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  • Red Tea, Black Tea, Dark Tea: Oxidation and Fermentation
  • Amy Covey
  • Tea ChemistryTea Varieties

Red Tea, Black Tea, Dark Tea: Oxidation and Fermentation

Red Tea, Black Tea, Dark Tea: Oxidation and Fermentation

The crafting of tea is an exacting process, developed over hundreds of years through trial and error. Today, scientific research has helped us define the chemical processes that occur during the various steps of crafting, but the long history and guarded crafting traditions have led to some common misconceptions and confusing terminology. In particular, “fermentation” is often used to describe the darkening of tea leaves during the crafting process, though very few teas undergo microbial activity during processing. Instead, most teas undergo what is more accurately called “oxidation”.

The confluence of these two words is compounded by confusing translations of the names of tea categories. For instance, the Chinese word hēichá translates directly to “black tea”, but this category is commonly translated to “dark tea” in order to avoid confusion with the black teas known in the west. Rather, an English black tea would be called hóngchá, or literally, “red tea”, in Chinese.

heicha, translated as "black tea" or "dark tea"

To clarify, understanding the distinction between oxidation and fermentation is essential. So what is the difference between the two, and how can we define them in the context of tea processing?

What is Oxidation?

Oxidation is an enzymatic reaction that causes a tea leaf to turn brown after harvest. To be specific, this reaction occurs when the cell walls are broken, and polyphenol oxidase is exposed to oxygen in the atmosphere. This chemical reaction is the same process that occurs when an apple or avocado turns brown after being cut. To stop the oxidation process, the leaves are heated, which “cooks” or denatures the responsible enzymes and halts the browning process. To use the same apple analogy, this is the same as baking the apple, perhaps into a pie.

Oxidation is the process that produces the color-coded categories we know in the west. Put simply, green tea is roasted quickly to prevent oxidation, while black tea is allowed to oxidize fully, turning brown before roasting. Oolong teas cover the range between the two extremes, with oxidation levels ranging from about 5% to 75%. During the process of oxidation, the leaves are carefully monitored, so that the reaction can be stopped at the desired level.

partially oxidized leaves on a bamboo tray are still mostly green.

To be clear, this process is entirely a chemical reaction. It is caused by the proteins in the leaf reacting with the oxygen in the air. Left unchecked, oxidation will result in full deterioration of the leaf, just as a bruised apple will eventually rot. Therefore, techniques developed to induce and control oxidation have been an integral part of tea crafting throughout history.

What is Fermentation?

The word “fermentation” is often used to describe oxidation, but it is in fact a totally different process. Part of the problem is that the word has taken on a number of meanings, each with it’s own context. One thing most definitions have in common, though, is that fermentation requires microbial activity. In tea, this is generally taken to mean any microbial activity, whether it occurs with or without oxygen present.

sheng pu-erh cakes are traditionally wrapped in bamboo and left to ferment with age.

While oxidation will occur naturally in any plucked tea leaf, only a few types of tea are truly fermented, and thus categorized as hēichá. Most notable and widespread is pu-erh, which is made in Yunnan, in the south of China. There are two variations of the fermentation process in pu-erh crafting, with the main difference being the speed of fermentation. Sheng, or “raw” pu-erhs, ferment slowly over time, in a process that was almost certainly discovered by accident. After heating to halt oxidation, sheng pu-erh leaves are steamed and compressed into cakes, which were easy to store and trade. Over time, in the humidity of southern China, naturally occurring microbes on the leaf broke down some components of the tea leaf, and created new substances, changing the chemical composition and flavor of the tea.

More recently, a process of accelerating this fermentation was developed to meet the rising demand for these unique teas. The resulting style is called shou, or “cooked” pu-erh. Directly after they are initially heated to stop oxidation, the unfinished leaves are heaped into a large pile. Frankly, the process looks a lot like composting. The large piles of leaves generate heat, killing off undesirable microbes and promoting the activity of others. During the process, the leaves are regularly turned to introduce oxygen and prevent excessive heat. The microbes in this process may still be naturally present on the leaf or in the environment, but some tea makers use a starter culture from a previous batch or otherwise purposely introduce microbes to make the fermentation process more consistent across harvests.

maocha, or unfinished leaves, are heaped to accelerate fermentation

Common Misuse and Confusion

It seems pretty clear that these two processes are unique and distinct from one another, given the privilege of microscopic knowledge. But it’s also important to remember that Chinese tea crafters were using both processes several hundred years before any of this was discovered. As both turned the tea leaves a dark color, conflation is, perhaps, understandable. Today, the confusion lives on, as the process of oxidation is referred to in Chinese as fāxiào, or literally, “fermentation”. To distinguish the microbial process used in pu-erh, Chinese uses the term hòu fāxiào, or literally, “post-fermentation”. Both translations are misleading, but still commonly used in English tea writings.

The difference between an oxidized tea and a fermented tea may look subtle, but the resulting flavors are wildly distinctive. While fully oxidized black teas typically have flavor notes of chestnuts, malt, or citrus, fermented pu-erhs are more often described with nuances of earth, peat, or camphor. Understanding the way these terms are used and misused will surely make the vast world of tea a little less confusing.

Want to learn more about fermented tea? Check out these 6 reasons to love pu-erh tea.


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  • Amy Covey
  • Tea ChemistryTea Varieties

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