Green tea is tea leaves that have undergone minimal oxidation during processing. Oxidation is a chemical reactions that takes place when tea leaves are picked and begin to wither and die. After tea leaves are picked they are left in an climate-controlled room where they turn darker as time progresses. This is due to the enzymatic break down of chlorophyll. Green tea is only 5-40% oxidized. It is heated immediately through steam or dry cooking pans once it reaches that level. It is usually processed within one to two days of harvesting.