Why Chilli Peppers are Spicy: The Chemistry of a Chilli

The idea for this graphic came to me whilst saving the chilli peppers from a forlorn looking, aphid-infested chilli plant that’s probably on its metaphorical last legs. I thought it would be quite interesting to look at the origin of the heat in chilli peppers, and how this can be quantified.

A family of compounds called capsaicinoids are responsible for the heat of chillies; within this family, several different compounds are found in the various varieties of chillies, but the dominant compound is capsaicin, a vanilloid, shown in the graphic.

The capsaicinoids found in chillies bind to a receptor in the mucous membrane of the mouth when ingested; this is the receptor associated with heat and physical abrasion, and hence this produces a burning sensation. Despite this, the compound does not produce any physical or tissue damage. If the compound is ingested repeatedly, the receptors that it binds to can become depleted, effectively allowing you to build up a tolerance; the pain produced actually produces endorphins, a class of compounds that act as natural painkillers in the body, and can also impart ‘a sense of well-being’. This possibly explains the draw of chilli eating competitions, although possibly not the draw of those that cause participants to vomit and faint midway through the contest.

Although no chilli has a capsaicin content high enough to be harmful (even the fabled ghost chillies), capsaicin is nonetheless a toxic compound. Experiments have estimated that in mice, the median lethal dose (the dose required to kill half of a test population) is around 47.2mg/kg.

As well as its presence in chillies, capsaicin finds a use in pepper sprays in low concentrations, as its inflammatory effects cause the eyes to close, incapacitating those it is sprayed at. They’re commonly used in bear attack deterrent sprays for the same reason, in which the actual percentage of capsaicin and related capsaicinoids in the formulation is recommended to be a minimum of 1%.

The heat of chillies can be measured in a couple of ways. The first method, known as the Scoville scale, is a taste test in which a measured extract of the dried pepper is incrementally diluted with a solution of sugar and water, until the heat is no longer detectable by a panel of five testers. Obviously, this is far from being a precise method, but it has been estimated that the concentration of capsaicin that one unit corresponds to is equal to around 18µmol dm^-3.

The other method through which the heat of chillies is measured is the rather more precise procedure of high performance liquid chromatography (HPLC). In this variety of chromatography, a solvent sample is forced through a column under high pressure, to achieve separation of the mixture.

Finally, there’s the oft mooted question of how best to soothe the fire of chillies. The long hydrocarbon ‘tail’ of the capsaicin molecule makes it insoluble in water; it is, however, readily soluble in alcohol and oil. That said, the small percentage of alcohol in beer sadly isn’t enough to have much of an impact. The best bet for removing the burning sensation of too much chilli is to drink milk – this contains a class of proteins called casein, which is lipophilic and envelopes the fatty capsaicin molecules, successfully washing them away and preventing them from further stimulating the receptors in the mucus membranes.

For further reading on the subject, you can follow the links supplied below. If you want to download a high resolution pdf of the graphic, you can do so here.

An adapted version of this graphic and article are included in the Compound Interest book, “Why Does Asparagus Make Your Wee Smell?”, available now!

Edit 03-02-2014: As an addendum, I thought it was worth noting some of the other alternatives to the Scoville heat index. In addition to HPLC, gas chromatography/mass spectrometry is a method of assessing the capsaicinoid content of different chillies.  In order to do this, a capsaicin compound must be used as an ‘internal standard’ – this is a known amount of compound, different from that being analysed, that can be added to the unknown. The signal from the compound being analysed is compared to that of the internal standard in order to determine the amount of the compound present; the use of the internal standard is important because this standard should experience the same changes as the compound of interest, minimising errors in the gas chromatography process. Thus, after calibration, the ratio of the internal standard and the compound being analysed will allow its concentration to be determined.

Chemists have been working on using capsaicinoid derivative compounds not naturally found in chilli peppers, such as that pictured below, to quantify capsaicinoid compounds present in chilli peppers. This can give more accurate results for capsaicinoid concentrations, and could eventually be used to characterise the minor capsaicinoids present in chillies.

Many thanks to Brandi VanAlphen for providing the information on the use of ethylated capsaicin derivatives in the quantitation of capsaicin levels – you can read more on her research on these derivatives here.

The graphic in this article is licensed under a  Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. See the site’s content usage guidelines.

References & Further Reading

The Chemistry of Chilli Peppers

Fire and Spice