Chemistry of Garlic

We’re all familiar with garlic’s distinctive odour and taste, but where does it come from? Harvard-trained chemist Dr. Eric Block has been studying garlic for more than forty years. His book, Garlic and Other Alliums: The Lore and the Science, brings to life the complexity of allium chemistry. Understanding garlic’s chemistry may just help your cooking, too.

Imagine the parts that make up nitroglycerin. Separately they’re inert and stable. But when mixed, they react violently. Something similar occurs in garlic when an invading pest or herbivore gnaws its way into the plant’s cell walls. That bite triggers a tripwire that activates the plant’s defense mechanism. Two substances stored separately in thousands of cells in the garlic plant are called into action. When the cell walls are breached by the invader, the enzyme alliinase and the compound alliin instantly react to form allicin. That chemical repels the invading pest, which can’t stand the odour. Allicin accounts for much of the smell and taste of garlic. While pests are repelled, we humans have adapted it into our cooking and medicine.

The amount of allicin created in preparation for cooking depends on the number of cell walls breached. And that depends on the method used, such as chopping, using a garlic press or other means. Most of the allicin is created in less than a second. However, additional reactions occur over minutes and hours. Each reaction begets a new chemical reaction, with unique corresponding flavours. Because the chemical composition differs among garlic varieties, their individual tastes and odours also vary.
Garlic boiled or roasted whole—where cell walls are not breached—gets its odour and taste from a different set of chemical reactions than what occurs in crushed garlic. In whole garlic, alliinase cannot react with alliin—they’re kept separate. The difference in flavour comes from the breakdown of alliin, isoalliin, methiin and related amino acid–derived compounds, plus other non- sulphur compounds.

Despite the efforts of botanists and chemists such as Dr. Block, garlic still holds many secrets. Even the most sophisticated machine cannot decipher every chemical process in a lowly clove of garlic.