Oolong: More Than Mid-Oxidized

When introducing someone to the world of loose-leaf tea, I often describe oolong1 as being sort of in-between green tea and black tea. If a slightly longer explanation is appropriate, I describe the process of making black tea as oxidation (AKA enzymatic browning), where chemicals in the leaves turn brown after the leaves are crushed, and that green tea has been heated to prevent this chemical reaction from happening. Oolong has been allowed to experience a little bit of oxidation before heating, so it’s in-between. But in reality, this is a gross oversimplification. The differences between oolong and black tea processing are much greater than simply the amount of time oxidation happens, and there’s a lot of different things going on physiologically and chemically with the tea leaves.

To ensure that tea is completely oxidized in black tea processing, tea leaves are typically crushed or rolled strongly after withering to break open leaf cells and release their juices so that catechins mix with enzymes in the presence of air to ensure an even and complete oxidation. This reaction relies on enzymes and precursors that are already contained in the leaf cells. The crushing just allows them to mix and react with oxygen.

Oolong tea processing, on the other hand, does not generally involve crushing tea leaves, but rather starts with a gentler bruising, often called yáo q?ng or “rocking the green”. This may involve “fluffing” the withered leaves by hand every so often, or maybe placing the leaves in a large basket tumbler that rotates slowly. Both of these actions only gently damage the leaf edges where they will gradually turn brown, while the inside of the leaf is still green. Not only is the center of the leaf green, but it is alive during this step of processing. This, I’ve come to believe, is the major distinguishing feature of oolong tea, not its oxidation levels. This makes oolong tea more similar to white tea, which only undergoes a slow air drying, than to either green or black tea.

Person doing the yaoqing step of Oriental Beauty oolong processing
Yáo q?ng, or “rocking green” step of oriental beauty production. Video credit: Eric Scott.

Oolong processing involves not only the oxidation of catechins in crushed leaf cells, but also a whole lot of other chemical reactions going on in the living, intact cells in the center of the leaf. These cells are experiencing a lot of stress—they’ve been severely wilted and their neighbors on the edge of the leaf are being damaged by yáo q?ng and this causes them to ramp up all sorts of chemical defenses. This is evident when you look at the gene expression (a measure of which and how many genes are turned up or turned down) of tea leaves undergoing oxidation in oriental beauty processing, reported by Cho and others in a 2007 research paper. They found thousands of genes were activated during yáo qÄ«ng—many of them associated with stress. Many of these genes were responsible for encoding the production of enzymes that make aroma chemicals.

In other words, tea leaves undergoing yáo qīng sensed damage that would ordinarily come from something eating them, and in a last-ditch effort they produced a bunch of defense chemicals that just happen to be really tasty to humans. Many of the genes that were activated in oolong production require intact, living plant cells to do their jobs, which are much more abundant in oolong processing compared to black tea processing where many more leaf cells are ruptured by rolling or crushing the leaves.

Cho and others also measured the aroma chemicals in leaves throughout processing and saw that many aroma chemicals increased dramatically during processing, some up to 400 times greater than in fresh leaves.  In my own preliminary research comparing tea leaves before and after processing, many important aroma chemicals increase in concentration as a result of processing in oriental beauty tea including methyl salicylate, which not only contributes to tea flavor (it’s the main flavor constituent of wintergreen), but is also an important plant hormone produced by plant cells after sensing damage.

Rather than simply being differentiated from green and black teas by what percentage of the catechins are oxidized, I’d argue that there is a more important distinguishing characteristic of oolongs. Many of the floral or fruity aromas of oolong teas that make them so alluring might be produced “from scratch” by living cells in tea leaves during the slow, methodical processing of oolong teas. The fact that oolong leaves are alive (and stressed) for longer than green or black teas is what makes them more than just “mid-oxidized”.


