In the 1940s scientists found that a substance in garlic called alliin was the ‘parent’ compound that must be broken down before antimicrobial effects are possible, and before the characteristic odour of garlic is evident. This catalytic breakdown is accomplished with the enzyme allinase, which is also naturally present in garlic. Alexandra Hicks (1986) wrote:
Simply stated, when a clove of garlic is cut or crushed, its extracellular membrane separates into sections. This enables an enzyme called allinase to come in contact and combine with the precursor or substrate alliin to form allicin, which contains the odoriferous constituent of garlic.