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What Is Oxidized Tea? Learn About Oxidation in Tea Processing.

What Is Oxidized Tea

Have you ever been curious about the processes that go into making your favorite tea? What is oxidized tea and how does it affect the flavor?

Today we are going to talk about the process of oxidation and how different levels of oxidation bring your favorite tea to life. Well, a tea may be from the same plant, but the amount of oxidation they undergo determines how it will taste and smell.

We will also be covering how fermentation and oxidation are completely different processes but can be used together to make a special type of tea.

So, if you are an avid tea lover, here’s a fun read for you.

What is Oxidation in Tea?

Oxidation simply refers to the process that changes the chemical composition of a compound by reacting it with oxygen. It’s the same process that apples go through when they are left exposed to air.

In the case of tea, the oxidation method involves exposing the tea leaves to air in order for them to darken. The leaves contain enzymes that react with oxygen and break down the chemical components present inside.

This is how oxidation affects the color, flavor, and aroma of different teas.

What Are Enzymes?

Let’s take a trip back to grade 5 biology.

Enzymes are proteins that help speed up chemical reactions. Similar to humans, plants also go through many biochemical reactions in order to live and continue all functions in their system. And since leaves are a part of the plant, enzymes are therefore naturally present in them to aid the chemical reactions.

When it comes to tea leaves, we are only concerned with two enzymes. They are chemically known as Polyphenol oxidase and peroxidase. These enzymes are responsible for the coloring of the tea leaves which ultimately contribute to the final taste of the tea.

After the desired level of oxidation is reached, tea producers take measures to deactivate the enzymes. This could be done by controlling the temperature or moisture levels that the tea leaves are exposed to.

How Is It Different From Fermentation?

Fermentation and oxidation can be confusing. That’s why it is important to not mix the terms as they are two entirely different processes.

Fermentation involves the use of microbial organisms to break down the chemical components of the leaves. This causes them to decompose with or without the presence of oxygen.

Oxidation, on the other hand, is a chemical process that utilizes oxygen to react with the enzymes present in the plucked leaves.

Different teas have different methods of production. For instance, white and green tea are oxidized whereas teas like Pu-erh are produced using a combination of oxidation and fermentation. 

Which Teas Are Oxidized?

Here’s a fun fact for you. Every tea you find on store shelves is oxidized, and here’s why.

The oxidation process actually begins from the moment the tea leaves are plucked. The process is just sped up by different tea producers to achieve the desired oxidation level for specific teas.

Here are some of the most popular teas around the world. Some of the teas in this list come from the same oxidation levels, but they are mixed with some extra elements to give the tea its own natural blend of flavors.

1.    White Tea

2.    Black Tea

3.    Oolong Tea

4.    Purple Tea

5.    Yellow Tea

6.    Green Tea

7.    Pu-erh Tea

8.    Red Tea

9.   Honeybush Tea

10. Milk Tea

Oxidation Levels For Different Teas

Now that you know that the oxidation level of your tea impacts its flavor and color, it’s time to look at how the oxidation levels of the various types of tea differ around the world.

White Tea

White tea, as the name suggests, goes through a bare minimum amount of oxidation. The main process of oxidation occurs naturally while drying the tea leaves, and it is stopped as soon as it starts.

People often make the mistake of thinking that white tea isn’t oxidized. However, as the tea leaves go through withering for a long period of time, it results in light oxidation.

This is why the taste of white tea can be noted as natural. The tea is also known to have a fruity and floral scent.

Green Tea

Green tea, when compared to white tea, is oxidized longer. The tea still has a significant amount of catechins and this gives it a stronger flavor than white tea.

Green tea originated in ancient China. It was discovered by a Chinese emperor who had mistakenly boiled water with a dead tea leaf. The tea became symbolic in China because of its delicious and refreshing flavor.

The taste of green tea will depend on the method of brewing. Green tea that is steamed is said to have floral notes whereas toasted green may have a more earthy flavor.

You may also like: Best Green Tea for Beginners

Oolong Tea

Oolongs teas vary in terms of oxidation levels. The tea leaf may undergo oxidation anywhere between 20% and 70% and the taste is dependent on the degree of oxidation.

