Oolong tea is mostly picked by hand during the spring and winter months in southeast China and Taiwan regions. Oolong teas are partially oxidized teas and undergo the most difficult and time consuming processing method of all tea types. Processed to be full-bodied teas, the leaves for oolong tea must not be picked too early as they will loose their main flavors and their rich taste and they must be processed immediately afterwards. The leaves are withered in direct sunlight and then shaken gently in bamboo baskets to lightly remove some of the plant cells found on the edges of the leaves. Next the leaves are air-dried in the shade until the surface of the leaf turns slightly yellow. This process has to be repeated a few times to ensure the best oxidation. The oxidation period for oolong teas is lower than that for black teas and depends on the type of oolong that was picked. It can vary from about 20% for a green oolong to 60% for a classic Formosa oolong. The oolong leaves are also pan-fired once the desired amount of oxidation has been achieved. Most tea manufacturers use higher heat levels for oolong than for other tea types – this reduces the moisture levels of oolong, which translates in longer shelf life.