Wednesday, April 4, 2012

The Oxidation of Tea And Classifying Tea As Black, Green, Oolong, Etc.

When I first got into tea, my level of knowledge was a lot more basic than it is now. I had heard the standard adage "black tea is fully oxidized" and its counterpart "green tea is unoxidized". I initially thought that the different classes of tea (black, green, oolong, etc.) were characterized or defined by their levels of oxidation. I was surprised when I found teas that seemed to be exceptions to this pattern. What I found when researching more deeply was that differing levels of oxidation alone does not define or separate the different classes of tea, and that these classes are usually defined by the production process as a whole, which usually, but not always corresponds to certain differences in levels of oxidation.

Reflecting on these exceptions, and wanting to create a better resource on the topic of tea and oxidation, I recently published a new article on RateTea about the oxidation of tea, in which I go into more depth about the level of oxidation among the various tea types.

I would encourage you to take a peek at that article, and if you think it is a worthwhile resource, to consider linking to it when you need a reference on the topic of tea and oxidation. In this post though, I want to delve more into some of the specific teas that inspired me to think in more nuanced ways about oxidation, and ultimately led to that article.

Very green black tea: Darjeeling first flush:

Darjeeling is a black tea, but it often is not fully oxidized. In some cases, it is as green in color (both of leaf and brewed cup) as a number of green teas. Look at the following examples:

Pictured here are, the first two from Upton Tea Imports, Arya Estate First Flush SFTGFOP1, and Thurbo Estate TGBOP Cl/Tip First Flush, then Makaibari Estate Darjeeling 1st Flush from Arbor Teas, and last, Adagio's Darjeeling #1. I featured these teas mainly because of their visual characteristics, although I will say that I've tried the first three and they're all delicious, and they all have a greener character when brewed. The greenest character in a tea presented as a "black tea", however, was, also from Upton, Castleton Estate TGBOP Ch. First Flush; this tea produced a cup lighter in color than a typical Chinese pan-fired green tea, and barely resembled black tea at all, with a very light character and tones of mint.

Very dark green tea:

Although there are plenty of green teas that are more moderate in their color, truly dark green teas, I have found, are rarer than "black" teas which exhibit a highly green character. One particular example stands out, a large-leaf green tea, produced in Thailand from the Assamica cultivar, which I purchased from Upton tea. Here are Upton's pictures of the leaf:

As you can see in the photo, which accurately depicted the tea as I sampled it, this tea was much browner than green in color. You can read my review of this tea for more about my experience with this tea.

The color of the brewed cup was also quite dark, closer to a typical black tea than other green teas. However, there was little about this tea's flavor, aroma, or other characteristics that resembled black tea in any way. I don't have a way of objectively measuring the oxidation level of teas. I wonder if the dark color of this tea were due to oxidation, or were just due to other factors. I honestly don't know what to expect about a tea like this.

Dark white teas:

Darker white teas, like shou mei and bai mu dan (white peony), are fascinating to me because they create problems for some of the older definitions of white tea, as I explore in my post definitions of white tea: raising eyebrows.

But I have found that learning about these white teas has also taught me a lot about the role oxidation plays in tea production, and the way varying production processes impact tea's characteristics through halting or allowing oxidation. For example, the fact that white tea's production does not denature the enzymes responsible for oxidation in the way that the heating does in the case of green tea, enables white tea to oxidize more than green tea, but because the leaves are quickly dried, this process is not allowed to carry out completely. But because larger leaves contain more moisture, the larger-leaf teas oxidize more. This phenomenon also explains why certain teas like moonlight white exhibit a lighter color in years of drought: when the leaves are dry, they dry out completely more quickly during production, thus allowing for less oxidation.

What do you think?

Let me know what you think of the new article on tea and oxidation. I'd appreciate any corrections or additions if you think it can be improved. I'd also like ideas if you have specific articles (including ones you may have written) that you think would make a good addition to my list of further reading at the end of the article. And if and when you think the article is an accurate, comprehensive resource on the topic of the oxidation of tea, I'd like to ask you to link to it as a reference when you mention the oxidation of tea. Thank you in advance!


  1. I think the classification of tea based on oxidation is not based on the outcome (how much the tea is oxidized) but based on processing - whether oxidation enzyme is dis-activated, when it's done and how it's done.
    If thinking from the processing aspect, there are less discrepancy (but there will always be some). But it's also harder for people to understand it this way. I think your article would be very helpful to those who care to learn more. And even for those who don't want to go deeper into biochemistry of tea, it's a good reading to break common myths such as thinking of white tea as snow white.

    About the above mentioned "darker green tea", I suspect its dark color is not because of oxidation. In green tea and puerh, when unwanted oxidation happens, the leaf appears red/brown (can be seen in spent leaves after brewing) and usually this happens in small spots or at the rims. If large part of the leaf turn dark brown, it's hard for the rest of the leaves to still maintain green. The Thai tea leaves still look dark green, so I guess it's just its color. But I've never had this tea. What's its liquor color and how does it taste?

    1. What you say about the classification being based on processing makes sense to me. The more I learn about tea, the more this method of classifying seems to make sense to me.

      One of the reasons I'm uncertain about the oxidation in this particular tea is that the leaves looked more reddish than is typical for green tea, after brewing. The liquor color is quite dark as well, much darker than a typical green tea.

      In terms of its flavor and aroma, it vaguely resembled the one example of Tai Ping Hou Kui that I've tried, and it also vaguely resembled a green tea I tried that was produced in Laos. However, it was quite unlike anything else I've tried and I do not know how to describe it more...I updated my blog post with a link to the review though if you're curious to read it.

    2. That sounds very interesting! Is it in anyway similar to puerh?

    3. that I think about it, perhaps, maybe a bit like some young sheng Pu-erh with larger leaves. Not that I have a huge experience with sampling Pu-erh to really say. It had none of the smoky or strongly edgy qualities that these teas often have though. But it was similarly bold.

      But it was quite a while ago that I sampled this tea that it's hard to recall and say for sure.