What is Oxidation in tea?
In the world of tea, you may have heard about oxidation as it’s a big part of the changes that occur in tea leaves to make a finished tea product. The production of tea encourages or discourages oxidation that help build the categories of tea like Green tea (which has little oxidation, or Red Tea (known as Black Tea in the West) is fully oxidated.
In this article we’ll cover what is oxidation in tea, and also address some of the large misconceptions around oxidation — trust me, there are a lot of them!
Oxidation in A Nutshell
Any dictionary will tell you that oxidation is a chemical reaction where oxygen atoms lose electrons. In the case of plants, when they are damaged, cut, or crushed, air gets in, and enzymes rapidly oxidize.
You can see this pretty easily when you cut apples and they turn brown. Tea makers can control this process which alters the taste of the tea leaf. With tea, this happens when:
The leaf is plucked from the bush or damaged.
Polyphenols reactions start, carried out by enzymes in the leaf.
When the tea leaf becomes oxidized, there are a number of changes to the leaf. Catechins (an astringent tasting antioxidant) begin to convert into theaflavins & thearubigins (sweet and savory antioxidants.) Generally speaking:
The less oxidized the tea the greener it will usually be.
The more oxidized the darker the tea will be.
Fats, proteins, and pigments break down into aroma and flavor compounds.
Is Oxidation The Same as Fermentation in Tea?
This is a difficult question to answer straightforwardly. In the world of tea, both terms are used interchangeably - but there are a few nuances between the words.
When tea is picked it begins a complex process known as enzymatic metabolism. While this term is the best description of what’s occurring in the tea, saying “enzymatic metabolism” over and over is a mouthful. So tea-makers need a shorthand. The two best candidates are “oxidation” and “fermentation.”
Oxidation is a strictly defined chemical process. The term does not begin to capture the wide breadth of changes occurring during tea’s complex chemical changes.
We think of fermentation as yeast, microbes - but it’s actual definition is much wider than that. The word “fermentation” is not as strictly defined and thus can be used more liberally to describe a larger range of complex changes in the tea leaves.
Thus fermentation is a much better term than oxidation to “shorthand” what’s actually going on: enzymatic fermentation.
When “Oxidation” Makes more sense for tea
Yes, fermentation is a better word to describe the changes in tea than “oxidation.” (Though, again, both terms are used interchangeably in the industry.) But there are times when the word “oxidation” is better than saying fermentation.
In exceptionally low-grade, mass-produced commercial teas - companies will cut up the tea leaves, and then expose them to air for 20 or 30 minutes to oxidize them. They will then call these teas “Oolongs” or “Black Tea” because they have been oxidized. They are not fermented, as they do not go through the proper, and much more complicated chemical changes in fine tea.
These mass-produced "teas” are not really processed correctly - and are akin to calling Kraft Singles “Cheese.”
Microbial Fermentation
Part of the confusion with fermentation in regards to tea is that we think of microbial fermentation that you see in bread making or winemaking when we say fermentation. And yes, microbial fermentation is the chemical breakdown by microbes, especially in regard to yeast. Oxidation is a completely separate process, as it's a chemical reaction from Oxygen.
But fermentation (as already mentioned) can describe a more generic kind of chemical change that does not involve microbes. Though it is worth noting that there are microbially fermented teas.
What are Fermented Teas?
There are some teas that do involve microbial fermentation such as Chinese Black Tea, known as “Shou Pu.” This is not the same as true Chinese Red Tea (which is confusingly called “Black Tea” in the West.”) Actual Chinese Black tea is a microbially fermented tea that isn’t something you see too often in the United States.
An example of Black tea you may have heard of is Shou Pu, which is composted and fermented, making an earthy flavor. There are also a few Japanese Black fermented teas too, though it’s more commonly seen in China.
Oxidation in Tea Categories
The enzymes in the leaf mix with oxygen to oxidize, and different tea categories have varying levels of oxidation thus changing the taste. We can generally expect that:
Green tea has minimal oxidization.
White tea will be slightly more oxidized than green tea.
Wu Long (Oolong) can be oxidized anywhere from 10% to 85%.
Red tea (known as black tea in the west) is 100% oxidized.
When it comes to taste, teas that are less oxidized tend to have characteristic flavors and aromas, while fully oxidized tea has it’s own set of expected characteristics:
Unoxidized tea like green tea will have more grassy, vegetable and bitter notes.
While full oxidized teas like red tea will have more tannic, malty, and fruity profiles.
Notes: For a long time, there was only Green tea. That’s because the Chinese who were the main tea providers of the world considered heavily oxidized teas like Red tea (Black tea in the West) as failed products. If you want the full story, check out our article A Brief History Of Tea here.
Tea Categories are not defined by Oxidization Levels!
This is likely one of the most important points in this article - mostly because there is so much misinformation out there about this! We often hear that the category of tea (Green, White, Yellow, etc…) is based on the oxidation level. This is not accurate.
