A bite of raw garlic, by any other name, would still taste as scorching. Now, researchers have determined that an unstable chemical in crushed, raw garlic titillates two receptors on nerve cells that trigger pain--the same receptors that are stung by hot peppers and menthol. The result settles the debate of which sulfur compound elicits the distinctive pungency, as well as explains why cooked garlic tastes mellower.
Many plants have evolved acrid compounds to dissuade animals from eating them. The sharp taste comes from chemicals that trigger receptors on pain nerves. For example, compounds found in hot peppers, cinnamon, and mustard set off particular receptors called TRP channels that are sensitive to heat. Menthol, on the other hand, trips a related TRP receptor that's sensitive to cool temperatures. Molecular geneticist Ardem Patapoutian at the Scripps Research Institute in La Jolla, California, wanted to know if the caustic sensation of raw garlic also worked via these receptors.
To find out, he engineered cells in culture to contain either the heat- or cold-sensitive TRP receptors, which when triggered change the biochemistry of the cell in a measurable way. Then he added extracts of raw, crushed garlic or extracts of oven-roasted garlic (400 degrees Fahrenheit for 1 hour and you can spread the cloves like butter) to the cultures. Baked garlic didn't trigger either kind of receptor. But the raw garlic extract got both kinds kicking.
The team then tried to identify the active ingredient. When raw garlic is cut or crushed, enzymes convert the nonsmelly compound alliin to allicin, which is then further modified into three other sulfur-containing compounds, all four of which contribute to garlic's delectable scent. When Patapoutian and his colleagues added each sulfur compound individually to cultured cells, only allicin triggered a fast, intense response like the raw extract did. The team also showed that cultured nerve cells responded to allicin. Additional experiments suggested that the concentrations of allicin found in raw garlic extract could account for all of garlic's effect on the nerve cells, further suggesting that allicin is the key component that makes your tongue burn. (Cooking garlic probably prevents the conversion of alliin to allicin, or breaks down allicin quickly.)
Calling the paper "terrific," sensory biology Barry Green at Yale University School of Medicine in New Haven, Connecticut, says the work settles a longstanding question on what chemical gives garlic its bite. More importantly, he says, the work is "filling out our understanding of how the TRP channels work."