphenols—also known as tannins—which are chemical compounds found in many plants. These tannins are also found in wine, and are instrumental in influencing the overall flavour of the tea. As with any other plant, as soon as the tea leaf is plucked, it immediately begins a slow process of deterioration. As the cellular structure of the leaf deteriorates, an enzyme called oxidase is released (this deterioration can occur naturally due to age, or intentionally during the production process, where the leaf may undergo “rolling” and/or “bruising” in order to accelerate the release of the oxidase enzyme). When oxidase comes into contact with the latent oxygen in air, it begins to break down the catechins (a type of tannin) in tea at the cellular level. How long this process is allowed to continue determines what “type” of tea is produced; in particular, the oxidation process determines how specific chemical aroma and flavour groups are expressed. At certain levels of oxidation, unique smells and tastes are produced, and at other levels they are destroyed and replaced with new ones. The oxidation process is stopped at the desired level during production by “firing” the tea leaves—that is, heating them in a manner that does not burn or destroy the leaves, but heats them to a sufficient temperature to destroy the oxidase enzyme and arrest the oxidation process.

All tea begins its life as white tea (which is not oxidized at all, nor processed much) but most teas are subjected to processing which changes them into something else. Tea that is 100% oxidized is known as “black tea”. This is the tea most of us are familiar with, and the type found in most teabags. Tea that is partially oxidized is known as “Wu Long” or “Oolong” tea, and is originally a Chinese specialty (although Taiwan produces some of the world’s finest). Chinese Oolongs tend to be about 10-30% fermented, while some, particularly those of Taiwanese origin, are about 60-70% oxidized. Green tea is tea that has been “fired” before the process of oxidation could begin, and as such retains most of the qualities of the freshly plucked leaf. For all practical purposes, green tea is 0% oxidized.

A cup of tea is the product of a complex chemical process, but at its heart is the process of oxidation. Through oxidation, the chemical compounds present in tea can give rise to all kinds of unique and wonderful flavours and aromas—from the burnt notes of a Tie Guan Yin oolong to the vegetal freshness of a Lung Ching green tea. The next time you hear the word “oxidation,” don’t think of decay, think of creation: namely, the wonderful variety of unique flavours and aromas it produces in tea!