Some people do feel that all oolong tea generally contains a floral and fruity flavor, however, it is hard to come up with an accurate flavor profile as it is completely dependent on how much oxidation the tea leaves have gone through.

Recent studies have shown that oolong tea has polyphenols that can help weight loss. In addition to this, the tea is also known to lower cholesterol, and blood pressure, and increase your immunity to flu and viruses.

Black Tea

Unlike white and green tea (which require almost no oxidation), tea producers allow the tea leaves to be oxidized until the color changes to brown.

Black tea is over 80% oxidized and contains the highest amount of theaflavins (anti-oxidants). This gives black teas a generally more dark and malty flavor. The color of the tea will range between dark red or an orange-colored brew.

This particular type of tea is common in the western world. It also happens to be the most popular type of tea around the world. In some places, black tea is consumed without the addition of milk as it can reduce the potential of its antioxidants.

Pu-Erh Tea

Pu-Erh tea differs from the rest of the teas in this list as it does not involve only oxidation. It also involves the use of microbial fermentation that gives the tea its unique scent and flavor profile.

The process of Pu-erh tea can be divided into two stages. The leaves are first oxidized to the same level as green tea and then left to undergo a fermentation process under high humidity. This process is also known as aging.

Since the tea is partially fermented, it has a mushroomy taste. The flavor profile can range between woody and tobacco, or even floral lawn notes.

Why Is Oxidation Important?

Oxidation has the biggest impact on the taste and smell of tea. If several leaves were plucked on the same day, different levels of oxidation would sort them out into different classifications of teas.

In addition to this, the level of oxidation can also affect the number of antioxidants in your tea. This will ultimately determine the health benefits you will get from having specific types of tea.

How Does Oxidation Affect Tea Flavor?

If the tea leaves are not oxidized, they would taste bitter and unpleasant. Oxidation is necessary for you to get the desired taste and flavor. It helps the tea unlock its hidden flavor by breaking the catechins and tannins.

The polyphenols that make tea leaves, particularly the catechins, are in itself flavorless. The tea leaves are more flavorful when they are broken down into tannins.

The catechins and tannins have different flavor profiles and hence the final taste will be different based on the number of catechins or tannins present. 

Oxidation and Caffeine

Teas are caffeine beverages. But how are caffeine levels impacted by oxidation? Let’s find out!

Does Tea Oxidation Affect Caffeine?

Yes, it does!

Tea oxidation can affect the amount of caffeine content in your tea. This is because the process of oxidation does not break down the caffeine molecules in the tea leaves. Instead, when the oxidation process is stopped, the process separates the caffeine completely.

The heat that is used to halt the oxidation process in the tea leaves produces a white powder residue. This is the caffeine that was present in the tea leaves. It simply means that the higher the level of oxidation, the more caffeine is present.

Hence, the caffeine content is higher in black teas than in green, white, or oolong teas.

The Process of Oxidation

Oxidation, in terms of tea, is a set of reactions that causes a change in the color of the tea leaves. This in turn changes the taste and scent of the tea that is to be produced.

When the leaf breaks and is exposed to air, certain chemical reactions take place that convert the chlorophyll in the leaf to pheophytins which darken the color of the leaves. The catechins, on the other hand, break down and give the tea its unique flavor.

There are various ways tea producers initiate this oxidation process and also stop the process when certain requirements are met.

Initiation of Oxidation

The process of oxidation is initiated by damaging the cells of the tea leaves. To do this, tea producers use a process called maceration. This is one of the most effective methods as it instantly turns the leaf inside out and exposes it to oxygen. This results in a better mixture of the enzymes and the chemicals that initiate the oxidation process.

Maceration can be done by hand or by using rotary machines. The rolling performed using these methods gives slow but gentle oxidation.

Oxidation can also be initiated by a method called tumbling. It is generally much more gentle and utilizes large cylinders to tumble the leaves.

No matter which method you choose for initiating oxidation, you should exercise great caution when handling the tea leaves as it can cause early oxidation and give rise to unevenly processed tea.