I’d like to thank Dr. Yang Zi-Yin from the Chinese Academy of Agricultural Sciences (CAAS) for inspiration and Dr. Li Xin from the Tea Research Institute in Hangzhou for taking pictures of Dr. Yang’s talk for me, translating it, and discussion.


Cho J-Y, Mizutani M, Shimizu B-I, et al (2007) Chemical Profiling and Gene Expression Profiling during the Manufacturing Process of Taiwan Oolong Tea “Oriental Beauty.” Bioscience, Biotechnology and Biochemistry 71:1476–1486. doi: 10.1271/bbb.60708


1 The correct Pinyin transliteration for this word is “wulong”, but I’ve chosen to use “oolong” because it’s probably a more common spelling in English speaking countries.

The science and nomenclature of tea processing. Part 2: Microbial ripening.

In the first part of this series I tackled the issue of what to call one of two very different tea processing steps both referred to as “fermentation.”  In this installment, I’ll discuss what you might call real fermentation.

One type of Chinese tea, called hēi chá (literally “black tea” but usually translated as “dark tea” to avoid confusion) is often described, confusingly, as “post-fermented”. This, again, is a literal translation from the Chinese term hòu fāxiào and doesn’t make a lick of sense in English. I don’t know much about the history of this term, but this type of tea is actually fermented, so if you’re calling enzymatic browning “fermentation”, then I guess you have to call actual fermentation something different? But I’m getting ahead of myself…

What’s going on:

Although there are several kinds of hēi chá, the most popular by far is shu puer tea (also seen—and heard—as “shou” puer). To make this tea, you start by making what is essentially a green tea, and that involves using heat to stop enzymatic browning by inactivating polyphenol oxidase. This dry, loose-leaf green tea is then piled in thick heaps on a factory floor, moistened with water, and covered with a tarp made of natural or synthetic materials. The tea may or may not be purposefully inoculated with microbes either from some bits of the previous batch of tea (a starter culture) or with specific isolated strains of microbes. If it isn’t purposefully inoculated, the microbes involved come from the tea leaves, workers hands, the tarp, the factory floor, or the air (microbes are everywhere!). The microbes doing the work here are mostly fungi, especially Aspergillus niger, a fungus which is also used in the production of soy sauce, and Blastobotrys adeninivorans, but many bacteria are present as well.

Dish cultivating microbes from puer tea
Photo Credit: Gabriela Garcia

This pile of tea quickly heats up from the metabolic activity of the microbes that are multiplying and eating and breaking down compounds in the tea. This heat is important—only very heat tolerant microbes can survive this process, and those happen to be the microbes you want for safety and flavor reasons. It’s also important to keep the pile oxygenated which is done by turning the leaves occasionally—again, this selects for “good” microbes over “bad” ones. Turning and selectively removing the tarp also keeps the pile from getting too hot and killing everything.  During this process, the microbes are not only breaking down compounds in the tea, but they are also producing compounds that otherwise wouldn’t be found in tea. For example, shu puer is often found to contain cholesterol-lowering statins produced by microbes.

After 30 or 40 days, the microbial process is slowed by removing the tarp and piling the tea into furrows to help it cool and dry. The finished product is drastically different from the starting material in appearance, flavor, and aroma.

What to call it:

There is substantial debate over what to call the tea itself in English—the “shu” in shu puer can be translated either as “cooked” or “ripe”.  As of late, my strategy is to avoid translating at all and just use the Chinese terms, but after deciding on what to call the process that creates this tea, we might just have some good solid reasoning for choosing one translation over the other.

Compared to the process I discussed in part one, this one is much more clearly deserving of the term “fermentation” because it actually involves microbes. “Post-fermentation”, the literal translation of what this process is often called in Chinese, doesn’t make any sense unless you’re going to call some earlier step “fermentation” or if this was something that happened after fermentation. I would personally suggest that “post-fermentation” has no place in any realm of tea discussion and should be totally phased out.  

