All teas are prepared by brewing cured leaves of the tea plant, the Camellia sinensis. The main difference between green and other teas is in the method used for curing the tea leaves: either fermentation or steaming and roasting.
With the exception of green teas, most teas (such as black or oolong) are made by undergoing varying degrees of fermentation, from several months to many years. Fermentation is basically an open-air oxidation process which alters the aroma and color of the tea, and typically intensifies its taste. The process also chemically alters the nature of tea's antioxidants.
In contrast, green teas are produced primarily by steaming or roasting the fresh tea leaves — thus able to maintain the maximum possible amount of tea's important antioxidant molecules called “catechins“, responsible for many of the health benefits of green tea.
Large number of scientific studies have linked regular consumption of green tea to beneficial health effects (As of the date of this writing, US National Library of Medicine website - Pubmed.gov lists over 3,600.)
These studies suggest a strong link between regular consumption of green tea and benefits such as
Reducing LDL “bad” cholesterol
Possible cancer cell growth suppression
Increasing immunity to viral infections
There are numerous type of antioxidants. Catechins are one type. The most abundant catechin in the plant is compound called EGCG (Epigallocatechin gallate.) A potent antioxidant, numerous studies have linked EGCG’s therapeutic properties to fighting many human disorders.
A large portion of the antioxidants in black teas are converted to other compounds during the fermentation process. Because of green tea’s minimal processing — its leaves not fermented like black and oolong teas — it contains by far more antioxidants. On average, green tea contains about 120 milligrams of catechins per 100 grams. In contrast, black tea contains about 25 milligrams in 100 grams.