Decoding garlic's pizzazz: extract stimulates taste, temperature receptors

Science News, June 4, 2005 by C. Brownlee

Despite garlic's widespread role in cooking--and in vampire tales--scientists had long failed to explain the pungent plant's burning taste. Now, a study of gustatory physiology suggests that raw garlic's characteristic spiciness stems from its capacity to open channels on nerve cells that react to both tastes and noxious temperatures.

Researchers have identified a family of neuron receptors, known as transient receptor potential (TRP) channels, that respond to diverse triggers including touch, pain, and pheromones. At least six of these receptors, a subset called thermoTRPs, react to both certain molecules and unpleasant temperatures by permitting a flood of calcium ions to enter nerve cells, which then fire.

Previous studies showed that thermoTRPs respond to several naturally occurring flavor chemicals, such as capsaicin in hot peppers and menthol, which gives peppermint its coolness. Although some researchers have hypothesized that raw garlic also stimulates thermoTRPs, Ardem Patapoutian of the Scripps Research Institute in La Jolla, Calif., notes that no experiment had confirmed that conjecture.

To test whether the TRP channels are responsible for raw garlic's strong flavor, Patapoutian's team used a kitchen garlic press and more-sophisticated equipment to prepare extracts of both raw and baked garlic purchased at groceries. The researchers added droplets of the extracts to cultures of hamster cells that were genetically modified to sport thermoTRPs on their membranes.

The team designed the experiment so that a cell would fluoresce when calcium passed through its membrane. A glow would indicate that the particular extract had activated the channels.

Patapoutian's team found that two types of thermoTRP channels, designated TRIA1 and TRPV1, reacted to the extract from raw garlic but not to the extract from baked garlic, which has a less pungent taste.

To investigate which chemical in raw garlic was responsible for activating TRPA1 and TRPV1, the scientists compared the chemical compositions of the extracts.

"We found that the major difference between the baked and raw garlic extracts was that the raw extract contains allicin, but the baked does not," says Patapoutian's Scripps colleague Lindsey Macpherson. Allicin, a chemical well studied for its role in garlic's health benefits, breaks down when garlic is heated. Further experiments with hamster cells bearing thermoTRPs revealed that pure allicin opens the calcium channels.

The researchers report their findings in the May 24 Current Biology.

The finding adds to a growing list of naturally occurring plant products that stimulate thermoTRP channels, says David McKemy, a researcher who studies the channels at the University of Southern California in Los Angeles. The next challenge, he notes, may be to uncover why the burn of garlic's allicin differs from that of other thermoTRP stimulators, such as those in hot peppers, cinnamon, or mustard.

Patapoutian notes that people have quite an unusual relationship to flavor chemicals such as allicin. While people crave foods containing thermoTRP-activating chemicals, most other animals tend to avoid them. Observes Patapoutian: "It's an interesting issue of how humans have come to like this [sensation]".