The Chemistry of Tea

Tea is one of the most popular beverages in the world. There are over 300 different types of teas falling into one of three general forms: unfermented green tea, partially fermented oolong tea and fermented black tea. The chemical composition of tea varies with species, degree of oxidation, harvesting season, climate, and the age at which the leaf was picked.

The benefits associated with tea are attributed to their antioxidant activity and their ability to scavenge free radical oxygen. Many studies have shown tea's inhibitory effects against tumor formation and growth. These benefits are associated with the polyphenols, which is a family of molecules including flavonols, flavandiols, flavonoids, and phenolic acids. Most of the green tea polyphenols are flavanols, also known as catechins. The major tea catechins are (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG), (-)-epicatechin (EC), (+)-gallocatechin (GC), and (+)-catechin (C). The polyphenols can account for up to 30% of the dry weight of green tea.

Another class of compounds found in tea is the alkaloids (methylxanthines). This family includes caffeine (3 - 5%), theobromine (0.1%), and theophylline (0.02%). Also present are phenolic acids such as gallic acid and quinic acid.
During the fermentation process to produce black tea, the monomeric flava-3-ols (catechins) undergo polymerization leading to the formation of bisflavanols, theaflavins, thearubigins, and other oligomers (short chained polymers). Theaflavins (about 1 - 2% of the dry weight) include theaflavin, theaflavin-3-O-gallate, theaflavin-3'-O-gallate, and theaflavin-3-3'-O-digallate. Theaflavins are red-orange compounds which give black tea its color and flavor. About 10 - 20% of the dry weight of black tea is from thearubigens, which are highly oxidized and polymerized compounds and have not yet been fully characterized.

Oolong tea, which has undergone only partial fermentation, contains monomeric catechins (8 - 20%), theaflavins, and thearubigins. Other compounds found in oolong tea are epigallocatechin esters, theasinensins, and dimeric catechins.

Maintaining Tea Quality in the United States

From 1897 to April 1996 all tea in the United States had been regulated by the Tea Act. The quality of tea was watched over by a group of four tea tasters funded by the FDA. The tea teasters rejected about one percent of the thousands of teas tested each year for reasons of adulteration or contamination. The Tea Association of the United States supported the Tea Act because it prevented inferior and unsafe teas from entering the marketplace. In 1996, Congress and President Clinton repealed the Tea Act to save $200,000 per year in the federal budget (Public Law 104 128), thus removing one of the key watchdogs to tea quality in the industry.

Now it is left to private companies to assure tea quality. Some parameters which can be monitored are chemical profiling for catechin, caffeine, and polyphenol content; microbiological contamination; and pesticide residue. Soon, laboratory techniques might be borrowed from the perfume industry and applied to tea to differentiate between tea types. By developing relationships with independent laboratories, tea companies will not only be able to maintain high quality products, but understand the chemistry of their products and market that knowledge.

Mark Lange, Ph.D., is a senior scientist for The Industrial Laboratories Co., an independent analytical laboratory in Denver. Dr. Lange can be reached at (303) 287-9691.

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