Vitamin B12, also known as cobalamin, is a polycyclic compound containing 3-valent cobalt. Four reduced pyrrole rings are connected together to form a Gulin macrocycle (similar to porphyrin), which is the only vitamin containing metal elements. Vitamin B12 is a red crystalline powder, odorless and tasteless. It is slightly soluble in water and ethanol. It is most stable under weak acid conditions with pH value of 4.5 ~ 5.0. It decomposes in strong acid (pH < 2) or alkaline solution and can be damaged to a certain extent in case of heat. Higher animals and plants cannot produce vitamin B12. Vitamin B12 in nature is synthesized by microorganisms. Vitamin B12 is the only vitamin that needs the help of an intestinal secretion (endogenous factor) to be absorbed.