Tea is an infusion of the leaves of the Camellia sinensis plant and is the most widely consumed beverage in the world, aside from water (1). Herbal teas are infusions of herbs or plants other than Camellia sinensis and will not be discussed in this article. Although tea contains a number of bioactive chemicals, including caffeine and fluoride, scientists are particularly interested in the potential health benefits of a class of compounds in tea known as flavonoids. In many cultures, tea is an important source of dietary flavonoids.
Types of tea
All teas are derived from the leaves of Camellia sinensis, but different processing methods produce different types of tea. Fresh tea leaves are rich in flavonoids known as catechins ( Figure 1). Tea leaves also contain polyphenol oxidase enzymes in separate compartments from catechins. When tea leaves are intentionally broken or rolled during processing, contact with polyphenol oxidase causes catechins to join together forming dimers and polymers known as theaflavins and thearubigins, respectively ( Figure 2). This oxidation process is known (incorrectly) in the tea industry as “fermentation.” Steaming or firing tea leaves inactivates polyphenol oxidase and stops the oxidation process (2). Although there are thousands of tea varieties, teas may be divided into three groups based on the amount of oxidation they undergo during processing.
White and green teas
White tea is made from buds and young leaves, which are steamed or fired to inactivate polyphenol oxidase, and then dried. Thus, due to minimal oxidation, white tea retains the high concentrations of catechins present in fresh tea leaves. Green tea is made from more mature tea leaves than white tea, and tea leaves may be withered prior to steaming or firing. Although they are also rich in catechins, green teas may have catechin profiles different from white teas, with slightly higher levels of oxidation products (3).
Tea leaves destined to become oolong teas are “bruised” to allow the release of some of the polphenol oxidase present in the leaves. Oolong teas are allowed to oxidize to a greater extent than white or green teas, but for less time than black teas, before they are heated and dried. Consequently, the catechin, theaflavin, and thearubigin levels in oolong teas are generally between those of green/white teas and completely oxidized black teas (2).
Tea leaves destined to become black tea are fully rolled or broken to maximize the interaction between catechins and polyphenol oxidase. Because they are allowed to oxidize completely before drying, most black teas are rich in theaflavins and thearubigins, but relatively low in monomeric catechins, such as EGCG (see Bioactive Compounds) (4).