So is it that easy? Can we all agree on “fermentation”? Well, that’s what I thought until I asked a microbiologist. Ben Wolfe is a microbiologist at Tufts University, and he provided a surprising and insightful comment after learning about shu puer and the microbial players involved in making it.  He asked, “Is this actually fermentation?”

Just like the term “oxidation” in chemistry, “fermentation” has a slightly different meaning in microbiology compared to everyday English. Fermentation isn’t just any process that microbes carry out—it specifically describes the way microbes get energy from their food when no oxygen is around. Fermenting microbes typically make lactic acid or ethanol as waste products of fermentation. As Professor Wolfe scanned a list of microbes found in shu puer, he noted that none of the most abundant species were typical fermenters.

So what do microbiologists call a food that is made by microbes but not by true fermenting microbes? Ripened foods. Salami, brie cheese, and katsuobushi (AKA bonito flakes) are all examples of microbially ripened foods where exposure to oxygen is necessary in production.¹ On the other hand, kimchi, sauerkraut, and kombucha are all fermented under limited oxygen exposure. Making shu puer is more like making salami than like making sauerkraut because of the importance of oxygen. Too much exposure to oxygen can ruin a batch of sauerkraut but too little oxygen will ruin a batch of shu puer.

If we want to be really specific and geeky, we can call this process microbial ripening which naturally lends support to calling shu puer “ripe” or “ripened” puer in English.

Closing words

Language is crazy complicated, especially so when you’re dealing with two very different languages, industry jargon, AND scientific jargon. I mostly intend this series as a fun, ultra-geeky delve into a few aspects of tea and not as a suggestion for how everyone in the tea world should talk or write. Honestly, introducing yet another set of terms to the tea world probably isn’t worth the confusion even if the terms themselves are less confusing.

My advice is to approach tea terminology with a goal of clear and friendly communication. As long as there is mutual understanding, there’s no need for correction or clarification. There are a few situations where I think clarity is especially important and confusing terms should be avoided. For example, when introducing beginners to the world of tea, “fermentation” is a really confusing term for what is actually “oxidation” or “enzymatic browning”—I’d avoid that term entirely unless you’re talking about real microbial fermentation or ripening. If you’re writing a scientific publication about tea that a microbiologist or a chemist might read, be clear about whether you mean microbial ripening, fermentation, or enzymatic browning (which all get called “fermentation” regularly).  

On the other hand, if a tea farmer from China is telling you about how they do the “fermentation” process for their oolong, suppress the desire to interject with “Actually…” and start taking notes instead!


¹As a side-note, the reactions that ripening or fermenting microbes use to get energy from their food would be characterized as oxidation reactions 😉

The science and nomenclature of tea processing. Part 1: Enzymatic browning.

Part of earning “geek status” in the tea world is learning a bit of the specialized jargon that goes along with it. As in any specialized field of industry or study, jargon can be problematic, especially when that jargon takes the form of using words that already exist in everyday English and applying new meaning to them. Perhaps the most frustrating example of this is the word “theory” which in everyday English means “a guess”, but in science means essentially the exact opposite—“a broad explanation of something that we’re really certain about”. The lexicon surrounding tea is no less confusing. In this series, I’m going to explore the jargon behind two different processes important for tea production that are both sometimes called “fermentation.” I’m also going to dive into the biology and chemistry behind steps of tea processing and suggest some new terms that could make the biology and chemistry even clearer.

In part one, I’m going to be tackling the problem of what to call the processing step that makes green tea leaves into black tea. In the tea industry, this is often called “fermentation” which is a direct translation of the Chinese fājiào. This is the process that makes green tea leaves darken in color to produce oolong or black tea.

What’s going on:

Almost all plant parts contain a suite of chemicals called catechins, one group of a larger family of compounds called polyphenols. They all share a similar structure that includes several alcohol (-OH) group sticking off of them. Catechins are colorless and have a variety of functions in plants including acting as antioxidants to snatch up DNA damaging free-radicals. They are generally kept in a compartment called a vacuole inside the plant cell. Plant cells also contain an enzyme called polyphenol oxidase. Like all enzymes, polyphenol oxidase is a large (relative to the catechins), protein machine designed to speed along a specific chemical reaction. Polyphenol oxidase is kept in a separate compartment from catechins, called a plastid, and it can only do its particular job when a plant cell is damaged, like when tea leaves are bruised during the production of oolong or black tea.

