In this context, fermentation can also be described as oxidation, or a blackening of the leaves, which occurs when the chemicals in tea react to heat. In green tea, no fermentation occurs. For oolong tea, leaves are fermented less than one hour. For black tea, the fermentation time is three to six hours. Other variations in the teas come from factors such as the soil, climate and weather conditions of the regions in which the tea leaves are grown.
All teas contain powerful antioxidants called polyphenols.
These substances may help protect us from a variety of conditions, including heart disease, stroke, cancer, arthritis and sun-related skin damage. At four cups a day, the risk dropped by 69 percent. In a study funded by the U.S. National Heart, Lung and Blood Institute, those who consumed a cup or more of tea a day had a 44 percent reduction in heart attack risk, when compared to nontea drinkers. The study did not compare the benefits of one cup versus two, three or four. However, much of the evidence about tea's protective qualities still comes from laboratory and animal studies, not human studies, according to the current issue of the University of California at Berkeley's Wellness Letter.