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Hot and Cold

By: Steve Herman
Posted: April 27, 2012, from the May 2012 issue of GCI Magazine.

Daisy rose, smiling faintly, and went to the table. “Open the whiskey, Tom,” she ordered, “and I’ll make you a mint julep.”

—From The Great Gatsby by F. Scott Fitzgerald

The spectators at the Kentucky Derby often sate their thirst with mint juleps, a mixture of bourbon, ice, sugar and mint. That mint component of the drink is a natural source of menthol, a key ingredient in Bengay. Bengay takes advantage of menthol’s cooling properties, because the sensory effect of menthol is cool, while other materials such as capsaicin and mustard oil are perceived as hot. And in fact, menthol is among the most important cooling ingredients in cosmetics and is one of the few materials with an immediately detectable impact. So how does it work?

Extensive information on menthol’s use in fragrance and flavor is available in an account from Leffingwell1.

Menthol has stereospecific isomers, meaning the same molecule can have a mirror image with different properties, similar to the relation of your left hand to your right hand. Nature usually makes the left-hand molecule of menthol, which is critical for its activity. The right-hand menthol molecule—or a mixture of right- and left-handed isomers, called “racemic”—won’t work. So when using synthetic menthol, it is important to know its “handedness.”

Sensing Sensory

Biologically active hot and cold molecules work by binding to receptor sites on sensory neurons, cells that send messages to the brain. Looking at these receptors in detail explains the difference in people’s response to temperature and certain chemicals.

You know when you go outside if it is hot or cold. Receptors in the skin detect temperature and send a message to the brain. Important to sensory response is a transient receptor potential (TRP) cation channel that acts as a transduction channel. Transduction means the receptor takes one signal—it could be a sensation of hot or cold, the smell of a rose or any number of things—and turns it into a message for the brain. 

Messages are sent along the nervous system by positive ions (cations), which are the biological equivalent of the electric current that turns on a light bulb. The full name of the menthol receptor, TRPM8, means “transient receptor potential cation channel subfamily M member 8.”