Allicin does not occur in garlic naturally. Instead, garlic cloves contain the amino acid alliin (S-allylcysteine sulphoxide):
Structure of Alliin
When garlic is crushed or otherwise damaged, the alliin reacts with the enzyme allinase, also found naturally in garlic. Allinase acts as a catalyst and results in the transformation of alliin into allicin (diallyl thiosulphinate):
Structure of Allicin
Allicin begins to break down quickly, expecially if heated. Conversely its breakdown can be slowed by refrigeration.
When allicin degrades it produces various diallyl sulphides, the most common of which is diallyl dishulphide:
Structure of Diallyl Disulphide
Although the diallyl sulphides do not have the strong anti-bacterial and anti-fungal properties of allicin, they are still believed to have medical benefits especially as regards circulation and cholesterol.