Oxidation radically alters everything we perceive in food. It changes the flavor, aroma, color, and texture of everything we eat and drink.
Without oxidation, tea would taste unbearably bitter. Oxidation is what unlocks tea’s hidden flavors by removing bitter compounds and transforming flavorless proteins into flavorful amino acids.
Oxidation does more than change the flavor of tea. It defines each category of tea. Think about Black tea (called “Red tea” in our shop and throughout this article) and Green tea. They look nothing alike and taste completely different, yet they both come from the same plant, Camellia Sinensis.
Thanks to oxidation, Red tea looks reddish-brown and tastes so different from Green tea. In fact, the entire category of Red tea is defined by its high oxidation level, whereas Green tea is defined by being unoxidized. Teas like White, Yellow, and Oolong live in between. Let’s take a closer look at oxidation and how it secretly defines the whole world of tea.
What Is Oxidation?
Oxidation is a type of chemical reaction. When a molecule is oxidized, it effectively loses electrons. The molecule donates its electrons to other molecules called oxidizing agents, like oxygen and sugar. The dance of electrons between molecules transforms oxidized molecules and oxidizing agents into new compounds.
In food products, those new compounds can taste and look radically different. Take fruit ripening, which is an example of enzymatic oxidation. When farmers pick bananas, they damage the cell walls of the fruit. The damaged cells release enzymes that mediate oxidation when the banana is exposed to air. When you buy a banana at the supermarket, it is largely unoxidized because of how it is stored. It is firm, bitter, and green. The more the banana is exposed to air, the more it ripens, becoming softer, tasting sweeter, and looking browner.
Not all oxidative reactions require an enzyme. Exposing a compound that is ready to be oxidized to low heat and an oxidizing agent kickstarts non-enzymatic oxidation. A famous example is the Maillard reaction, which occurs when browning food. Proteins mix with sugars (the oxidizing agent) under low heat. The proteins break down into amino acids, each of which has a more distinctive flavor. Think about cooking dry rub chicken wings under low heat. The Maillard reaction produces a rich, savory flavor that is absent from boiled chicken.
More Oxidation Means More Flavor, Color, And Mouthfeel
Without oxidation, all brewed tea would taste bitter and look slightly green. Freshly picked leaves are full of bitter-tasting compounds like catechins and caffeine as well as flavorless proteins. The bitter compounds mask some of the flavors the tea leaves pick up from their environment. Farmers use oxidation to remove undesirable flavors and unearth new, tasty ones.
Oxidation reduces undesirable bitterness in tea. Removing bitterness makes flavors from the terroir, like minerality, detectable in the brewed tea. It is what distinguishes unoxidized Green tea from teas with low oxidation (White and Yellow) and from teas with higher oxidation (Oolong and Red).
Oxidation turns bitter catechins into more complex polyphenols known as tannins. Tannins like theaflavins and thearubigins dominate oxidized teas. It's been observed that higher weight polyphenols like tannins are less bitter than lower weight catechins. The underlying reason is still a mystery.
Tea oxidation also affects caffeine content, which affects bitterness. Unlike catechins, processing tea leaves does not turn caffeine into new, less bitter molecules. Instead, the process used to halt oxidation removes caffeine from tea leaves, which reduces their overall bitterness. When farmers roast tea, a white powdery substance appears on the outside of the leaf. That’s caffeine. This is also why teas that have been highly processed have less caffeine than less processed tea. More processing involves more heat, which removes more caffeine. Contrary to popular belief, Red teas like Darjeeling actually have less caffeine on average than White or Green teas because they are so highly processed.
Oxidation brings out new flavors in tea leaves by increasing the leaves’ amino acid content. Amino acids are the building blocks of proteins. On their own, proteins are not very flavorful. Amino acids and their salts, however, exhibit all the fundamental flavors: sweet, salty, bitter, sour, and savory. Oxidizing proteins release some of their amino acids. More oxidation means more amino acids means more flavor.
Oxidation affects more than flavor. It also affects the color and mouthfeel of tea. Catechins are colorless. Their byproducts from oxidation, theaflavins and thearubigins, give Oolong and Red teas their reddish and orange hues. Even the mouthfeel of theaflavins and thearubigins is different from catechins. Theaflavins feel creamier on the mouth than catechins.
Oxidation also degrades chlorophyll, which changes the color of brewed tea. Central to all plants, chlorophyll looks green. By degrading chlorophyll, oxidation reduces the greenish color of tea. Chlorophyll’s byproducts increase the brownish color of tea.
