Capsaicin, and other closely related compounds (the capsainoids) are responsible for the heat of chillis. For some time, scientists have thought that Chillis produce capsacin in order to protect themselves from fungal infection. This hypothesis seems quite reasonable: after all, capsaicin has no obvious function in the plant, and so may have evolved to have an effect on other organisms, perhaps to protect the chillis from being eaten. Furthermore, Chillis are dependent on many animals to disperse their seeds, and any defense that developed against vertebrates would be likely to interfere with this, having a detrimental effect on the plant. In contrast, no microorganisms feeding on the chillis benefit them.
But the strongest evidence for this idea was presented in a paper in the Aug 19 2008 issue of Science 1. The authors showed that fungal infections are indeed a major cause of damage to chillis, and that there is a correlation between number of insect bites in a particular fruit and the severity of subsequent fungal infections. The effect of additional insect bites is less significicant for peppers producing more capsaicin, which also contain less fungus, suggesting that capsaicin has a protective effect. To show that it was indeed capsaicin that was responsible for this difference, the scientists then grew fungus in two media, differing only in amount of capsaicin and dihydrocapsaicin - these compounds reduced fungal growth by 33%. Finally, they compared chillis growing across an area of 1,600 square kilometers in Bolivia. The scientists expected that in areas where chillis have more insect bites, they are exposed to the fungus more frequently, and will therefore be more likely to produce capsicin to protect themselves. And that is exactly what fieldwork found.
Not all chilis are equally hot, and their heat is compared with the Scoville Scale, developed by Wilbur Scoville. Each chilli pepper, or chilli sauce, can be given a score according to the number of times it must be diluted with water before its heat is no longer felt by a panel of taste-testers. The problem with this method is that it is time consuming, and the testers must be representative to the population. Some have tried to use more objective techniques from analytical chemistry, but initially had little success. High Performance Liquid Chromatography was tried, but its use was limited by its high cost. Last year, Richard Compton and his colleagues at Oxford University announced2 that they had developed a technique for the voltammetric measurement of capsaicin concentration. In short, they placed electrodes into the sample, and measured how the current flowing between them varied as they changed the voltage across them. By repeating this several times, they could assign a Scoville score to the sample. This method is potentially much cheaper and more widely usable than existing methods.
The reason why capsaicin tastes hot is that it binds to a receptor called TRPV1 on specific nerve cells. This permits the entry of sodium or calcium ions into the cell, resulting in the transmission of an electrical signal to the brain. The opening of this recptor could have another application: the targeted entry of anesthetics. The local anesthetics currently used by dentists target all nerve cells in the mouth, not just the pain-causing nociceptors. Consequently, they make the mouth feel numb, and speech slurred, for hours after the operation. A study3 conducted on rats by scientists in Massachusetts found that when an anaesthetic called QX-314 was applied after capsaicin, it would enter nociceptors through the TRPV1 channel, reducing pain, but have no effect on other nerve cells. Capsaicin has also be used alone as a local anaesthtic.
Some have suggested that people initially began eating chillis as medication, because of their anti-fungal properties. And as the paper by Compton observes they have “antioxidant power, in addition to anti-tumoral, anti-mutagenic, antibacterial and anticarcinogenic properties. . . protective effects against cholesterol and obesity”. Despite their usefulness in healing, they are also used to harm: many of the less-than-lethal pepper sprays used to disperse crowds or disable attackers contain capsainoids as their active ingredients.
Tewksbury et al. Evolutionary ecology of pungency in wild chilies. Proceedings of the National Academy of Sciences of the United States of America (2008) vol. 105 (33) pp. 11808-11 doi:10.1073/pnas.0802691105 ↩
Kachoosangi et al. Carbon nanotube-based electrochemical sensors for quantifying the ‘heat’ of chilli peppers: the adsorptive stripping voltammetric determination of capsaicin. Analyst (2008) pp. 15 doi:10.1039/b803588a ↩