When you think of jalapeños, you’ll probably picture the immature green variety. Since it is picked imaturely, the heat hasn’t fully developed, resulting in a milder flavor and crunchy texture. However, if the jalapeño is kept on the vine it will slowly change in color and consequentially, in heat. Red jalapeños are the hottest and green are the mildest, with orange and yellow peppers lying in between the two.
Once you’ve determined which type of pepper you need for your dish, external markings should be the next criteria you consider. Peppers that are improperly cared for will likely have white stretch marks. They become more frequent with age, so a green pepper with white marks is older than a unmarked pepper and therefore spicier. This doesn’t mean that you should avoid marked peppers entirely, but be aware that they’ll have a bit more kick.
Like most produce, its important to pick a jalapeño without obvious blemishes. However, if you’re stuck with a jalapeño that’s bruised or has shriveled stems, you can usually count on it being older. Also, blemished jalapeños are often hotter, but since they’re lower in quality, avoid them whenever possible.
To further estimate the hotness of your jalapeños, run your fingers around the entire pepper. The smoother the skin, the milder the flavor. Note that this is not a hard and fast rule; its quite possible for smooth peppers to be spicy, but most varieties will adhere to this criteria.
Some may claim that the heat of jalapeños corresponds to a smaller size, but we don’t find it to be a great indicator from jalapeño to jalapeño. Instead, comparing size is better suited when working with different types of peppers. For example, the smaller Serrano pepper is always hotter than a jalapeño, while bell peppers or poblanos will be milder.