My wife’s graduate studies are winding down, so I’ve been taking advantage of her university privileges to access some tea books and journal articles. I recently read “Pu-erh tea tasting in Yunnan, China: Correlation of drinkers’ perceptions with phytochemistry,” by Ahmed et al., from the Journal of Ethnopharmacology, and my scattered reflections follow.
The authors studied 10 samples of puerh (ripe, new raw, and >10-year-old raw; plantation-grown and agro-forested), infusing each sample 10 times in gongfu fashion. Each infusion was described and rated by a panel of Yunnan experts, and also analyzed in the laboratory for its constituent chemical content.
I learned several things just from the article’s introduction. The Pu, an ancestral people of the Bulang, Wa and De’ang, are considered the first cultivators of the tea plant. I imagine the town of Puerh got its name from them, before becoming famous as a trading center for compressed tea. What a happy accident for this variety of tea to be indirectly named for those ancient tea pioneers! Of course, it may be instead that the Pu’s claims for discovery of tea are falsely advanced on the strength of this “evidence.”
I object to the authors’ statement, “Gongfu cha dao developed in eastern China and draws from earlier tea preparation methods documented in Lu Yu’s Cha Jing.” This is not patently false, but it’s about as meaningful as saying “Punk music developed in Europe and draws from structures developed by Baroque composers.” If the authors have been exposed to that particularly over-reaching kind of Lu Yu-worship, they may even deserve credit here for repeating such claims so tepidly.
The authors set out a classification of the 4 environments where puerh is grown, which mirrors those I have seen elsewhere:
- Forests (composed of wild trees or formerly cultivated trees that have become feral)
- Agro-forests, ”forest areas (0.5–3.0 ha.) thinned for tea cultivation or swidden areas where plant regeneration is fostered…. The multi-storied vegetative structure of agro-forests provides soil fertility and pests and disease control.”
- Mixed crop fields, created in agro-forests by “replacing associated woody plants with rice or other crop production and by pruning tea trees to increase irradiance and promote crop growth.”
- Terrace plantations, that easy-to-hate intensive monoculture.
In the production of ripe puerh, I was interested to learn that “leaves may intentionally be inoculated with selected microorganisms such as Aspergillus sp. (Trichocomaceae)…. The microorganisms in black pu-erh oxidize polyphenol compounds more completely than the enzymatic oxidation of other black teas, and create fermentation-derived compounds known as statins.” I would love to know more about these chemical processes. I remember seeing the term “theabrownin” while browsing journal articles, so this may be one of the compounds created in polyphenol oxidation by microorganisms. Statins are a class of compounds that are used medically to inhibit cholesterol formation.
Previously, I could not find the crucial distinction between green tea manufacture and puerh manufacture. In both, the leaves are withered, fired, rolled, and dried. If these steps are the same, then is puerh merely compressed green tea? The most provocative claim I found in the Ahmed paper is that “the deactivation of enzymes is not as complete for pu-erh [as it is for green tea]. Consequently, pu-erh has a distinct oxidizing process and develops a smooth taste with age.” This is to say that in green tea, the leaves are fired–in Chinese, shā qīng or “kill-green”–to kill enzymes that would cause oxidation; while in puerh, the firing is briefer, leaving some of the enzymes intact to perform a more gradual oxidation. The satisfaction of this crucial insight lasted only a few days, until I subsequently found a post by Tea Urchin that claims that the enzyme-deactivating stage is not categorically less complete than that of green tea. Tea Urchin claims the difference comes from the sun-drying step at the latter end of manufacture. I’m not convinced by his technical explanations, although I’m not in a position to refute them point-for-point. It also occurs to me there are sun-dried green teas that would confound this distinction.
The Ahmed study focused primarily on catechin content (aka polyphenols or antioxidants) and methylxanthine content (caffeine and its relatives), and how these correlate with tasters’ perceptions. On average, agro-forested puerh in the study had much more catechin content, while terrace-grown tea had the edge in methylxanthines. New raw tea had the most catechins, “aged” raw tea had less, and ripe had the least. Measurements of antioxidative activity paralleled catechin content. A taster’s favorable perception was proportional to a tea’s catechin content, likely informed by its bitterness. Methylxanthine content was about the same across production varieties.
As I look at the results of this study, I have a familiar frustration. The authors have analyzed a heroic amount of infusions, but it still seems like the data is not robust enough to isolate each variable in play, and therefore no binding generalizations can be made about manufacture and chemical content. For instance, when I compare the charts, I can identify 6 infusions that are very high outliers on the methylxanthine scale. These 6 infusions turn up in the “aged” green category and also as terrace-grown. I’m going to guess that these 6 infusions all belong to one tea sample, since other charts indicate that methylxanthine content doesn’t vary drastically from one infusion to the next. So which factor contributes the most to a high methylxanthine level? Does terrace farming somehow produce high methylxanthine content? Could methylxanthine actually increase during the aging process? Or is it something else entirely, such as the plant cultivar used, the harvest time, or the leaf grade?
It seems to me that a longitudinal study design would be much more effective in understanding these factors. For example, harvest a batch of single-origin tea leaves, and manufacture some of it as raw puerh, and some as ripe. Manufacture some as green and black tea while you’re at it. Analyze multiple samples of the different teas, and also save some of the raw puerh to be analyzed after it has aged. This is a long time to wait, it’s true, but any solid research started now on the effects of puerh aging would not be too late!
This kind of study might require an intimacy with tea manufacturers of several different regions and their methods, that many Western food science researchers don’t possess. And to be fair, the primary goal of the Ahmed study seemed to be establishing objective chemical markers of tea quality, and then using those markers to advocate for lower-intensity agro-forest cultivation, which the authors have performed admirably. (Evaluating raw, “aged,” and ripe puerh on the same quality scale does confound things a bit, but the averages should still be meaningful.) But I’m sure the information is out there–I just need to keep looking!
Ahmed, S., Unachukwu, U., Stepp, J.R., Peters, C.M., Long, C. & Kennelly, E., 2010. Pu-erh tea tasting in Yunnan, China: correlation of drinkers’ perceptions to phytochemistry. Journal of Ethnopharmacology, 132, p. 176-185. doi:10.1016/j.jep.2010.08.016