The sensation of “spicy” is not a true taste like sweet or sour, but rather a chemical irritation that activates pain receptors in the mouth. When a food that is universally mild triggers this burning sensation, the cause must lie outside of the pepper’s natural chemical makeup, originating either from external contamination or an internal biological anomaly within the consumer’s sensory system. Understanding this distinction is the first step in determining why your personal experience deviates from the norm.
Why Bell Peppers Are Chemically Not Pungent
Pungency, the characteristic heat in chili peppers, is caused by a group of compounds called capsaicinoids, with capsaicin being the most well-known. These chemicals specifically bind to the TRPV1 receptor, a protein in your nerve cells that is primarily responsible for detecting heat and pain. Bell peppers, which belong to the species Capsicum annuum just like hot chilies, lack the ability to produce these compounds due to a genetic mutation.
This lack of heat is traced to a recessive gene that prevents the full biosynthesis of capsaicin. Specifically, bell peppers have a non-functional version of the Pun1 gene, which is necessary for the production of capsaicin. While some analyses suggest bell peppers contain trace levels of capsaicinoids, they register a score of zero on the Scoville Heat Unit (SHU) scale, which measures the concentration of these irritating chemicals.
External Causes: Residue and Cross-Contamination
The most common explanation for unexpected heat is the transfer of capsaicin residue from a truly pungent source. Bell peppers are frequently handled and processed near chili peppers, creating multiple opportunities for contamination.
Cross-contamination can occur as early as the farm or distribution center, where bell peppers and chili varieties might be packed or stored in adjacent containers. In the grocery store, the proximity of mild bell peppers to jalapeños, serranos, or other hot peppers in the produce display is a common source of residue transfer. The chemical can rub off onto the bell pepper’s skin through simple contact.
The most likely source of contamination is often the kitchen environment. If the same cutting board, knife, or even the same pair of food handling gloves are used to prepare a hot pepper before a bell pepper, a microscopic but potent amount of capsaicin will transfer. Even a small residual amount can be enough to activate the TRPV1 receptors in the mouth, which are highly sensitive to capsaicin.
Internal Causes: Sensory Hypersensitivity and Oral Pathology
If contamination is ruled out, the cause of the burning sensation shifts to how your body is interpreting the pepper’s non-pungent compounds. This phenomenon is often rooted in a heightened sensory response known as chemesthesis, which is the chemical sense that detects irritation, temperature, and pain through the trigeminal nerve. The trigeminal nerve is what capsaicin activates, and in certain individuals, it can be overly responsive to other, milder compounds in the bell pepper.
In cases of sensory hypersensitivity, the trigeminal nerve might be triggered by certain volatile organic compounds or the physical texture of the pepper’s skin and flesh. This causes the nerve endings to send a pain signal to the brain, which the consumer interprets as “spicy” or burning, even though no capsaicin is present. The nerve’s heightened state can result from previous exposure to high levels of capsaicin, which can sensitize the nerve and lower its threshold for activation.
Another biological cause is Oral Allergy Syndrome (OAS). OAS is a form of contact allergy where the immune system mistakes proteins in certain raw fruits and vegetables for similar proteins found in environmental pollens, such as mugwort or birch. Bell peppers are known to be a trigger for individuals with mugwort pollen allergies.
The symptoms of OAS are typically localized to the mouth, lips, and throat, and can include tingling, itching, minor swelling, or a distinct burning sensation that mimics spiciness. These non-capsaicin-related irritations are caused by the release of histamine in response to the pepper’s proteins.