Ever bite into a chile pepper expecting a pleasant kick and gotten a mouth full of fire instead? It turns out the amount of heat a pepper packs comes down partly to the way it’s built. Researchers at New Mexico State University’s Chile Pepper Institute have discovered that super-hot peppers use their interior space differently than mildly spicy ones.

According to the NMSU research team, peppers with more than one million Scoville Heat Units make the most of their interior space in a serious way. All chili peppers come with some amount of the chemical compound capsaicin, which makes them spicy. In relatively mild peppers like jalapeños, capsaicin is found in vesicles (yellow sacs) attached to the fruit’s placenta near its seeds (likely giving rise to the myth that the seeds are the spiciest part of a chili pepper). But in super-hot peppers like the Trinidad Moruga Scorpion chile pepper, capsaicin vesicles are also found on the pepper’s inside wall in large quantities. The spicier peppers make use of their interior surface area, packing in more capsaicin than milder peppers, which confine the compound to the placenta.

Researchers found the source of super-hot pepper’s heat by making the capsaicin vesicles fluoresce before taking a look at them under an electron microscope. “What we were interested in finding was why super-hot chile peppers are able to get that hot,” explained NMSU Regents Professor and Chile Pepper Institute director Paul Bosland. “There, you could see that the jalapeño was only fluorescing on the placenta, while the super-hots would fluoresce all over the wall. It’s a very dramatic image to see. Right now, we’re assuming this is a genetic mutation in super-hots because we’ve never seen this in wild chile peppers.”

While these findings are fascinating in their own right, they may have practical applications, according to Bosland. Understanding what makes some peppers super spicy may be useful to companies that extract heat compounds for use in medicine, as well as for pepper breeders interested in engineering super-hot peppers to delight and torture consumers.

[h/t: Science Daily]