Black tea is oxidized the longest and results in the familiar dark leaf. It generally produces a correspondingly darker liquor, ranging from a beautiful light amber to a dark reddish brown. Because of this longer oxidation, its chemical properties are quite different than either oolong or green teas. One result is that most of the caffeine is freed from its chemical bond with the tannins, this gives black tea its unique flavor as well as its healthful properties. Even though black tea may have more caffeine than the other types, it will still generally contain only a quarter to half the caffeine in a typical cup of coffee.
Oolong requires several minutes longer brewing time than either black or green teas.
The finest oolongs are generally considered to be those from Formosa. An exception is Ti Kuan Yin, from China, one of the best oolongs. It receives no fermentation whatsoever and because of this, most of its caffeine remains bonded with its tannins. Experts believe this causes much less caffeine to be released. Research indicates that green tea may be very useful in the treatment of certain types of tumors and cancer.