A few of the recipes in GOOD FOOD call for the use of a Serrano chili. Use a Jalepeno anytime you cannot find a Serrano in the grocery store. This tip may help you decide which pepper to choose.
"Ever take home a jalapeño chili 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 are the most hot, because they've been maturing the longest. "
"If you are trying to avoid the hottest jalapeños, pick the chilis without any striations. If you are looking for heat, find a red jalapeño, or a 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 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 chilis their heat, is concentrated in the seeds and ribs. The flesh of the chili that is closer to the seeds will be hotter than the flesh near the tip. This is established fact. Perhaps chilis 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 red or 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."
To view the picture above, simply double click on it for a larger image. You will see the "stretch marks" or "striations" more clearly.