Why does mint ice cream seem colder than other flavors? Why does some ice cream seem to melt faster than other kinds? asks a reader.
Wondering about your ice cream? Just think about the toothpaste you use after finishing your cone. Even when minty toothpaste is at room temperature, the brain interprets the mouth sensation we're feeling as "cold." What gives your tongue that tingly chill is menthol, a component of peppermint oil. And peppermint oil or extract is the main flavoring in most mint ice creams.
Menthol is a pungent plant chemical called a terpene. (Terpenes also give pine needles and turpentine their memorable scents.) Why does menthol create a cold sensation? Scientists found clues in how chili peppers make the tongue feel hot, even when the peppers aren't warm.
The "heat" sensation depends on channels in nerve cells in places like the mouth and skin. These "ion channels" open to allow electrically charged sodium and calcium atoms to enter the nerve cell (neuron). Signals travel from neuron to neuron to the spinal cord and brain, which decodes the signal as "heat." Interestingly, both heated food and chili peppers cause the channels to open. Result: that warm-to-fiery feeling.
Until recently, the equivalent "cool" receptors hadn't been discovered. Then researchers found an ion channel on neurons that opens at temperatures of 46 to 82 degrees. (A temperature of, say, 54 degrees feels downright chilly to our nearly 100-degree bodies.)
Just as the chili pepper chemical called capsaicin opens the hot channels, so menthol opens the cool channels. Like a mouthful of ice, menthol causes neurons to send a "chilly" signal to the brain. Result: Your frozen mint ice cream provides an extra burst of cold, courtesy of menthol.
But whether your favorite flavor is double-cold mint chip or chilly plain vanilla, it's always a race to eat your scoop before it melts. Unlike a frozen block of plain water, how quickly ice cream melts depends on much more than air temperature.
According to food scientists, ice cream is a frozen foam, a whipped mixture of milk, cream, sugar and flavorings, with plenty of thin air. Premium, high-fat ice creams contain the least air, while bargain brands may be more than half air by volume.
Ice cream melts as it absorbs heat from the outside air, with ice crystals on the surface melting first. Scientists say that ice cream's melting rate depends on the amount of whipped-in air, the size of its ice crystals and its framework of fat droplets (courtesy of milk fat and, sometimes, egg yolks).
Whipped-in air actually slows melting, since heat transfers more slowly into air than into water (and milk is mostly water). Ice cream with larger ice crystals also melts more slowly, just as ice cubes melt more slowly than crushed ice. (However, ice cream with large crystals is grainy and crunchy, a texture manufacturers try to avoid.) And dairy scientists say that how evenly fat globules are distributed in the mixture, encapsulating and stabilizing the mixture's air bubbles, also affects the melting rate.