Allicin was first isolated from garlic in 1944. Chemists Chester J. Cavallito and John Hays Bailey identified it while investigating the antibacterial properties of Allium sativum, the scientific name for cultivated garlic.
Allicin is not stored in an intact garlic clove. Crushing or chopping garlic damages its cells and brings the enzyme alliinase into contact with the sulfur-containing compound alliin. The resulting reaction produces allicin, which gives freshly cut garlic much of its sharp aroma and flavor.
The compound quickly breaks down into other sulfur compounds, so pure allicin is unstable and difficult to store. Its antibacterial activity made it scientifically important, but laboratory effects should not be confused with proven medical treatment: evidence for using allicin supplements against infections or the common cold remains limited.
A common mix-up is treating allicin as garlic's only active compound. Garlic contains many sulfur compounds, and its chemistry changes with preparation. Heat, storage time, and whether garlic is crushed or left whole can all affect how much allicin forms.