By Dr. Doris Kimbrough
You grab a bag of corn chips and a bowl of salsa out of the refrigerator and settle in to watch TV. The salsa is hotter than you expected and after about five chips, your mouth is on fire. Big gulps of ice water don’t help, so you head back to the kitchen to look for sour cream or a glass of milk. What is going on in your mouth? How can cold salsa from the fridge burn your mouth? Why doesn’t cold water help the way it would for hot soup or hot tea? How does the sour cream or whole milk solve the problem?
To answer these questions we have to look at some special nerve cells (neurons) and the chemicals found in hot peppers. In addition to the nerve cells that help you move (motor) and control body functions (autonomic) you have lots of different kinds of sensory nerve cells. Sensory neurons are responsible vision and hearing and all your other senses. There are many different types of neurons involved in touch. Some can detect pain; others respond to pressure, heat, cold, or itchiness to name a few. The nerve cells that detect heat are the ones we need to focus on for this story.
Heat detecting neurons don’t work at or below normal body temperature; think of them as sleeping until you touch a hot stove when they wake up and tell you “Ouch, pull back! Pull back!” Hot peppers like jalapeños contain a chemical called capsaicin (cap-SAY-shin) that fools these nerve cells. The capsaicin binds to the nerve cells and wakes them up. Your brain gets signals that something is burning you even though nothing that is actually hot (in temperature) is involved. There are other chemicals that can do this: piperine and sabinene are chemicals found in ground pepper and curry spices.
So why doesn’t a nice cold drink of water help with the burning? Capsaicin is a chemical that is hydrophobic—literally: “water fearing”. Capsaicin doesn’t really fear water; it can’t because it is a molecule, which cannot have feelings. However hydrophobic compounds, like capsaicin, do not dissolve in water. Other hydrophobic substances are vegetable oil, wax and gasoline. So when you gulp cold water because spicy salsa is “burning” your mouth, the capsaicin stays bound to your neuron and your brain still gets signals that your mouth is burning. Hydrophobic compounds do dissolve in other hydrophobic substances, like oils or fats. You may have heard the expression, “like dissolves like”. The fat in sour cream and whole milk will dissolve the capsaicin and remove it from the nerve cell. This turns off the signaling to the brain and lets you get on with your life.
About the author: Doris Kimbrough is a chemistry professor at CU Denver. She grew up in Atlanta, GA, and went to college at the College of William and Mary in Virginia and to graduate school at Cornell University in Ithaca, New York. She has loved science and chemistry since she was a little girl when her chemist father let her play (safely!) with stuff in his lab.