Capsaicin & the Scoville scale

When a chile makes you sweat, cry and regret your life choices, there is no magic: there is chemistry. Behind all the burn sits a single molecule, and for over a century there has been a way to put a number on it. Let’s take them one at a time.

Capsaicin, the molecule of fire

Capsaicin was first isolated in 1816, though it was not synthesised in a lab until 1930. It is a curious molecule: it has no taste, colour or smell. In other words, spice is not a flavour: it is capsaicin triggering the very receptors in your mouth that detect heat and pain. Your tongue, quite literally, thinks it is burning.

Chile does not make capsaicin to annoy you: it is a defence mechanism. It deters mammals, which crush the seeds with their teeth, but it does not bother birds, which cannot sense the heat and spread the seeds intact. Put another way, chile burns to choose who gets to eat it.

That is why milk and not water. Capsaicin is hydrophobic: it does not mix with water, so drinking water just spreads it around your whole mouth (thanks for nothing). The casein in milk, on the other hand, grabs it and carries it away. Yoghurt, full-fat dairy or a bit of bread work just as well.

The Scoville scale

In 1912, the pharmacist Wilbur Scoville devised the first way to measure heat. His method, known today as the Scoville organoleptic test, was charmingly simple: he diluted a chile extract in sugar water and gave it to a panel of tasters, adding water until no one could feel the burn any more. The more dilution needed, the hotter the chile. That is where Scoville Heat Units (SHU) come from.

The method had an obvious flaw: it relied on human palates, which tire and disagree. Today it is measured with high-performance liquid chromatography, which quantifies capsaicin in parts per million and converts it to SHU. To give you an idea: pure capsaicin sits at around 16 million SHU.

To place yourself, a few reference points:

• Bell pepper: 0 SHU
• Sriracha: ~1,000-2,500 SHU
• Original Tabasco (sauce): ~2,500-5,000 SHU
• Jalapeño: ~2,500-8,000 SHU
• Habanero: ~100,000-350,000 SHU
• Carolina Reaper: up to ~2,200,000 SHU
• Pure capsaicin: 16,000,000 SHU

With these two ideas in your back pocket (what burns you and how much) you can look at our chile grid with new eyes. And, above all, understand why you ordered that glass of milk.