Tea leaves contain tannin and caffeine, which cause bitterness and astringency, as well as theanine, an amino acid that provides an overall umami (savory taste) and sweet complexity. If tea is brewed at a high temperature, tannin and caffeine will dissolve into the water and can make the tea taste slightly bitter. However, if tea is prepared at a low temperature slowly, the tannin and caffeine will hardly dissolve into the water, and theanine, an amino acid, can be accentuated. This is why tea prepared with cold water is rarely bitter or astringent, and embodies a mild yet rich flavor, the inherent nature of tea leaves.

Black tea has no vitamin C. Theanine is contained in high concentrations in green tea, especially Japanese green tea, but not much in black tea. For black tea, the astringent taste of catechin is appreciated more highly than the umami (savory taste) of theanine.