Oxidation is a term which is commonly used in the context of tea production and tea culture. Oxidation is usually presented as the primary difference between black tea (which is fully oxidized) and green tea (which is unoxidized). This article explores a little bit of the biology of what is happening when tea oxidizes.

Oxidation vs. fermentation (Fermentation is NOT the correct word here)

Some tea sources, particularly older ones, use the term "fermentation" when talking about the process that converges tea leaves into black tea. This term is technically incorrect. Fermentation is a process through which sugars are converted to acids or alcohol, and it is the key process in production of beer and wine, as well as foods like yogurt and kimchi. It is also transported out in our muscle cells when short on oxygen.

What happens in tea is different; it is carried out out by yeast or other organizations, but by enzymes that exist in the tea leaf, and the key chemicals being converted are not sugars but rather polyphenols and other chemicals responsible for the flavor, aroma, and health properties of tea.

What is oxidation?

In a very general sense, the oxidation is one half of "reduction-oxidation" reactions, also called "redox reactions", a broad class of chemical reactions which includes any chemical reactions in which atoms, molecules, or ions gain or lose electrons.

In biology, redox reactions include both part of healthy function, and reactions associated with stress or decay. A few examples are respiration (cells burning sugar for fuel), oxidative stress or reaction with free radicals, and many types of metabolic processes that invelve conversion of one chemical to another, often catalyzed by enzymes.

What happens in tea?

The oxidation in the tea leaf is a reaction that changes numerous chemicals. Several things need to be in place for the oxidation to be carried out. The enzymes need to come into contact with the chemicals to be oxidized, and there needs to be enough moisture for there to be a reaction.

During this process, the antioxidants in tea, Catechins, became a new class of chemicals, Theaflavins, which are also antioxidants. The colors change from green to brown, and the aromas change from fresh leafy, vegetal and floral aromas, to the deer, richer aromas of malt or fruit characteristic of classic black teas.

Manipulating the oxidation process

Through how tea is processed, the natural oxidation can either be sped up or facilitated, or inhibited or halted. The most thorough way to halt the oxidation process is through heating.

Heating stops the oxidation of tea, because it denatures the enzymes that carry out the oxidation reactions. Enzymes are protein, and just like protein-containing foods like meat or egg causes the protein to change texture, heating enzymes to a certain point causes them to stop being able to catalyze their reactions.

Green tea is one type of tea produced in this way; by heating the leaf, through steaming (as with Japanese teas), or pan-firing or baking (with Chinese ones), the oxidation can be thoroughly halted. This is why green teas retain both their vibrant green color, and their fresh, vegetal and grassy aromas.

Black tea, on the other hand, is produced by encouraging oxidation. This is accomplished by bruising the leaf. Bruising the leaf breaks cell walls, causing the enzymes to come into contact with more of the total leaf, leading most of the chemicals in the leaf to oxidize. The tea is then heated after the leaf has darkened, stopping it from breaking down entirely.

Climate and oxidation

Just as humans can manipulate the oxidation process, the climate exerts its own influence on this process as well. Thin, dry air, especially at high altitudes, can cause the tea leaf to dry out (and then oxidation to stop) before the reactions have completed.

This is why Darjeeling first flush teas (harvested in the spring after only light rains, well before the peak of monsoon season, and grown and processed at high altitudes) are very light in color, whereas Assam first flush teas (grown and processed at a lower altitude where the air is thicker and more humid) or Darjeeling teas harvested in wetter seasons, have a darker color.

Other teasides black and green

White tea is the least processed type of tea, but it is not the least oxidized. White tea is typically only allowed to dry or naturally wither, or is very lightly heated to dry the leaf. Because it is not heated enough to denature the enzymes, it still oxidizes somewhat, but because it is not brewed like black tea, the oxidation only carries out partly. This is why white teas smell less like fresh vegetables or grass, and a little more like autumn leaves or dried flowers, and it is also why they have a more silvery, brownish, or pale green color, contrasting with the vibrant green of green teas .