Ever take home a jalapeño chile pepper from the grocery store and have it either be so lacking in heat it may just as well be a bell pepper, or so hot a speck will create a raging inferno in your mouth? Here’s a quick tip for choosing jalapeños that can help you decide which ones to pick. Jalapeño chilies progressively get hotter the older they get, eventually turning bright red. As they age, they develop white lines and flecks, like stretch marks running in the direction of the length of the pepper. The smoother the pepper, the younger, and milder it is. The more white lines, the older and hotter. Red jalapeños can be pretty hot, if they have a lot of striations, but they are also sweeter.
If you are trying to avoid the hottest jalapeños (say for a stuffed jalapeno dish), pick the chiles without any striations. If you are looking for heat, find a red or green one with plenty of white stretch marks.
Note that this is just a guideline. There is still plenty of variation among individual peppers. You can find hotter-than-Hades peppers without any white lines. But your chances of picking a mild one are better if you go for smooth. Or if you are looking for heat, you will more likely find that in a pepper with lots of lines.
I would like to clarify here that this tip is based on absolutely NO scientific evidence. I was complaining to a Mexican chef friend of mine one day that I kept on buying jalapeños with no flavor and no heat, and he pointed out to me that I should look for peppers with a few striations (but not too many). I have seen this approach mentioned by others (online), but who knows what is really going on? I do know that they are developing much milder variety of jalapeños these days. I also know that the capsaicin, the chemical that gives chiles their heat, is concentrated in the seeds and ribs. The flesh of the chile that is closer to the seeds will be hotter than the flesh near the tip. This is established fact. Perhaps chiles that are more mature have more of their capsaicin distributed throughout their flesh than the younger ones? Perhaps hotter varieties of jalapeños develop striations and milder ones do not. I have never eaten a mild striated jalapeño. But several times I have bought perfectly smooth, beautiful jalapeños only to be disappointed in their complete lack of flavor and punch.
So, please take this tip with a grain of salt. Since using this approach I have not encountered a dull jalapeno. But as I said, I don’t really understand the how’s and why’s of it, and am only taking guesses at what might be going on.