Tea Oxidation: What Is It?

You may have heard tea connoisseurs talking a lot about tea’s oxidation and how it impacts the overall flavor. But what exactly is tea oxidation?

Tea oxidation is the chemical process that occurs when oxygen reacts with the enzymes in plucked tea leaves, causing them to turn brown and break down over time. Oxidation begins as soon as leaves are plucked and is often accelerated by crushing or rolling the leaves.

Below, we’ll explore a little bit about how oxidation impacts tea flavor, the oxidation levels of common teas, and where in the tea process oxidation happens.

Oxidation in Tea Processing

Oxidation is arguably the most important part of tea processing when it comes to the flavor achieved in the final product. It completely changes the tea leaves’ chemical makeup.

Tea oxidation is the same process that occurs when cut fruit begins to brown, but rather than just changing the leaves’ color, it actually changes their flavor as well. Just like a piece of fruit will continue to brown over time, a tea leaf will continue to oxidate indefinitely if not stopped. 

There are two main ways that tea processors alter the oxidation levels, depending on what type of tea they’re producing:

  • Accelerating the oxidation process
  • Halting the oxidation process

Increasing the Amount of Oxidation

For teas that require higher oxidation levels, leaves are often rolled or crushed to expose more of them to the air, resulting in faster oxidation. This can be done by hand or by machine, depending on the size of the batch being processed.

Oxidation plays such an essential role in tea flavor that even leaves picked from the same plant on the same day will result in entirely different types of tea if their oxidation process isn’t the same.

Tea leaves used for more oxidized teas are also left to oxidize for more extended periods before the process is stopped using the methods described below.

Stopping the Oxidation Process

As mentioned above, the process of oxidation will go on indefinitely if not stopped. The way to prevent oxidation—or stop it—is by applying heat.

Tea harvesters can stop oxidation by applying heat in various ways, depending on the region, culture, process being used, and tea type. The most common methods are:

  • Dry roasting on a wok or skillet
  • Baking
  • Steaming
  • Sun drying
  • Microwaving

The amount of time a tea leaf is left to oxidize will vary depending on the type of tea being produced, as we’ll see later. This can be almost immediately after the leaves are plucked, or not until the leaves are almost completely oxidized.

How Does Tea Oxidation Affect Flavor?

Now that you know a little about the oxidation process, let’s get into how oxidation affects tea flavors.

A very simplified explanation is this: as tea leaves oxidize, the polyphenols that make up the leaves, specifically the catechins, are turned into theaflavins.

Catechins and theaflavins have completely different flavor profiles, so the resulting tea will taste different depending on whether there are more catechins or theaflavins present due to more or less oxidation.

Teas with less oxidation (or more catechins and fewer theaflavins) tend to be described as:

  • Mellow
  • Bright
  • Light

Whereas more heavily oxidized teas tend to be more:

  • Robust
  • Malty
  • Rich

Teas that have been lightly oxidized will contain flavor profiles of both catechins and theaflavins.

(Source: Red Blossom Tea Company)

tea oxidation on trays

Oxidation Levels of Common Teas

If you know your way around tea types, you can probably already guess which teas are the most or least oxidized during processing just based on the flavor profiles above.

But if you’re new to the world of tea, no worries—we’ll break down the most common types below, so you can make an educated guess next time you’re out tea shopping.

Keep in mind, all the teas listed below are “true teas” or those made from the camellia sinensis. “Untrue teas” are herbal teas, yerba mate, and rooibos, which aren’t made from camellia sinensis leaves, so aren’t actually tea in a technical sense.

So all the teas listed below are made from the exact same plant. Even though they look and taste completely different from one another, the only difference between them is how much the leaves were oxidized during processing.

White

White tea is known for being the lightest of the teas and is described as:

  • Floral
  • Fruity
  • Delicate

White tea, as you may have guessed, is barely oxidized at all. The main oxidation that occurs happens naturally during drying, and it’s halted very quickly. It still contains most of its natural catechins, and therefore tastes most like the leaves in their natural, untouched state.

Green

The next step up from white tea, green tea is popular for its many health benefits and mellow taste.

The flavor profiles of green tea are:

  • Mellow
  • Grassy
  • Bright

Oxidized just a tiny bit more than white tea, green tea still contains a lot of catechins. This causes its flavor to be stronger than white tea, but it still has a very “leafy” taste that can take black tea drinkers some getting used to.

