Tea Terms: Fermentation vs. Oxidation

Black Tea - fully oxidized
Black Tea - fully oxidized

Among the more tricky issues when it comes to writing about tea, or for that matter, when it comes to being a tea lover in general, is that there are many popular myths that have sprung up about tea — not to mention the fact that the terminology of tea is not always as exact as it could be. The latter is an issue that I explored recently in an article that also appeared at this site.

One of the semantic issues when it comes to describing how tea is processed is the question of what is fermentation and what is oxidation. Though the former term is typically used to refer to the process black and oolong teas undergo to make them what they are, that process should more correctly be referred to as oxidation. Black tea is known as a fully oxidized tea, while oolong is typically referred to as semi-oxidized.

Of the six processing stages that black tea undergoes, oxidation is third, after withering and rolling. During oxidation, tea leaves are spread out in layers in a temperature and humidity controlled room, which initiates a chemical process that breaks down various compounds in the leaves. Among the results of this process are thearubigins and theaflavins, compounds which give black tea its distinctive taste and which cause the leaves to darken.

Oolong tea - semi-oxidized
Oolong tea - semi-oxidized

In the case of oolong tea, the process is interrupted at some point before the leaves are fully oxidized, with the exact amount of oxidation dependent on the variety of oolong being processed. These can range from very light varieties that are close to green tea all the way to heavily processed oolongs that are just a step or two removed from a proper black tea.

Outside of black and oolong, fermentation does actually have a place in the glossary of tea, specifically when used to describe the processing of the type of tea known as puerh (insert your favorite alternate spelling here). There are essentially two different types of puerh, one of which is a green or “raw” variety that is compressed into cakes or bricks and ferments naturally. The other may be known as black or “cooked” puerh and it’s a type in which the fermentation process is accelerated.

For a more in-depth discussion of oxidation and fermentation, as it relates to tea, look here.

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5 thoughts on “Tea Terms: Fermentation vs. Oxidation

  1. Pingback: St. Patrick and Green Tea | Tea Blog

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  4. THANK YOU so much for clarifying this. With this explanation, it’s easy to see how people get confused over the whole thing.

    And it makes sense, too, that fermentation is what gives puerh its earthy, almost musty flavor.

  5. Pingback: St. Patrick’s Day Tea Time Goes Green! « Tea Blog

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