Controlling Oxidation

Certain strategies need to be implemented to control the oxidation process. This is important as we know how oxidation affects the taste and the quality of the produced tea.

One way to control the oxidation process is to constantly add more warm air that is enriched with oxygen to the tea leaves. The temperature has to be maintained throughout the oxidation process as well.

Fixing or Halting Oxidation

When an oxidation process is stopped, we refer to it as ‘fixed’ or ‘halted’. Halting involves heating the tea leaves so that the enzymes that are responsible for the oxidation process get denatured.

To successfully denature the enzymes, you would need to use extreme heat on the leaves. The heat destroys the active sites that enzymes use to complete the chemical reactions and makes them useless.

Another method of halting the oxidation process is to remove oxygen completely. This is the reason why you should always store your tea in an air-sealed bag to halt the oxidation process.

Most Common Fixing Methods

There are a few common methods that tea producers around the world use to halt the oxidation process. All of these methods use the concept of heat denaturing the enzymes.

Steaming The Tea Leaves: Forcing steam to be pushed through the leaves. 

Frying The Leaves: Using a large pan or a wok to heat the leaves using fire.

Baking The Leaves: Utilizing an oven or kiln to heat all the tea leaves.

Tumbling: Using a pre-heated tumbler to heat the leaves.

Less Common Fixing Methods

Traditionally, the tea leaves have been halted using a number of techniques before the arrival of new technologies. These methods also use heat to denature the enzymes and halt the oxidation process.

The methods have been used extensively in the past, but are not so common right now.

Boiling the leaves: This method involves pushing the leaves into water that is at boiling temperature.

Sun-drying: Uses the heat from the sun to dry and heat up the leaves.

Microwaving: Utilizes the power of electromagnetic waves to heat the leaves.

One interesting aspect of halting is that no matter what method you choose, the leaves will still undergo oxidation. However, the rate of oxidation will be extremely slow.

Passive vs. Active Oxidation

We have explained earlier how the process of oxidation starts from the moment the tea leaves are plucked. This is called passive oxidation.

Passive oxidation is actually perfect for teas like White and Green tea which oxidizes by itself after plucking, requiring minimal effort for the oxidation process.

However, this type of oxidation is not feasible for teas like Black and Oolong tea as the oxidation process is too slow after a certain period.

To fix this problem, manufacturers roll the leaf or apply pressure to it, increasing the surface area of the leaf to speed up the oxidation process. This process of speeding up the oxidation process by artificial means is known as active oxidation.

Active oxidation makes the oxidation process relatively faster so that it is easier to produce the desired tea.

Wrapping Up

The journey of a tea leaf, from its origin to your cup, is a long long one. It’s something you can dwell on while you are looking at tea bags or ordering your favorite cup of tea.

To some, tea is not just a drink, it’s a culture. And dedicated tea lovers always want to know more. And today you’ve learned how important the role of oxidation is in producing different types of teas.

No matter what is, when it comes to savoring a diverse flavor profile of real teas, little details can make all the difference. So, the next time you have a cup of tea, do think of the chemical processes that went into producing that particular type of tea.


1.    Which tea has the most oxidation?

Black teas are almost fully oxidized and result in a darker cup of tea. They are often macerated during the oxidation process, enabling all the parts of the leaves to be exposed to air.

2.    Which tea has the least oxidation?

White tea is the least oxidized tea. The tea itself does not require much processing as they are almost ready to go right after plucking.

3.    Is tea fermented or oxidized?

Teas can be both fermented and oxidized. White and black teas are common examples of oxidized tea whereas Pu-erh teas can be a mixture of oxidation and fermentation.

4.    Is oxidized tea good for you?

Studies have shown that oxidation increases the number of theaflavins in teas. Theaflavins are strong antioxidants that can help prevent cancer and other chronic diseases.

5.    What is a semi-oxidized tea?

Semi-oxidized tea comes with a varying level of oxidation that can range anywhere between the oxidation levels of black and green tea.

6.    How do you stop the tea from oxidizing?

Storing the tea in a tightly sealed jar that cuts off contact with air will stop the tea from oxidizing.

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