Black

Black tea is the most popular tea in the West, making up classics like English Breakfast and Earl Grey.

The taste of black tea is said to be:

  • Rich
  • Hearty
  • Bold

Black tea is over 80% oxidized and contains more theaflavins of any other tea type. This takes its flavor profile to the opposite end of the spectrum from white and green teas, giving it a more earthy, astringent quality.

Oolong

Oolong tea describes the wide range of teas that are oxidized to a degree in between green and black teas. They can be anywhere from 30%-70% oxidized and taste more like green or black teas depending on where they are on the oxidation scale.

Because the oxidation levels, and therefore the catechins and theaflavins, in oolong can vary so drastically, it isn’t easy to pinpoint a flavor quality that will accurately describe them as a group.

Oxidation Levels & Health Benefits

Green tea is often touted in the West as being healthier than black tea, but in reality, both have health benefits such as antioxidant properties.

Keep in mind, though, for some people, the higher levels of caffeine in more oxidized teas might outweigh any potential benefits. Additionally, according to Penn Medicine, health benefits vary depending on the type (or oxidation level) of tea.

White Tea

White tea, the least oxidized of the teas, is said to:

  • Help fight cancer with its high antioxidant value
  • Promote dental health due to fluoride content

The imperceptible amount of caffeine is good for those with caffeine sensitivities, too.

Green Tea

Green tea, the tea touted as a miracle in the West, does have some real benefits, including:

  • Antioxidants that may fight some types of cancer
  • Promotes heart health
  • Slows blood clotting
  • Reduces bad cholesterol
  • Anti-inflammatory properties

Although drinking a cup of green tea here and there isn’t going to guarantee good health, it can be a great addition to other healthy lifestyle choices.

Black Tea

Black tea, most commonly drunk to get a quick boost of caffeine to start your day or get through that afternoon slump, is also good for you.

It has been found to:

  • Improve immune system function
  • Reduce inflammation

This makes black tea an excellent alternative to coffee and sodas for caffeine.

Pu-erh Tea

Pu-erh and other fermented teas (which we’ll get into below) are also said to have many health benefits, such as:

  • Weight control
  • Reduced bad cholesterol

However, as with many fad health foods, no food or drink is an all-powerful cure-all, so it’s a good idea to take such claims with a grain of salt and do your own research.

Where Does Oxidation Fit into Tea Processing?

Now that we know what tea oxidation is, how it affects the type and flavor of the tea, and how it’s controlled during processing, it might be useful to know where in the process oxidation actually takes place.

This will help you get a better, more complete picture of just what happens when a tea leaf goes from bush to cup.

Tea processing has been around for centuries and looks a little different depending on where in the world it’s taking place and what type of facility is doing the processing.

In many places, tea processing is still done entirely by hand in small batches, although as demand continues to grow, machines have started to take over in some facilities.

However, the basic steps of how tea is made are the same, no matter what. They are:

  • Harvesting (picking) the tea leaves
  • Withering or wilting
  • Leaf disruption
  • Oxidation
  • Fixation
  • Rolling and shaping (sometimes)
  • Drying

Tea Harvesting

Tea leaves are picked, usually twice a year, by hand or machine. Picking by hand is still more common, but it largely depends on the cost of labor, the amount of tea being produced, and the tea quality.

Higher quality teas are more likely to be picked by hand since the leaves are less likely to be damaged this way.

Teas that will undergo a lot of oxidation are more likely to be picked by machine, since leaf breakage is less of a concern. The more highly oxidized leaves are likely to be disrupted later anyway, so it’s not as big of a deal if they get broken by a machine.

Leaf Withering

Leaves are left out to wither, or wilt, in the sun or an airy room where they will lose much of their moisture content. This is no different from picking a bouquet of wildflowers and forgetting to put them in water—they naturally start to shrivel once they’re disconnected from the root system.

Some oxidation is already taking place at this point. Some teas, like white and some greens, will go from this step right to fixation. The rest will go on to leaf maceration or disruption.

Leaf Disruption or Maceration

The darker the tea, the more leaf disruption there will be in this process. Leaf disruption is just a fancy term used to describe breaking the leaf wall to allow more oxygen to penetrate it and accelerate oxidation.

Leaf disruption can be done by hand or machine, and often involves:

  • Rolling
  • Tearing
  • Mashing
  • Chopping

The more broken up a leaf is, the faster it will oxidize, resulting in a darker, stronger tea. For example, picture an apple cut in half versus chopped into tiny pieces; you’ll see how much more of the chopped pieces will brown than the half apple. 