Epigallocatechin gallate (EGCG), the most abundant catechin in tea

When the leaf is damaged, polyphenol oxidase mixes with the catechins and speeds along a reaction converting catechins into quinones by removing the hydrogen atoms (H) and some electrons from those alcohol groups (-OH) with the help of oxygen from the air (Oâ‚‚). Quinones are then able to bind with other quinones or other polyphenols forming larger, more complex polyphenols that tend to have a reddish brown color.

However, if you apply heat early on in tea processing, the enzymes (being made of protein) “cook”, which makes them inactive. Thus, if the leaves are heated before they are bruised, this chain of events never happens even though the catechins and oxygen are present. The simple version: when a leaf is bruised, the contents of two compartments mix and a chemical reaction occurs that creates a reddish brown pigment. If you heat the leaf enough, this reaction won’t happen. Or I should say, the reaction won’t happen quickly since catechins can oxidize without polyphenol oxidase present; it just takes a lot longer¹.  

What to call it:

In English, “fermentation” always involves microbes in some way, so when English speaking tea geeks first learn about the lack of microbes in this step, they generally switch to using the term “oxidation” instead as a more correct, scientific alternative.  Unfortunately, this term has some jargon issues as well. In everyday English, “oxidation” is a reaction that happens when things are exposed to oxygen. Rust formation, the Statue of Liberty turning green, and apples browning might all be called “oxidation”. In chemistry though, “oxidation” has a different meaning.

An oxidation reaction is any chemical reaction where something loses some electrons. That’s it! It doesn’t even have to involve oxygen! So for a scientist, calling this step of tea production “oxidation” isn’t terribly helpful since that word can describe so many different chemical reactions going on all the time. A food scientist would probably call this process “enzymatic browning,” a term that I’ve surprisingly never seen come up in any discussion about tea². If you read about browning of fruits and veggies though (which is exactly the same chemical reaction), it becomes clear that “enzymatic browning” is the most precise term for what’s going on here without going into a full on description of the exact enzymes and chemicals involved. It’s unclear to me why tea scientists and tea industry professionals have tried to reinvent the wheel with the terminology for this process, since “enzymatic browning” appears to be already widely used and understood in food science.

When deciding on what to call this process, it’s important to think of who your audience is.  If you’re talking to tea professionals who are already familiar with tea processing, it probably makes sense to call it “oxidation” or maybe even “fermentation” (as long as it’s clear you’re not talking about real fermentation).  However, for neophyte tea geeks, “fermentation” is often a very confusing term, and I would avoid using it entirely (with the exception of warning them that other people might use it).  If your audience is scientifically minded, please at least avoid using “fermentation”, and why not use “enzymatic browning”? It’s clear, concise, and it lets you tap into a large established body of food-science literature.


¹ For this reason, I believe it’s unnecessary to invoke the “partial kill-green” theory of what makes raw puer different from green tea. Catechins oxidize over a period of years just fine with no enzymes present. It is, however, entirely possible that higher temperatures in processing kill the microbes present on fresh tea leaves and slow or prevent the ripening of raw puer tea because of reduced microbial colonization.

² Tea scientists seem to be as confused as everyone else. They sometimes use “fermentation” with an explanation that this step doesn’t actually involve microbes. Other times they use “oxidation”, but explain what specific chemical reactions are going on. Still other times they use these terms without any explanation that they have a different meaning than the scientific one!

Bug-bitten teas: why are leafhoppers only sometimes a good thing?

