All types of "true" tea (as opposed to herbal tea) come from the same plant - Camellia sinensis. The major difference between black tea, green tea, and oolong tea is how the leaves are processed after they are picked.
Black tea is fully fermented which means that the leaves are allowed to oxidize after picking. Oolong tea is a semi-oxidized tea and green tea is un-oxidized.
Since the oxidation process starts immediately after the leaves are picked, the first step in green tea processing is to halt the oxidation as soon as possible. This is done by heating the leaves by steaming or roasting them.
Steaming is a very precise procedure and requires the supervision of a tea master. Steaming time is measured in seconds, and if done incorrectly can ruin an otherwise acceptable batch of tea leaves.
The tea leaves are spread out to an even depth and exposed to non-pressurized steam. As well as stopping oxidation, steaming also removes the grassy smell, and the steaming time affects the color and taste of the tea liquor.
The leaves must be cooled as quickly as possible after they have been steamed. If they remain at a high temperature they will lose flavor and color. The leaves are brought down to room temperature with air conditioned forced air.
After the leaves are cooled they are pressure-rolled to remove excess moisture. This is followed by at least two cycles of drying and rolling the leaves to give them shape and to bring the moisture content down to about 5%. The final result is Aracha or crude tea. Aracha tea may be consumed as is or can be further processed to a refined tea by roasting the leaves and sorting them by size.
Shiagecha (Refined Tea) Processing
Aracha tea has a strong taste that some tea drinkers prefer, but further processing of the tea leaves gives the tea a smoother taste and the leaves are sorted to a uniform size.
Aracha tea has leaves of various sizes, so one of the steps of refined green tea processing is to sort the leaves by size. This sorting can be done before or after the leaves have been fired. Machine processed leaves are usually sorted after firing, but tea masters processing tea in the traditional manner may sort the leaves before firing them and then fire each batch separately. This produces a more uniform, higher quality tea.
If the leaves are fired after sorting, the larger leaves are fired at higher temperatures than small leaves. This brings out the best qualities of the particular grade of green tea. Firing also produces a more uniform taste to the final product.