How Capsaicin Works: The Truth Behind Spicy Food

That burning sensation in your mouth isn’t heat at all, but a neurological sleight of hand courtesy of a compound called capsaicin that’s very good at lying to your nerves.

Have you ever wondered what makes your food spicy? A simple enough question, sure, but one with an answer far stranger than you might expect. Despite your tingling tongue and watering eyes telling you otherwise, what registers as heat when you bite into a chilli pepper isn’t actually a taste at all. In fact, it’s a reaction of the body that is not very different from if you’d pressed your hand against a hot stove.

The culprit behind this apparent malfunction of your senses is a compound called capsaicin, which is primarily found in the white membranes and pith inside chilli peppers. Capsaicin works by binding to something known as TRPV1 receptors, which are heat-sensitive nerve receptors that normally exist to warn you when something is genuinely hot enough to cause harm. Capsaicin, through a most fascinating mechanism of action, essentially hijacks this system. 

When it binds to these receptors, it sends a signal to the brain that says, “hey, there’s something hot happening here, and you should be concerned,” this being despite the fact that there has been no actual increase in temperature. What then follows is that hellishly familiar sensation that leaves you sweating, turning red as a beet, and reaching for the nearest glass of water (or milk, if you’re smarter than the average bear). Your mouth might not be on fire, but your nervous system, through capsaicin’s ingenious trickery, has been led to believe otherwise.

Of course, not all chillies are created equal, and to measure just how much capsaicin a pepper contains, we use something called the Scoville Scale, named after the American pharmacist Wilbur Scoville. Measured in Scoville Heat Units, or SHU, the scale ranges from things like bell peppers, which sit comfortably at zero, meaning no heat, to peppers with a little bit of kick to them like jalapeños that range between 2,500 and 8,000 SHU, to tear-inducing aberrations like habaneros, which can reach anywhere from 100,000 to 350,000. 

Originally, Scoville measured heat by repeatedly diluting pepper extracts until people could no longer detect the burn, but nowadays, laboratories use a more precise chemical analysis, which is honestly probably better for everyone involved.

As if this wasn’t enough, there is a brand of absolute sadists who have dedicated years of their lives towards the express goal of making peppers even hotter. Through selective breeding, growers have crossed particularly fiery varieties over multiple generations in an effort to increase capsaicin concentrations. 

The result is a class of peppers often referred to as “super-hots”. Among them are names you’ve probably heard before, such as the Carolina Reaper or Pepper X, the current incumbent atop the infernal throne of the world’s hottest pepper. These peppers are so potent that eating them has become something of a competitive sport, though why anybody would willingly subject themselves to such an experience is beyond me.

It is, when you stop to think about it, a rather absurd state of affairs. A pepper evolves to stop things from eating it, and what do we do? We hold competitions to see who can eat the most aggressive version of it. Yet another quirk that characterises our at times deeply confusing species.