Vitamin B12, also known as cobalamin, is a polycyclic compound containing cobalt 3. Four reduced pyrrole rings are linked together to form a golum ring (similar to porphyrin), which is the only vitamin containing metal elements. Vitamin B12 is a red crystalline powder, odorless and tasteless, slightly soluble in water and ethanol. It is most stable in pH 4.5 ~ 5.0 weak acid conditions. It is decomposed in strong acid (pH<2) or alkaline solution, and can be destroyed to some extent when heated. Higher plants and animals cannot make vitamin B12, which is synthesized by