shutterstock_19620757 copy 2

Fermentation Vs. Oxidation

Written By

Perhaps while browsing through various tea sites, you’ve come across the words “oxidized” and “fermented”. The uninitiated tea drinker might not know what to make of them. Understanding the difference can be confusing, especially since they seem to be used interchangeably.

The more tea-savvy, however, can use these classifications to discover which companies really know their tea, and which are simply spreading misinformation. In this article, we’ll discuss the difference between the two and which teas fall into what category. Knowing the difference for yourself can help you to avoid getting hoodwinked by second-rate tea distributors.


This term explains the most common form of preparation for tea leaves, and is correct in most cases. Once a tea leaf has been plucked, it begins to wither and become oxidized. Certain enzymes in the cells of the leaves react to the oxygen in the air. This breaks down chlorophyll in the leaves, causing them to darken and changing their shape and flavor. Heating the leaves can interrupt the process, allowing tea growers to control how much oxidation the leaves experience. The amount of oxidation is one of the primary factors that determines what type of tea is made with the leaves.

milosk50 /


Though some people will use the term fermentation to refer to most teas, what they actually mean is oxidation. Fermentation occurs when tea leaves are left in large piles for extended periods. Microbes begin to alter the chemicals in the leaves, similar to what happens in the making of yogurt, cheese and wine. Tea leaves that have undergone the fermentation process are called “post-fermented teas”. A post-fermented tea may sometimes be oxidized, but an oxidized tea is rarely fermented.

Learning the Tea Types

It’s actually quite easy to remember the tea types once you’ve had a bit of practice. Here are the five most common tea types and what makes them unique.

Green Tea

Green tea leaves are the least oxidized of all teas. Shortly after picking, the leaves are either steamed or pan-fired to preserve their fresh, natural flavor. This process is called “killing the green” (or kill-green). It is why the leaves maintain their rich green shade.


Black Tea

shutterstock_143332633 copy

The opposite of a green tea, black tea leaves are allowed to completely oxidize before they are processed. The leaves turn dark and curl, and they present a deep red brew with a strong flavor. Only western tea drinkers call these “black teas” for their leaves; Asian cultures refer to them as “red teas” because of the color of the tea after steeping.

White Tea

This class of tea is often mistakenly called the least oxidized, but is actually the least processed. White tea leaves are plucked from the youngest tea leaves and buds, giving a lighter color. They are then allowed to air dry for one-to-three days before they are lightly heated. The resulting tea has a light, fresh and often sweet taste.

shutterstock_59808529 copy

Oolong Tea

Beware sources that call this a “partially fermented” tea, as this definition is wrong on two counts. First, it is impossible for a tea to be partially fermented; once the fermentation process begins, all tea leaves from that batch are post-fermented.

Second, oolong tea leaves are almost never fermented, but they are partially oxidized. The leaves are allowed to oxidize until they have begun to wither, but are heated before they become fully oxidized. The result is a tea somewhere between a green tea and a black tea. The amount of oxidation will vary depending on the type of oolong, whether it is a light oolong or a dark oolong.


This tea variety is the only one that is always post-fermented. There are two classes of pu-erh: raw (green, or sheng) and ripe (black, or shou). These are not the same as the green or black teas mentioned above, but they explain how the tea was prepared.

Both kinds of pu-erh undergo withering, kill-green, rolling, drying, steaming and shaping. From there, the ripe tea leaves will undergo an extra cooking stage, while the raw leaves go straight to fermentation. Both teas are piled up and left to ferment for extended periods, then are packed up

shutterstock_260877782 copy

and aged. After at least three months of aging, the teas are either sold in loose-leaf form, or pressed into various shapes.

These teas are prized for their deep, smooth and complex taste. The flavor truly begins to mature after aging for at least five years, and a well-aged pu-erh cake is highly prized.

Other Fermented Teas

Aside from pu-erh, which is always fermented, it is possible to ferment other types of teas. Since pu-erh tea leaves are not oxidized, tea leaves that are oxidized, then fermented will belong to a different category. Partially-oxidized leaves would become a fermented (or aged) oolong, and fully oxidized leaves would be a fermented black tea.

This may seem confusing, but most tea companies will explain how the tea has been processed to clear up any confusion. Because of their rarity, these teas also tend to be more expensive than standard oolong or black teas. If you see a “partially-fermented oolong” priced the same as other oolong teas, it’s typically a good indicator that this tea is not actually fermented at all.

Choose Your Teas Wisely

To be fair, not every source that uses these terms incorrectly is automatically a bad source. It is possible that some companies, especially those in Asia, simply suffer from mistranslations. Others, however, may learn from such mistranslations and continue to repeat them – an indicator of how little they know about their own product. Once you know which teas are which, you can use this knowledge to help you discern which sources are valid, and where you’re likely to get the best cup of tea.


Still confused about fermentation and oxidation? Feel free to ask about it in the comments below!