Green tea is the least oxidized of the six tea types. Green teas are crafted with the application of heat in a step called firing. This heat denatures the enzymes that cause tea leaves to change in color from green to yellow, to red and final brown in a natural process known as polyphenolic oxidation. Firing neutralizes these enzymes, halting oxidation and fixing the green color. This is what gives the category “green tea” its name. The degree of oxidation that is allowed before firing is the primary defining characteristic used to distinguish the six tea types. 

​

In Japan, firing is most often conducted with steam heat, which rapidly blanches the tea leaves and fixes their bright green color. By comparison, in China and other origins, “dry heat” techniques such as pan-frying, tumble-roasting, oven-baking, and sun-curing are used.