Several things make a jalapeno hotter, and fire-roasting is one of them. If you fire-roast them whole, the hottest inner parts will infuse the entire chili's flesh with heat, whether you keep it whole or remove the spicy seeds and membranes. Fire-roasting also enhances the jalapeno's flavor by imparting a smokiness to the pepper.
Jalapeno heat may seem like a subjective experience, but there's actually a standardized way to measure the heat of peppers. The Scoville Scale measures pepper heat, ranging from zero, the strength of a bell pepper, to 16,000,000, the strength of pure capsaicin, which is the component in peppers that gives them their heat. Jalapenos have a respectable heat level, at 2,500 to 5,000 Scoville units. They aren't nearly as hot as some of the top mouth burners. Habaneros and Scotch bonnets measure at 100,000 to 350,000 units.
Jalapeno heat can vary according to variety, and even from one pepper to another from the same plant. If you're looking for a spicier pepper, the 3 1/2-inch "Jalapeno M" variety measures 4,500 units on the Scoville Scale, on the high end for a jalapeno, according to the University of Florida website. Growing "Jalapeno M" plants will give you a supply of consistently hot jalapenos. Alternatively, substitute hotter peppers such as serranos or habaneros for all or part of the desired fire-roasted jalapenos for your dish.
How you prepare a jalapeno for fire-roasting has an impact on the pepper's hotness. To reduce the risk of burning your eyes or getting the burning juice in any cuts or tender places, wear gloves when you handle jalapenos. Wash them under cool running water. Unless you plan to fire-roast them whole, use a paring knife or other small, sharp knife to cut the cap off. The cap is the part of the pepper that looks like a little hat where the stem comes out. For the hottest flavor, leave the ribs -- the white membranes inside the jalapenos -- intact, and leave at least some of the seeds. For milder jalapenos, remove all the ribs and seeds then rinse out the capsaicin released during the removal.
Fire-roasting blisters the outer skin, easing its removal. The inner flesh becomes tender during the few minutes it takes to roast. The flame heat caramelizes the natural sugars resulting in a smoky sweetness. A gas or charcoal grill will work. At 400 to 450 degrees Fahrenheit, the peppers will blister and become tender in six to eight minutes. Use metal tongs to turn the jalapenos and expose each side to the flames for even roasting.
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