Why chillies are hot? The science of heat

By Beau Gamble September 29, 2011
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Aussies have made the world’s hottest chilli, but what makes chilli hot, and what’s the best cure for the burn?

YOUR EYES ARE watering, your nose is running, and your mouth feels like an inferno. Instinctively, you reach for the glass of cold water in front of you and slosh the liquid down your throat. To your dismay, the water does almost nothing to douse the flames. If only you’d had a glass of full-cream milk – after all, that’s the common cure for chilli heat. Or is it? 

Humans have been cultivating chillies for 6000 years, but we are still learning new things about the science behind their heat and how it reacts with our body.

In the late 1990s, scientists identified the pain nerves that detect capsaicin: the chemical in chillies responsible for most of the burn.

But it’s only during the last few years that scientists have also learnt why chillies evolved to be spicy in the first place, and they have managed to cultivate new varieties that are up to 300 times hotter than the common jalapeno.

What we have known for more than a century, though, is that the capsaicin compound (pronounced cap-say-sin) is hydrophobic, meaning it doesn’t dissolve in water but readily dissolves in fats and oils. And this explains why full-cream milk, and not water, is the traditional choice for quelling the fire. 

“Something with a lot of fat in it – like yogurt or milk – is going to dissolve the compound and wash it away,” says Mark Peacock, a plant scientist from the University of Sydney, who this year helped to cultivate the world’s hottest chilli, the Trinidad Scorpion Butch T. “My favourite remedy is olive oil,” he says, “but it’s not the most pleasant.”

Why did chillies evolve heat?

The hottest part of a chilli is not the seeds, as many people think, but the white flesh that houses the seeds, known as the placenta.

But why did chillies evolve to be hot in the first place?

Most scientists believe capsaicin acts mainly as a deterrent against would-be mammal predators such as rodents. “If a mammal eats a chilli, the seeds are completely destroyed by the mammalian digestive system,” says Mark.

But recent research suggests this may not be the whole story. US scientists working in Bolivia have studied how hot and mild chillies differ in their susceptibility to a certain harmful fungus. It turns out that the hotter the chilli, the better its defences against the fungus, leading the researchers to propose that heat may have evolved to help chillies deal with harmful microbes, as well as hungry mammals.

Birds, unlike mammals, are not bothered by capsaicin, and their digestive systems actually encourage chilli seeds to germinate. So when birds fly away and spread their droppings, they help the parent plant to disperse its seeds.

Maximising the heat

Some varieties of chilli are naturally hundreds of times hotter than others, but Mark says they all have a “maximum genetic potential” that can be achieved through clever growing techniques.

Working with NSW business The Chilli Factory, Mark used liquid runoff from a worm farm to fertilise a particularly spicy chilli known as the Trinidad Scorpion Butch T. Months later, the Butch T claimed the Guinness World Record for the hottest chilli.

Like all fertilisers, ‘worm juice’ is rich in nutrients like phosphorous and nitrogen, but what makes it particularly effective for bringing out the heat are the bodies of insects that have decomposed in the worm farm.

“The insects in there are living and dying pretty rapidly, and bits of their shell will break down,” says Mark. “When you apply the juice to the plants’ roots, they think they’re getting eaten by insects.” In response, the chillies produce more of their defensive compounds like capsaicin. “It’s like getting an injection to boost your immune system,” he says.

Why chillies are unpleasant

Chillies are inherently unpleasant to humans – capsaicin is technically a neurotoxin – and yet every day, more than two billion people around the world consume them. Our obsession with chillies can be at least partly explained by the fact that our body releases endorphins in response to the burn. These are the same chemicals released during orgasm and the so-called ‘runner’s high’.

Curiously, humans appreciate a number of flavours that are inherently distasteful, says Professor Joel Bornstein, a neurophysiologist from the University of Melbourne. “Sour and bitter are supposedly unpleasant but we’ve converted them in our minds into pleasant tastes,” he says. “It’s similar with capsaicin.”

Fooled into thinking chilli is hot

Capsaicin is a special case, however, in that it can fool our bodies into thinking chillies are literally ‘hot’. “The nerve cells that have the receptor to detect capsaicin, some of them also sense changes in temperature,” says Joel. “So when they’re activated, they tell the brain there is a hot stimulus and it actually feels like your mouth is burning.”

The more concentrated the nerve receptors in a particular part of your body, the more sensitive that part is to capsaicin. That’s why getting chilli in your eye can be unbearable, and why you should protect your hands when touching the inside of a hot chilli. Handling the outer layer, however, is usually not painful.

Fortunately for heat-seekers, it appears capsaicin does not cause permanent tissue damage, even in high doses.

“It’s what I call ‘fake pain’,” says Mark. “It doesn’t actually cause you physical harm, even though it feels like it.”