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.
As they age, some peppers develop white lines and flecks, like stretch marks running in the direction of the length of the pepper. The stretch marks are also indicative of the amount of stress the pepper plant has endured.
A pepper plant that is stressed, having the soil get dry between infrequent waterings, appears to have an impact on the the hotness of the pepper.
The older the pepper, and the more stress the plant has been under, the more white lines you'll see, and the hotter the pepper will be.
The smoother the pepper, the younger, less stressed, and milder it is.
Left on the plant (and even after picked) green jalapeños will eventually turn red. So red jalapeños are older than green jalapeños. The red ones can be pretty hot, especially if they have a lot of striations, but they are also sweeter than the green.
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. Make sure to taste test a chili before using it in a recipe!
The best way to taste test?
Capsaicin, the chemical that gives chiles their heat, 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.
So the best way to taste a potentially hot chili is to cut off a small piece at the tip and have a nibble (you'll have less chance of burning your tongue if the chili is really hot).
For cooking, if you want to lower the heat of the chiles, cut the peppers in half, scrape out and discard the seeds and inner ribs (use gloves and don't touch your eyes). If you want more heat, just add back some seeds with the rest of the jalapeño.