Oxidation vs Fermentation of Tea

Oxidation is a process in which tea leaves are exposed to oxygen so that an enzymatic reaction can occur. Tea leaves that are fermented are instead exposed to microbes that change the composition of the leaves.

It is these two processes that make tea types different from each other despite coming from the same plant. Read on to discover more about these processes, as well as the various health benefits of oxidation.

What is oxidation?

Whether you’ve known what it is or not, you’ve likely seen oxidation in action before. For example, the Statue of Liberty is green because of oxidation. It’s also what turns the inside of an apple brown.

Broadly speaking, oxidation is when an atom loses an electron. In tea, catechins are processed and converted into other polyphenols such as theaflavin.

To put it differently, a specific polyphenol (polyphenol oxidase) reacts with oxygen to darken the color of the leaves. It’s also the process that changes the flavor, chemical composition, and strength.

This process was not fully understood in the early years of tea making. Today, the process is highly controlled to create specific types of tea. 

What is fermentation?

Despite usually being thought of only in terms of alcohol, fermentation is also used to make tea. However, it’s only used to make certain types.

Fermentation in tea leaves requires the presence of microbes that will consume certain compounds to create others. This creates a unique flavor for the end product.

The term fermentation is often used interchangeably with oxidation. Even though they are different processes you will sometimes see oxidized teas labeled fermented teas. Truly fermented teas are then called post-fermented.

Overview of tea processing

While the two processes are very important when making tea, they are not the only steps. In fact, some types of tea don’t go through either, except for what is unavoidable. 

The types of teas and the level of oxidation or fermentation they go through will be discussed in the next section.

The first step that all types of tea go through is called withering. This step involves drying out the leaves to about 60% or 70% of their original water. Usually, this is done by placing them on a rack in a room with a specific temperature and humidity.

Withering allows tea makers to further dry the tea in a later step instead of cooking it. 

After withering is bruising. This step exposes more of the leaf to oxygen for the next step which speeds up the process. Traditionally, tea leaves are hand-rolled. Today, they are bruised mostly using one of two methods.

The first is rolling by a machine. This achieves a similar effect as rolling by hand. The other method is less gentle. It involves a machine that shreds and curls the leaves.

Regardless of which bruising method they went through they move on to the oxidation step. Oxidation is similar to withering when done commercially. Tea leaves are left out in a room with a highly controlled atmosphere.

This is where the distinction between types of tea becomes apparent. Green tea does not go through this step. It will still have oxidized on its own, but the change will be so small as to be unnoticeable. The rest of the tea types will go through this step for various lengths of time.

After oxidation is a final heating that further drys out the leaves and stops the oxidation process. Then the leaves are ready to be packaged and sent out.

Major differences in tea categories

Now you know the general process and how similar most of it is for different types of tea. The reason that the teas are different is mostly because of oxidation. 

As mentioned previously, green tea does not go through any oxidation. The different tastes within green teas are because of the heating method that stops oxidation from happening. Green tea typically has low caffeine.

White tea is another type that is not oxidized. What makes it different from green tea is when it is harvested. White tea is harvested before the leaf buds are fully open. White tea has very little caffeine. 

Oolong tea is in the middle when it comes to oxidation. Oolong teas can be oxidized for varying lengths of time which gives them a wide range of flavors and colors. Like its oxidation, oolong tea can vary in caffeine amount.

Then we have black teas. These are allowed to fully oxidize. They are as dark as tea leaves can be without beginning to rot. Black tea is typically high in caffeine. 

Pu-erh tea is one of the few that is fermented. This type is not oxidized, however. Pu-erh tea has a similar caffeine content to black tea.

Health benefits and impacts of oxidation

While all forms of tea have some health benefits, different benefits come from oxidation. The theaflavins and other nutrients produced by oxidation can help lower cancer risks, reduce blood pressure, and a few other benefits.

Most of the chemicals created by oxidation in tea are antioxidants. These antioxidants can help remove free radicals from the body. If allowed to build up in your body, free radicals can damage and affect cells.

The removal of these molecules is one reason why oxidized tea has so many benefits. Free radicals can increase your chances of atherosclerosis.

Closing Thoughts:

Though they seem similar, oxidation and fermentation are not the same in tea. They require different processes and result in different types of tea. However, while important, they are not the only important processes in tea production. As you now know oxidation is the reason for many of the health benefits of tea. Now you have something new to think about over your next cup!

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