Feel the heat: chilli tasters could soon be relieved of their eye-watering duties. Credit: BILL TARPENNING/USDA

How hot is hot? When it comes to chilli peppers, the answer tends to be subjective — even official hotness ratings are based on the reliability of human testers or expensive high-tech machines. But now nanotubes may provide an objective, and cheap, check on how hot that sauce you’re ladling onto your kebab is.

A chilli's hotness comes from chemicals known as capsaicins. The higher the concentration, the hotter a chilli tastes.

The conventional method to determine the heat of a chilli or chilli sauce was devised in 1912 by chemist Wilbur Scoville. Scoville ratings are worked out by diluting a chilli-containing sauce to the point at which a team of five expert tasters can no longer detect the heat.

The level of dilution is given as the official heat of the pepper. Scoville ratings therefore go from zero, for the humble bell pepper, to 30,000–50,000 for Tabasco sauce, to 855,000–1,041,427 for the world’s hottest pepper, the fearsome Bhut Jolokia.

Hot topic

But Richard Compton, a chemist at the University of Oxford, UK, and his colleagues have developed a way to get a Scoville rating for a sauce while sparing the tasters' tongues, and without blowing a cook's budget.

Compton says that his method is cheaper than the only other available tester-free method, called high-pressure liquid chromatography (HPLC). The HPLC technique involves separating out the capsaicins in a sauce and working out their concentrations individually — an expensive process.

“HPLC is overkill,” says Compton. He points out that the different capsaicin types that are painstakingly separated and measured with that technique are then bundled back together to give a Scoville-like rating anyway.

Compton and his colleagues have instead made a device that measures the accumulated concentrations of the capsaicins in a chilli simultaneously. The technique involves coating electrodes covered in carbon nanotubes with the chilli sample. The nanotubes have a high surface area, which means that they can absorb lots of the sample.

The whole device is then dunked in an ethanol-based solution to oxidize the capsaicins, which causes current to flow. Stronger chillies mean more electric current, says Compton, whose work is published in The Analyst1.

Cheap as chillies

By using mass-producible electrodes screen-printed with carbon nanotubes, Compton reckons he can bring down the cost of a sampler to around £30 (US$60). By comparison, an HPLC machine costs around £40,000.

Compton tested his electrochemical device on a range of hot pepper sauces, including 'Mad Dog’s Revenge', which has a Scoville rating of around 1 million. Compton's nanotube method gave a value of 991,700, showing that the technique works. “Ours is an easy version of Scoville,” Compton says.

Compton has applied for a patent for his device and is attracting commercial interest for what he envisages as a cheap, disposable hand-held heat detector that keen cooks could use in their own kitchens.

Any taste-testing on the crazy-hot sauces was left to Compton's students to experiment with, however. "I get off at about the level of Tobasco," Compton says.