The burning sensation experienced from a spicy meal is not a flavor detected by the taste buds, but rather a physical feeling of pain and heat. This sensation is known as pungency, a somatosensory experience involving general nerve endings throughout the mouth and throat. The feeling of heat is triggered by a chemical compound in chili peppers that interacts directly with these sensory nerves, mimicking actual thermal exposure.
The Chemical Culprit
The fiery sensation in chili peppers comes from capsaicinoids, primarily capsaicin, which is the most abundant and potent member. Capsaicin is a colorless, odorless alkaloid concentrated in the white pith or placenta of the chili pepper. It evolved as a defense mechanism to deter mammals, ensuring that birds, which are unaffected, disperse the seeds.
The intensity of a pepper’s heat is measured using the Scoville Heat Unit (SHU) scale, developed in 1912 by pharmacist Wilbur Scoville. This scale quantifies the concentration of capsaicinoids present. While the original method involved dilution and taste tests, today high-performance liquid chromatography (HPLC) is used to analytically measure the capsaicinoid content. This objective assessment is then converted to the SHU value.
Pure capsaicin registers at approximately 16 million SHU. For comparison, a common jalapeño pepper typically ranges between 2,500 and 8,000 SHU. The concentration of capsaicinoids has a linear correlation with the SHU score. Other capsaicinoids also contribute to the overall heat, though capsaicin accounts for 50 to 70 percent of the total capsaicinoids in most peppers.
The Biological Mechanism
The mechanism by which capsaicin produces a burning feeling involves a specific protein receptor found on nerve endings: the Transient Receptor Potential Vanilloid 1 (TRPV1). TRPV1 receptors are nociceptors, specialized sensory neurons designed to detect and transmit signals related to potentially damaging stimuli.
The ordinary function of the TRPV1 receptor is to act as a heat sensor, opening its ion channel when the temperature reaches approximately 108°F (42°C) or higher. When activated by actual heat, the channel opens, allowing an influx of positive ions into the nerve cell. This ion flow generates an electrical signal that travels to the brain, interpreted as burning pain.
Capsaicin binds directly to the TRPV1 receptor, stabilizing the open state of the ion channel. This action mimics the effect of noxious heat, chemically tricking the nerve ending into firing a pain signal. The brain registers the sensation as heat, despite the absence of actual thermal stimulus.
The binding process occurs on nerve endings throughout the mouth, throat, and digestive system. This deceptive activation causes the body to initiate responses typically reserved for overheating. Repeated exposure to capsaicin can temporarily desensitize these TRPV1 receptors, which is the basis for building tolerance to spicy foods.
Immediate Physiological Reactions and Relief
Once TRPV1 receptors are activated, the body interprets the signal as a threat and triggers involuntary physiological responses. The brain’s thermoregulatory center initiates cooling mechanisms, resulting in vasodilation (widening of blood vessels) and flushing of the skin.
The body begins to sweat profusely for heat dissipation, known as gustatory perspiration. The pain signal also provokes a stress response, leading to a runny nose and watery eyes. Paradoxically, the perceived pain prompts the release of endorphins, the body’s natural pain relievers, sometimes creating a sense of pleasure referred to as a “spice high.”
Water is largely ineffective for relief and may spread the capsaicin. Capsaicin is an oil-based, nonpolar molecule, meaning it does not dissolve in water, a polar solvent. Effective counteragents are substances that can dissolve or neutralize the oily capsaicin molecule.
Dairy products, such as milk, work well because they contain casein protein. Casein acts like a detergent, breaking up the capsaicin molecules and washing them away from the TRPV1 receptors. Other nonpolar liquids, like high-proof alcohol or vegetable oil, can also dissolve the capsaicin. Starchy foods like bread or rice may offer temporary relief by acting as a physical barrier.