Oct 3, 2007

The froth of the liquid jade

Thirteen centuries ago, Tibetans started to enjoy the drinking of tea. However, the tea plant (Camellia sinensis) did not grow on the Tibet plateau, so the product had to be brought from the neighbor regions of Sichuan and Yunnan (present southwest China). Around the year 1000, a large-scale commerce was already established: tea, sugar and salt came in exchange of horses, furs and other Tibetan goods. All these products were transported through a very mountainous terrain, with mules and horses following a path known as the Ancient Tea and Horse Caravan Road, or the Ancient Tea-Horse Road, which was heavily used until mid 20th century. Given the difficulties of the trip, merchants compressed the tea leaves as much as possible, so fewer horses were needed for the transport. With time, instead of loose leaves, tea started to be traded in the form of hard, dry cakes of various shapes, including tea bricks (which were used as tea money in several Asian regions).

A renowned area of tea production was Pu-Erh county, in Yunnan. Today, Pu-Erh tea (or just “pu-erh”) is generally compressed into cakes or bricks, and has become very appreciated among tea connoisseurs all over the world. Pu-erh is made from a “broad leaf” variety of Camellia sinensis (var. assamica), and the best tea is said to come from old wild trees growing in the Famous Tea Mountains. Many wild and cultivated trees, known as “Tea Tree Kings,” are more than a thousand years old. The traditional elaboration converts pu-erh into an unusual tea, because it can be stored for years and its quality improves with aging (if conditions are adequate). In other words, pu-erh is a “living” tea, which matures with age due to the activity of certain microorganisms. The custom of aging the tea is most likely reminiscent from the times of caravans, when tea cakes had to endure several months of transport across the mountains and were traded as highly valuable goods for years. Similarly to other teas, pu-erh might have some beneficial effects on human health: antioxidant, anticancer, and lowering cholesterol, blood pressure and blood sugar. Concerning its flavor, some experts say that it is “strongly earthy but clean, reminiscent of the smell of rich garden soil or an autumn leaf pile, sometimes with roasted or sweet undertones.”

ResearchBlogging.orgIn a less poetical tone, a group of researchers from Taiwan has studied the effect of microbial fermentation on the quality and chemical composition of pu-erh. First, they isolated a number of fungi and bacteria from two types of high-quality pu-ehr, which were 20-25 years old. They used these microbes to inoculate fresh tea leaves (previously sterilized), which were then fermented under controlled conditions for 7 months. Next, the teas were evaluated by a group of experts, assessing
flavor and quality. As a result, a number of bacterial strains, belonging to the Actinoplanes and Streptomyces genera, were found to contribute to pu-erh characteristic taste and flavor. The researchers observed that the fermentation of fresh tea leaves with some of the Streptomyces microbes produced a tea with at least some of the characteristics typical of aged pu-erh. The characteristics included color of tea infusion, antioxidant activity and content of several compounds (statins, GABA, polyphenols) that may be involved in the alleged health benefits of tea. These studies will contribute to a better understanding of the process of pu-erh aging, and eventually might lead to a controlled production of teas with healthier properties.

Jeng, K., Chen, C., Fang, Y., Hou, R.C., Chen, Y. (2007). Effect of Microbial Fermentation on Content of Statin, GABA, and Polyphenols in Pu-Erh Tea. Journal of Agricultural and Food Chemistry, 55(21), 8787-8792. DOI: 10.1021/jf071629p

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This post is my contribution to Science Linked: Bacteria, a Group Writing Project at Science in Review.

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