There’s simply more surface area open to the air. 

tea processing in China

Oxidation

The oxidation process is mostly a waiting game once it starts to take place. Leaves are left to oxidate, often in rooms designed to control the process by adjusting:

  • Heat
  • Humidity

Sometimes oxidation takes place outside in the sun, depending on the region and climate.

As mentioned before, lighter teas will oxidate for less time and darker teas for longer. This process is carefully managed, as the speed and duration play a considerable part in the finished tea’s resulting flavor profiles.

Fixation, or Stopping of Tea Oxidation

Fixation is the process of halting oxidation in its tracks by applying heat. A good way of thinking of it is that when you cook fruit, it no longer turns brown when exposed to air because the enzymes responsible for it are no longer active.

As mentioned, heat can be applied in a variety of ways, including:

  • Dry roasting on a wok
  • Baking
  • Steaming
  • Sun Drying
  • Microwaving

Care needs to be taken to apply enough heat to stop the oxidation process, but not so much that the tea’s flavor will be changed.

Rolling and Shaping

Depending on the specific tea, some leaves are shaped or rolled by hand to enhance flavor and aesthetic value.

Leaves can be rolled into little balls that will unfurl when water is added, simply curled, or even knotted.

Drying of the Leaves

Once the tea process is complete, the leaves are dried to preserve them. Like fixing, this can be done in several ways, usually by:

  • Baking
  • Sun drying
  • Air Drying

Leaves are then sold loose, divided into tea bags, or pressed into bricks or cakes.

Oxidation vs. Fermentation

For decades, people have used the terms ‘oxidation’ and ‘fermentation’ interchangeably when talking about tea, but they’re actually two completely different things.

As we learned, oxidation is a natural chemical process that occurs when oxygen breaks down the cell walls in a tea leaf. Fermentation, on the other hand, is caused by yeast or bacteria breaking down organic matter. 

oxidation levels of tea

There are two main types of fermented tea (also called post-fermented):

Hei Cha describes all post-fermented teas apart from Pu-erh. Pu-erh is made only in China’s Yumman province. Like champagne, teas made using the same leaves and process still aren’t technically Pu-erh, from a traditional standpoint.

Hei Cha

Hei Cha is an umbrella term for most post-fermented teas, which include:

  • Shou pu-erh
  • Liubao
  • Liu An
  • Hua Juan
  • Ting Juan
  • Tian Jian

After the tea process described above, tea leaves are stored in wet piles, which allow bacteria and microorganisms to start breaking them down. This process is, of course, fermentation.

The fermentation usually lasts for several weeks, after which the leaves are again dried in the sun or by other methods. This type of tea is said to get sweeter and more mellow with age and is often kept for long periods, similar to wine or bourbon.

Pu-erh

Sheng (raw) Puerh is made a little differently than Hei Cha teas like Shu (ripe) Pu-erh. When the tea is initially processed, it is fixed at a lower heat and dried in the sun rather than by machine. This dramatically slows down the oxidation process but doesn’t stop it completely.

Even after the leaves are dried, they continue to oxidize for decades, eventually fermenting if kept long enough. Raw Pu-erh tea is usually pressed into bricks or cakes rather than being kept loose, to keep too much air from reaching the leaves as they mature.

A Note About Kombucha

Kombucha is a fermented tea drink made with tea, sugar, bacteria, and yeast. The mixture is left to ferment for months or years, resulting in a slightly alcoholic, slightly carbonated tea drink marketed as having many digestive and health benefits.

Although kombucha is more of a tea drink than a tea, it’s worth mentioning since it’s what many Westerners think of when they hear fermented tea.

Final Thoughts On Tea Oxidation

Setting out to learn more about tea will take you on a fascinating journey, but it’s easy to feel daunted at first, since there’s just so much to learn. Processing tea is an art form perfected centuries ago, so it makes sense that there’d be a lot to cover.

Now that you know a little more about what exactly tea oxidation is, you’re well on your way to understanding what all goes into making a cup with just the right flavor profiles for your palate. 

And you can choose a tea to suit your every mood, and know all about how it came to taste the way it does. 

If you love tea as much as I do, pin this Tea Oxidation to your favorite tea-loving Pinterest board and pass it on for others to enjoy! Pinkies up!

tea oxidizing in a tea processing plant