Immature jalapeños are hot when they are green but gain pungency and a red color as they ripen. They also increase in sweetness as they mature. These bullet-shaped peppers reach between 2 and 3 inches (5–7.5cm) in length and up to 1 inch (2.5cm) in width. When smoked, dried jalapeños are called chipotle. They are preserved by smoking rather than air-drying because of their thick flesh.
Jalapaños are medium-hot peppers that range in heat between 2,500 and 5,000 Scoville heat units. How high a chile pepper scores on the hotness scale is determined by high-performance liquid chromatography measurement of how many parts per million of capsaicin it contains. (Capsaicin is the compound that gives chile peppers their fiery bite.) This figure is then converted into the historic Scoville heat units that signify how much dilution is necessary to drown out the chile’s heat. The heat level of a chile is given as a range because it varies with how and where the pepper was cultivated.