The category of tea is based on the processing method. The amount of oxidation in the tea is a by-product of the processing method. That might sound like I’m being pedantic, but this is an important distinction.
For example, I’ve seen charts that say, “A tea that is 11-15% is a white tea. A tea that is 15 to 80% is an Oolong.” White teas have aging potential and will gradually oxidize naturally over years. Does that mean a white tea that is 30% oxidation becomes an Oolong because it crosses that 15% barrier? No.
Tea Categories
The way we differentiate tea into different categories is based on processing steps that are unique to each category of tea. It’s true that these steps result in tea with an expected range of oxidation levels. You can think of oxidation levels more like a guidepost, it can give you clues as to both the category of the tea as well as whether the tea was processed correctly.
Passive VS. Active Fermentation
As mentioned before, the moment the tea leaf is picked the enzymes in the leaf begin to oxidize it. This is a slow process, known as “passive oxidization or fermentation.” This is perfect for White tea which naturally oxidizes while it’s drying, and will continue to slowly oxidize for decades after.
But this is hardly efficient for Red tea, where we want a fully oxidized tea with the enzymes fully exhausted in the leaf. To help speed things up, tea manufacturers bruise or roll the leaf to rupture the cell walls. This causes the spilling of the enzymes and juices throughout the leaf causing a speedy oxidation process.
This process of intentionally speeding up oxidation is known as “active oxidization or fermentation.”
Stopping Oxidation in Tea Processing
What separates Green tea from most other teas is as little fermentation/oxidization as possible. The bigger question is, how do you stop oxidation if you don’t want it? In order to achieve this, all the enzymes in the leaf must be denatured and rendered inert. This can be done pretty easily with a processing method is known as Sha Qing (杀青), or literally "kill green."
The wilted leaves are hit briefly over high heat to "kill" the enzymes and stop them from causing oxidation. Put simply: hit the green leaves with high heat to stop them from turning brown.
Subcategories of Green Tea
When we talk about killing green, there are 4 distinct methods to do this. Each method to denature the enzymes within green tea actually creates the 4 subcategories of green tea! The 4 methods are:
Chao Qing “stir-frying” - literally cooking the tea leaves in a wok, and drying the leaves in the wok.
Hong Qing “baked” - cooking the tea leaves in a wok first, then finishing the drying process in an oven or over a fire.
Shai Qing “sun-dried” - Only dry the leaves in the sun. This is done most famously with green tea known as “Sheng Pu Er” from Yunnan province in China.
Zheng Qing “steamed” - Steaming the tea, which is less common in China and is the most common with Japanese teas such as Matcha, Sencha, Gyokuro..
By doing this, Green tea retains its bright green color and original taste after processing and drying. It's important to note though: technically Green tea does have a negligible amount of oxidation because as soon as the leaf is picked the process starts.
Encouraging Oxidation in Tea
As you've seen, oxidation brings a lot of changes to a tea leaf. Let’s cover a few more changes, especially with teas more heavily oxidized.
Oxidation in Wu Long (Oolong) Tea
Wu Long (Oolong) teas and Red teas (known as Black Tea in the West) are processed in a way that encourages or exhausts the enzymes in the leaf. Usually with active oxidization methods like bruising and rolling.
In Wu Long (Oolong), the amount of oxidation can range.
The lighter the oxidization, the "closer" in flavor profile to a green tea we can expect.
The darker the oxidization, the "closer" in flavor profile to Red tea.
Red Tea Oxidation
Red tea has effectively had all of its enzymes exhausted. Thus, we can say that Red tea is fully oxidized. This brings out many of the savory and sweet flavors that we associate with Red Teas, and seems particularly suited for the Western palate where Red Tea is king. (In the East, green tea is favored.)
Red tea is also associated with higher levels of L-theanine, a savory-tasting amino acid that promotes relaxation. Though in some Japanese green teas such as Matcha, the tea plant is shaded with tarps that increase the amount of L-Theanine. Anyhow…
Enzymes are What We’re Talking About
One final, but important, note. When we talk about fermentation in tea leaves we are primarily speaking about the potential of the enzymes in the leaf to carry oxidization out (though, not exclusively! Always nuance...)
After the enzyme is killed like in Green tea or is fully exhausted like in Red tea – further oxidization can still occur. It’s just not at the hands of enzymes but from natural electron loss in the presence of oxygen (Like rust). So we can be more specific and say that in tea we’re talking about enzymatic oxidation rather than just oxidation.
Can Tea Go Stale?
Yes, tea can go stale. After tea has been processed it will further oxidize when exposed to air. Green tea, left out in contact with oxygen, will oxidize and lose its luster.
Red tea will oxidize too, and lose its taste and nuance. This type of oxidization is unwanted in tea. And will muddy the flavors.
In Closing
Oxidation is an important part of the tea-making process. It's fascinating to see how it can alter the taste, smell and color of tea. And also how tea makers control the level of oxidation to produce a specific flavor profile.
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