Fermented Tea Vs. Oxidized Tea
Fermentation and Oxidation; what do these two words really mean in the world of tea? What is the difference? In order to thoroughly understand, we must be aware of the process of manufacturing tea, which consists of a series of integrated steps that begins with the freshly plucked tea leaves and ends with the finished product that we see on the store shelves.
In the world of tea, oxidation has been the divider of the two main categories of tea for its drinkers. In fact, oxidation is the chemical process that must occur in the manufacture of several of the classes of tea and must be prevented in others. At this point, you may be asking: “What does oxidation really mean?” According to Dr. Robert J Heiss, "Oxidation is a biochemical, enzymatic activity during which oxygen is absorbed by and subsequently causes changes to the host physical matter." In our case the physical matter is the tea leaf. Oxidation can be spontaneous or controlled. During the manufacture of tea, both types of oxidation occur. Spontaneous oxidation occurs during the withering phase of the manufacture of white, oolong, and black teas. While green and yellow teas are prevented from oxidizing by meticulous steaming, drying and/or frying techniques often called “de-enzyming”.
When considering black tea, one thing that to keep in mind is that after several preliminary steps of the manufacture, prepped leaf is ready for the controlled oxidation process that is often incorrectly referred to as “fermentation.” Fermentation is a microbial activity involving one or more types of bacteria. By definition, fermentation occurs most readily in absence of oxygen. Fermentation is an important component in tea but only in a few classes, such as Pu-erh Tea. That is why any discussion of fermentation in tea manufacture should ideally focus on the manufacture of Pu-erh Tea.
Now that we have a better understanding of what Fermentation and Oxidation are, let us classify the six major classes of tea.
Green Tea – No Oxidation
Yellow Tea – No Oxidation
White Tea – Slight, spontaneous oxidation (8-15%)
Oolong Tea – Partial oxidation, controlled by a tea master (15-80%)
Black Tea – Fully oxidized, controlled by a tea master.
Pu-erh Tea – Always fermented, not always oxidized.
Further Reading: Oxidation and Fermentation In Tea Manufacture by Dr. Robert J. Heiss