How Tea Producers Control Oxidation
Tea processing is all about manipulating oxidation to extract and eliminate certain flavors. There are several techniques for starting and stopping oxidation. Each has its own impact on flavor.
Tea producers kickstart enzymatic oxidation by damaging tea leaves. Simply picking tea leaves is enough to damage cell walls that release enzymes required for oxidation. As the enzymes are exposed to the right conditions, the tea oxidizes.
However, minimally processed teas never achieve high oxidation levels, even if farmers do nothing to stop oxidation. For example, White teas are minimally processed; the farmers let the leaves oxidize from picking but do not do anything to stop oxidation. Yet, White teas are typically only 3% - 4% oxidized. To achieve higher levels of oxidation seen in Oolongs and Reds, farmers intentionally damage the leaves by rolling or tumbling them. These methods can oxidize teas up to 95%.
Enzymatic oxidation does not happen on its own. It requires the right conditions to make an impact. In particular, farmers need to apply just enough heat to activate the enzymes. Farmers activate the enzymes by heating tea leaves between 80 and 90 degrees Fahrenheit.
Farmers do not always want to produce fully oxidized teas when they use rolling and tumbling to damage the leaves. To control the amount of oxidation a tea has, they need a way to stop oxidation. Their tool is heat. While low heat activates the oxidative enzymes in tea leaves, high heat denatures them. When the enzymes are denatured, they are deactivated. As the farmers cool the tea, the enzymes can no longer mediate oxidation.
There are a few ways farmers halt oxidation, each with a different impact on flavor. Many Oolong teas are roasted. The roasting process halts oxidation and also imparts a roasty flavor on the tea leaves. Other teas, like Green teas, are traditionally steamed or pan fried. Both methods halt oxidation, but neither imparts as distinct a flavor as roasting does on the tea leaves. The method farmers use is largely stylistic and determined by what type of tea they want to produce.
Fermentation Vs Oxidation
A common misconception in the tea world is that oxidation and fermentation are the same thing. This is entirely false. Oxidation is a type of chemical reaction. Fermentation is a biological reaction. When fermentation occurs, microbes like bacteria and yeast eat the tea leaves to transform the compounds in tea.
Oxidation has been mistaken for fermentation in teas like Oolong because of a translational mishap. One of the technical words in Mandarin that refers to oxidation is similar to the word for “fermentation” — fa xiao. This mistranslation has given Westerners the false impression that fermentation is central to creating Oolong teas.
Despite being falsely mistaken for oxidation, fermentation is critical for understanding oxidation in one type of tea: hei cha. Literally translated as “Black tea”, Westerners better know hei cha as “Dark tea”. Think pu’er. Hei cha is an umbrella term for fermented teas. Fermentation competes against oxidation, which does not thrive while microbes process tea. As the microbes die, more oxidation kicks in. The tradeoff between fermentation and oxidation is something farmers unintentionally exploit for flavor and appearance when crafting hei cha.
Oxidation Is The Difference Between Each Tea Category
Oxidation is so important to making tea that the tea category names are shorthand for oxidation. Each tea category encodes how much oxidation, and which type, goes into producing that tea.
Green Tea (Lu Cha) - No Oxidation
Green tea is fried or steamed after picking. Frying and steaming kills the enzymes that cause oxidation. There is a slight amount of oxidation in Green tea because there is a small amount of time between picking and frying.
Because Green tea is unoxidized, the flavors experienced by Green tea drinkers are fewer and less distinct. Most Green tea has a flavor that is grassy, vegetal, or astringent. The main lever farmers pull to alter the flavor is roasting. Nonetheless, most Green tea has a light to medium roast, at most.
White Tea (Bai Cha) - 3% - 4% Oxidation
Unlike Green tea, White teas are not fried or steamed after picking. Most White teas are first processed by placing them in the sunlight, creating the ideal conditions for enzymatic oxidation.
White tea does not require heat to stop oxidation. Instead, oxidation naturally stops. Because farmers do not damage White tea leaves through rolling or tumbling, there are not enough enzymes to achieve high levels of oxidation in White tea.
Aged White Tea (Lao Bai Cha) - 5% - 6% Oxidation
Low water content in some White tea encourages oxidation to slowly occur over time. After about seven years, White tea is considered “aged” and has achieved higher levels of oxidation than average.
