Researchers believe that caffeine works as a natural defense system in plants as a way to ward off insects and other herbivores with its bitter taste. The young leaves of a tea plant manufacture higher amounts of caffeine than more mature leaves. Also, the growing region of the plant, the plant’s age, leaf age, field conditions, soil nutrients, length of the growing season, amount of rainfall, and stress by weather all influence how much caffeine content there will be in a specific tea.
Because of all the variables involved, it is difficult to determine exactly how much caffeine is in tea. However, the amount of oxidation that the leaves go through definitely determines the final amount of caffeine present in a cup of tea. The more the leaves are oxidized the more caffeine there will be. Therefore, white tea will contain the least amount of caffeine, while black tea will contain the most.