There is only one foolproof way to know how spicy a jalapeño is: ¡take a bite!
You don’t want to buy a jalapeño chile that lacks in heat so much that it may as well be a bell pepper. On the other hand, you don’t want one that is so hot that the smallest bite will set your mouth on fire. There is a trick that will help you determine how spicy a jalapeño is before you buy.
As peppers age they develop white lines and flecks, like stretch marks along their length. These striations are indicative of the amount of stress the plant has endured. This stress, generally due to the soil being allowed to get dry between infrequent waterings, appears to impact the level of spiciness of the pepper.
The older the pepper is, and the more stress it has endured, the more white striations it will have, 
and the hotter it will be. The younger chiles will be smoother, less stressed, and milder.
Left on the plant, green jalapeños will eventually turn red and their sweetness will increase. So will their level of hotness, especially if they have a lot of striations.
If you’re trying to avoid the hottest jalapeños, go for the green ones without striations. If you’re looking for a bit of extra heat, go for the red or green jalapeños with the most stretch marks.
Chiles get their heat from a chemical called capsaicin which is concentrated around the seeds and in the ribs. The flesh of the chile that is closer to the seeds will be hotter than the flesh near the tip.
Keep in mind this is merely a guideline, there is plenty of variation among individual peppers. While cooking, be sure to taste the chile to determine if it’s too spicy for you. The best way to do that is to cut off a small piece at the tip and have a nibble. If you find the pepper to be too hot, put on some gloves, cut the chile in half lengthwise, and scrape out the seeds and inner ribs. And don’t touch your eyes! If you feel you have reduced the spiciness too much, simply add some seeds back with the rest of the jalapeño.
Leave a Reply