If you like to cook with chile peppers--and so many people do these days, given the popularity of such spicy cuisines as Mexican, Asian, and Caribbean--you've probably heard of the Scoville scale. Chile aficionados will brag that their favorite chiles are the hottest, with a scorching 300,000 on the Scoville scale, while that jalapeño you've just learned to love only measures a wimpy 4,000. But what exactly do these numbers mean, and how do they translate into useful information?  

Scoville took the guesswork out of judging chiles. Let's start with the invention of the Scoville scale, and then we'll look at how different varieties of chiles rank in this heat hierarchy. In 1912, a man named Wilbur L. Scoville was working for a company that made an ointment for aching joints in which capsaicin, the heat-causing compound in chiles, was an important ingredient. The company was constantly frustrated because the heat level in chiles varied so much. Scoville devised a formal test in which exact weights of chiles were dissolved in alcohol and then added to sweetened water in precise measures. Tasters were asked to determine how much water was needed to neutralize the heat. A rating number was assigned, according to how many units of water were added before the chile's heat became imperceptible.  

Scoville's test was used for the next six decades, yet it wasn't totally reliable, given the fact that human testers' palates are different and easily fatigued by repeated tastings of hot food. In 1980, a more objective test was introduced, the High-Pressure Liquid Chromatography Test, in which powdered chiles are dissolved and then analyzed through a light beam that shows the heat compounds as fluorescent. Most large producers use this test today, but because the Scoville name has been so deeply ingrained in the industry, they make a conversion and still express the pungency in Scoville units. 

The Scoville scale ranks fire but not flavor. So what does any of this have to do with the flavor of chiles? Not much. These tests isolate only the heat-causing compounds, but tell us nothing about the overall flavor. The heat of a chile is found in the inner membrane, while the flavor comes from the meaty pod itself and makes all the difference in how we experience the heat. These tests also do nothing to discern how the heat is felt. As anyone who has eaten a lot of chiles will tell you, some, such as the habanero, deliver sharp, quick bursts of heat, while others, such as the fiery red Thai pepper, burn and linger. Some hit you up front on the lips and tip of the tongue, while others scorch your entire mouth and throat. Even the researchers themselves will admit that for all their accuracy, the pepper is a fickle plant: its heat varies widely from pod to pod, plant to plant, garden to garden, and season to season. Even on a single bush, a pod from the sunnier side will be hotter than one from the shady side.   

The Scoville rating provides a good general measure of the relative heat of different chiles. In other words, you can be assured that a cayenne will be hotter than a poblano. But ultimately, taste remains a subjective experience. There's no substitute for breaking open a chile and tasting it yourself (carefully) for  flavor and, of course, for firepower.