Oxidation of Tea

Wikipedia: Tea_processing | WikiCHA: Oxidation
Last Updated: Mar. 26, 2015
Green and black teaOxidation is the main factor distinguishing black from green tea.
Oxidation is a natural process that is important in tea production. In the past, oxidation was often inaccurately called fermentation. Although the term fermentation was in widespread use for a long time, it is misleading as there is no true fermentation (involving yeast or other microorganisms) happening when tea oxidizes. Varying levels of oxidation are one of the main characteristics that distinguish black tea from green tea.

What is oxidation?

In a very general sense, oxidation is a chemical process in which atoms lose electrons. Oxidation reactions include such diverse reactions as the burning of fuel, the rusting of metal, and numerous biological reactions. Oxidation can include both normal, healthy biochemical reactions, such as the metabolism of energy sources, and reactions associated with stress or decay, such as oxidative stress.

Oxidation in tea production

A photo, over 100 years old, of tea being steamed to stop the oxidation process.
The oxidation of tea is a key step in the production of tea that radically alters the flavor, appearance, and chemical composition of the tea leaf, and that is instrumental in distinguishing the different types of tea from each other.

Oxidized teas, such as black tea and oolong, are processed in a way that bruises the leaf, breaking cell walls and enabling enzymes in the leaves to cause natural oxidation reactions. These reactions are either allowed to carry out to completion, as in the case of most black tea, or are stopped by heating, as in the case of partially-oxidized oolongs. Unoxidized teas, like green tea, are heated earlier in the production process, denaturing the enzymes in the leaf that cause oxidation before the leaves are able to oxidize.

Teas that are not bruised in this manner may still undergo some oxidation, during the time period in between when they are harvested, and when they are heated. Yellow teas, for instance, are allowed to sit while damp, and the yellowing corresponds to a mild oxidation. White teas also oxidize, because their minimal processing does not halt the oxidation process the way the heating does in the case of green teas.

The oxidation process results in profound changes in the flavor, aroma, color, and chemical composition of the tea. As a general rule, tea tends to become darker in color as it oxidizes, due to the presence of tannins, dark chemicals absent in unoxidized tea. It is hard to generalize about flavor: both oxidized and unoxidized teas alike can be either mild or strong, bitter or smooth. However, the aroma of teas changes dramatically. Unoxidized and less oxidized teas often tend to have more grassy, vegetal, and herbaceous aromas, similar to those of fresh leaves, as the chemical composition of these teas are closer to those of fresh leaves.

Tea, like most fresh foods from plant-based sources, is rich in antioxidants, chemicals which often inhibit or serve as a buffer against oxidative stress. When tea is oxidized during production, the antioxidants are transformed but not destroyed, although the Vitamin C (which is one type of antioxidant) does tend to be destroyed. The page on antioxidants in tea explains this in more depth.

Tea types and their level of oxidation

The oversimplified explanation of the oxidation of tea that is usually given is that black tea is fully oxidized, whereas green tea is unoxidized, and oolong tea is partially oxidized or semi-oxidized. However, this explanation is not strictly true for all examples of these teas.

Also, the different classes of teas are not fully characterized by their oxidation level. That is, there are many semi-oxidized teas that are not oolongs, fully-oxidized teas that are not black tea, and unoxidized teas that are not green teas. Although level of oxidation is an important quality in distinguishing the different classes of tea from each other, we classify tea primarily based on its production method and not strictly on its oxidation level.

Measuring oxidation and subjective descriptions of oxidation level

It is possible to quantify the oxidation of teas by measuring the portion of various unoxidized chemicals and their oxidized counterparts. For example, catechins become theaflavins upon oxidation. However, because laboratory studies are costly, tea companies selling semi-oxidized teas usually make a subjective estimate of oxidation level of their teas based on color and flavor, or what is known of their processing, such as claiming that a particular oolong tea is 30% oxidized or 70% oxidized. These claims are usually arbitrary estimates, but can be good coarse indicators of the character of a particular tea.

Oxidation of herbal teas:

Rooibos(Red) rooibos is allowed to oxidize in its production, much like how black tea is oxidized.
Most herbal teas are not processed in the more involved, multi-step processes by which tea is processed; typically, herbal teas are produced by harvesting and then drying the herbs. There are, however, a few exceptions, in which herbs are processed in ways analagous to the way teas are processed.

The two most common examples of herbal teas allowed to oxidize like tea are rooibos and honeybush, both of which originate in South Africa. Both of these herbs are traditionally processed by methods that allow them to fully oxidize, turning them a rich red color. Both of these herbs can also be processed in ways inspired by green tea production, heating them to prevent oxidation. This leads to green rooibos and green honeybush, which shares some characteristics in common with green tea.

Further reading on the oxidation of tea:

Oxidation (fermentation), an article by the Tea Research Association.


What is Oxidation? by Tony Gebely of Chicago Tea Garden.


Oxidation of Tea 101 by Pei of Teanamu.


Oxidized or Fermented Tea? by Adam Yusko on the English Tea Store Blog.


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