Plenty of people like a good hot curry. I am not one of them, but I think that most people have met, or know, someone who likes wolfing down the hottest curry in the house as a matter of pride. I do know someone who likes to munch on the same kind of hot curry but exquisitely slowly. Either way, it is safe to assume that pretty much anyone who has had a strong curry, and either enjoyed it or not, will remember the flavour forever.

There has even been a certain amount of study on this topic: some time ago, the compound capsaicin (Figure 1) was identified as the cause of the hotness. Several related compounds have also been identified, some of which are ‘hotter’ than others. This led to the desire to measure the ‘hotness’, resulting in the Scoville Heat Unit, and the Scoville Scale.

There is also the well-known opportunity for a schadenfreude with curry flavours. As it can be a strong flavour, when someone bites on something unexpectedly teeming with chopped jalepeños, the shock on their face is palpable. However, this shock can also be turned on its head with respect to public order. Recent anti-capitalist protests in America have given rise to some disturbing images of people sprayed with a capsaicin formulation (commonly known as pepper spray) either intentionally, or apparently not.

While these are shocking, and the mental and physical distress caused by the use of this ‘riot-control agent’ are readily understood, other factors are also important. The use of pepper spray as a weapon of self-defence, against a rapist or criminally violent attacker for example, seems not unreasonable. However, the link between pepper spray and deaths in people exposed to it who also have compromised respiratory function, increases the interest in managing the use of pepper spray, both politically and scientifically.

Figure 1. The structure of capsaicin, showing a more polar section (left) and a more lipophilic one (right), giving rise to a comparison with lipid structures.

One way of taking things further is to understand the science behind what is happening when pepper spray is used. A judgement can then be made about safety and appropriate conditions for use. The structure of capsaicin (Figure 1) suggests that it has a lot in common with what we know about lipid structure – a relatively polar (hydrophilic) section as one end, and a lipophilic hydrocarbon chain as the other. However, it is not just the lipid-like properties of capsaicin and its related compounds that give rise to the effect we remember so readily – after all, we eat lipids of one sort or another in almost every mouthful and most do not have the same effect as a vindaloo on our taste buds. This hot sensation is due to an effect of the capsaicin on nerves that feel heat (thermoception) and pain (nociception).  Recent work has suggested that there is a direct impact on the activity of calcium channels in nerves and earlier work has found that such exposure was responsible for permanent damage to the cells involved.This is quite sobering when looked at from a riot-control angle. If a chemical is able to cause innervation, as measured by pain, it is arguable that it is a drug. If it is a drug, strict licensing laws would apply. This also influences the dose(s) that can be used legally. Perhaps we need to reflect on the use of pepper spray of indiscriminate dose, as a crowd control agent?