Types of tea are distinguished by the processing they undergo. Leaves of Camellia sinensis soon begin to wilt and oxidize if not dried quickly after picking. The leaves turn progressively darker because chlorophyll breaks down and tannins are released. This process, enzymatic oxidation, is called fermentation in the tea industry although it is not a true fermentation: it is not caused by micro-organisms, and is not an anaerobic process. The next step in processing is to stop the oxidation process at a predetermined stage by heating, which deactivates the enzymes responsible. With black tea this is done simultaneously with drying. Without careful moisture and temperature control during its manufacture and thereafter, fungi will grow on tea. This form of fungus causes real fermentation that will contaminate the tea with toxic and sometimes carcinogenic substances and off-flavors, rendering the tea unfit for consumption.