Higher oxidation means more distinct flavors. Even a difference of a few percentage points in oxidation makes aged White teas very distinct from their younger cousins. Aging also makes White tea very expensive. Check out our Enemy’s Demise Lao Bai Cha if you’re curious about aged White tea.
Yellow Tea (Huang Cha) - 5% - 6% Non-Enzymatic Oxidation
The oxidation we discussed thus far is all due to enzymatic reactions. Yellow tea is different. It is the only category of tea dominated by non-enzymatic oxidation.
Like Green tea, heat is applied to Yellow tea right away. However, water is removed through drying and the tea is wrapped in a special paper and stored in a setting with tight controls on humidity and temperature. This process, known as “smothering”, is where non-enzymatic oxidation occurs. How it works is a trade secret. Even though we have been going to China for decades to observe how farmers process tea, we haven’t been let in on the trade secret!
Non-enzymatic oxidation in Yellow tea is important because it yields a completely different flavor than enzymatic oxidation. Yellows tend to taste like Green teas, but they can have a sweet aftertaste that is stronger than the overall sweetness of lightly oxidized Oolongs.
Many people have mistaken the flavors in Yellow tea as resulting from slight fermentation because the flavor is so unique. This is 100% false. It is all from non-enzymatic oxidation.
Oolong Tea (WuLong Cha) - 10% - 80% Oxidation
Oolong tea has a complicated relationship with oxidation. Unlike Green, White, and Yellow tea, Oolong teas have a much bigger oxidative range. Moreover, it is impossible to achieve 10% oxidation, let alone 80%, by only picking the tea leaves.
After withering the tea leaves post-picking, Oolong teas are tumbled and lightly rolled. Tumbling and light rolling is enough to release more oxidative enzymes that, if left unchecked, would make most Oolong teas achieve very high levels of oxidation.
The amount of oxidation farmers allow is more of an art than a science. Using intuition, scent, and touch, farmers know when to halt oxidation to achieve the flavor profile they seek.
Most teas traditionally made in Taiwan are lightly oxidized. Examples include Iron Goddess (Tieguanyin) and Golden Osmanthus, ours being from mainland China. In our shop, we primarily carry Wuyi Rock Teas, many of which are more oxidized. Some of the most oxidized Oolongs we carry are Fusion and Black Dragon.
Red Tea (Hong Cha, a.k.a. “Black Tea”) - 80% - 95% Oxidation
Westerners know Red tea as Black tea. This is a mistranslation of hong cha, which literally means Red tea.
Most Red tea is 90% - 95% oxidized. That means that Red teas tend to be sweeter than other teas. Red teas achieve oxidation past 80% through heavier rolling than Oolong teas. For artisanal reasons, we have only seen Ancient tree Red teas with oxidation between 80% - 90%. Ancient tea trees are over a thousand years old and can achieve more distinct flavors due to their age. Dusky Python and Black Beauty are two of our Ancient Red teas that we particularly enjoy.
Fermented Tea (Hei Cha, lit. “Black Tea”)
The exception to the rule mapping tea names to oxidative levels is hei cha, literally translated to “Black tea” but often called “Dark tea” to avoid confusion in the West. Pu’er is a type of hei cha that is more common to Westerners.
Oxidation is not the main driver in processing Hei Cha. Instead, fermentation is. Fermentation competes with oxidation because as microorganisms actively process tea leaves, the byproducts do not undergo enzymatic oxidation.
However, farmers can halt fermentation and induce oxidative methods with the remaining unfermented tea compounds through rolling and tumbling. The competition between fermentation and oxidation gives hei cha a rich range of flavor profiles.
How To Tell If Your Tea Is Oxidized
Once you know what oxidation is, it becomes much easier to see it at play in your tea. If your tea is sweet without adding any sugar to it, then it has been very oxidized. If your tea is grassy or vegetal and does not taste very distinct, then it has not been oxidized. The more range and distinctness the flavor, the more oxidized the tea is.
If you have a hard time tasting the precise flavors, you can still tell how oxidized the tea is by its color. Brownish-red hues are a dead giveaway of oxidation. More brownish-red coloring means more oxidation.
Stay Tuned For More
This was our first article on tea processing in a new series we are rolling out. More articles are coming soon on the science behind tea and how it’s made. If you enjoyed this article, join our mailing list. We’ll share the articles as they’re published and keep you in the loop about new things we’re doing online at Sophie’s!