Leafhopper responsible for bug bitten teas

For most crops, insect damage is a bad thing—both for yield and quality.  With tea, however, we have this somewhat unique case of the so-called “bug-bitten” teas where attack by an insect actually improves the quality compared to an un-attacked plant by inducing chemical changes that are said to improve the aroma of the finished tea.  Famous examples of this are Oriental Beauty (东方美人, dong fang mei ren), Concubine Wulong (贵妃, gui fei), and Honey-Aroma (蜜香, mi xiang) black teas.  All three of these teas are originally Taiwanese, so is there something special about Taiwan?  Well, when you dive into the scientific literature on leafhoppers on tea plants, most of the studies done in Taiwan are on a species of leafhopper known as Jacobiasca formosana, while most of the studies in mainland China talk about a leafhopper known as Empoasca vitis.  So it seems safe to assume that maybe one of these insects causes “good” chemical changes in the tea leaves that increase the quality and the other species causes “bad” chemical changes that decrease quality, right?  Well, according to a few recent studies, it turns out there is very strong evidence that they’re all the same species (Empoasca onukii), so the leafhopper responsible for Oriental Beauty is actually quite widespread!

And it’s not something specific about geography either, as farmers in mainland China are beginning to adopt this technique to produce bug-bitten teas as well. So why is it that insect damage is a good thing only for these few teas? Why aren’t there any bug-bitten green teas on the market?  I’ve done a bit of thinking about this, and it seems like there are only a handful of ways to explain why leafhoppers are considered pests on green teas, but can improve the quality of some wulongs like Oriental Beauty.

  1. The leafhopper causes the same chemical changes in all tea plants, but those changes are considered “good” in wulongs and “bad” in green teas.  Green teas and wulong teas obviously have different criteria for judging, so maybe what makes an wulong good is actually undesirable in a green tea. This seems unlikely to me since there is a lot of overlap in the list of characteristics that make green tea and wulong tea good, but it is certainly the simplest explanation for this phenomenon. It may also be a simple matter of leaf appearance which is more important for a green tea like Longjing where any blemishes are easy to see compared to an oxidized, twisted leaf tea like Oriental Beauty.
  2. The effect of the leafhopper depends on processing method. In this scenario, the leafhopper still causes exactly the same chemical changes in all tea plants, but those chemicals get modified by different processing methods to create differences in the processed tea. For example, a leafhopper might cause a tea plant to produce some compounds that when left   unoxidized—as they would be in a green tea—produce undesirable flavors, but when they are oxidized in wulong processing they become compounds with desirable characteristics in the finished tea.
  3. Leafhoppers do different things to different cultivars. Maybe different cultivars of tea plant respond differently to leafhoppers.  For example, maybe Longjing #43 defends itself by producing more caffeine (an insecticide), which would lead to a more bitter tea, but Qing Xin defends itself by producing hotrienol, a chemical that smells nice to us and maybe attracts predators of the leaf hoppers.*
  4. Leaves of different ages react differently to leafhopper damage.  Ok, bear with me on this one. Most green teas are produced with only one or two leaves and a bud while most wulongs are produced with even older leaves included in the plucking. Maybe the leafhopper produces undesirable chemical changes in young leaves and desirable changes in older leaves.  Then, if you make a green tea, you’re only getting the bad changes, but if you pluck for an wulong, maybe the good changes outweigh the bad ones.  This one might seem like a stretch, but research shows that it is common for young leaves to respond to damage differently (and more intensely) than older leaves, so I think this is entirely possible.

I’m hoping that you—the knowledgeable tea buyers, farmers, and consumers of the tea world (AKA Tea Geeks)—can rule out some of these possibilities or maybe add some that I’ve missed.  Please let me know what you think in the comments!

*These are just examples! Leaf hopper damage has been shown to increase hotrienol concentrations in wulong cultivars, but as far as I know, no one knows what it does to caffeine or what leaf hoppers do to chemicals in green tea cultivars.

Image credit: By HectonichusOwn work, CC BY-SA 3.0