After nearly 5,000 years since humankind experienced an initial chance encounter with tea, we maintain an adoration of this amazing drink. We remain captivated by its delicacy, its aroma, its smoothness, and its boundless health qualities.
Our journey into the world of tea begins in 2737 BCE in China, where lived a man (although some believe a god) named Shennong. Emperor Shennong was a legendary ruler and hero, who taught the people of his day about agriculture and herbs. He is thought to have identified hundreds of herbs useful for medicinal purposes, leading the way for the development of Traditional Chinese Medicine. One day, while outdoors preparing a pot of hot water, camellia sinensis leaves from a burning twig were carried up from the fire, landed inside the pot of boiling water, and infused into tea. Shennong saw the darkened water and being used to tasting strange concoctions tasted the hot water that had become “contaminated” with camellia leaves and enjoyed the taste. Emperor Shennong made this mixture into his regular drink. As tea became the drink of choice among aristocracy, the populace at large readily embraced this new drink as well.
During the next nearly 4,500 years, tea will remain a vital position in the social structure of humanity – always being treasured for its delicacy, its aroma, its smoothness, and its boundless health qualities. As with all things of great value, tea has been fought over for ideological causes, traded among cultures as currency, and even idolized with a cult-like devotion.
Today, tea is the most popular beverage worldwide, second only to water, and per capita worldwide consumption is approximately 40L/y. About 3 billion kilograms of tea are produced and consumed annually and growing at a rate of 2.1 percent a year.
Commercial teas come in three major categories: unfermented green, fully fermented black, and semi-fermented oolong. Most of the tea consumed in the world is black tea, with green tea accounting for 20 percent. Tea contains more than 4,000 chemical compounds that may affect the human body, but most attention has been focused on the polyphenolic compounds.
Green tea is rich in the polyphenolic compounds – bonded benzene rings with multiple hydroxyl groups. Polyphenols are classified by structure into flavonoids and non-flavonoids, and those in tea are mainly flavonoids. Catechins make up 80 percent of the flavonoids in green tea. The levels of catechins are generally 30 to 50 percent lower in black teas , as some catechins are converted into theaflavins.
Tea catechins exist as two stereochemical isomers (forms), catechins and epicatechins, depending on the configuration of the 3′,4′-dihydroxyphenyl and hydroxyl groups at the 2- and 3- positions of the C-ring (see Fig. 1). Each stereochemical isomer, in turn, exists as two optical isomers:(+) and (-), which are mirror images of each other; (-)-catechin can be modified by esterification with gallic acid. Theaflavins are formed by enzyme-catalysed dimerization (two molecules joining) of catechins.
Tea polyphenols are strong antioxidants, but they seem to have a wide variety of biological effects not directly related to antioxidation (see main text). Evidence also suggests that the biological effects of green tea are over and above those that can be ascribed to individual catechins such as EGCG, or even a mixture of green tea polyphenols.