Is that chile pepper hot or not?

People who’ve made the mistake of biting into a spicy chile pepper often end up with a burning question:  How can you tell if a pepper is hot or not before you taste it? 
The world has at least 6,000 types of chile peppers, but only a small fraction of those varieties can be found in Texas grocery stores and gardens. Even this small set displays an exhilarating range of shapes, colors and spiciness levels.
“Peppers are quite diverse. If you look at a wild species, you might not even know it’s a pepper,” said Kevin Crosby, Ph.D., Texas A&M AgriLife Research professor with Texas A&M College of Agriculture and Life Sciences Department of Horticultural Sciences, Bryan-College Station.
One such unusual example is the little, round, intensely spicy chiltepin pepper that grows wild in the southern U.S. and Mexico.
The genetic diversity among pepper varieties gives plant breeders lots of opportunities to create varieties best suited to different climates — and tastes.
“We develop all kinds of peppers, but our main priority is stress- and disease-resistant varieties,” Crosby said. “Many wild species of peppers don’t get plagued by these problems, and so we’re trying to breed those genetic traits into the peppers that we like to consume.”
In the past 20 years, Crosby and his colleagues bred a mild jalapeño and a mild habanero, as well as other new chile pepper varieties.
How to gauge a pepper’s spiciness
Crosby tastes a fair number of peppers. But to officially measure spiciness levels, plant breeders run samples through equipment in the lab. They measure the levels of a substance called capsaicin and related compounds that are unique to chiles. From these levels, the researchers derive a rating of spiciness known as the Scoville value, also referred to as Scoville Heat Units, or SHUs.
Capsaicin feels “hot” because it plays a trick on our senses. It turns out capsaicin activates sensors in our body that normally detect high heat. In fact, capsaicin helped scientists learn more about how our ability to sense heat works, a discovery that was celebrated by the 2021 Nobel Prize in Physiology or Medicine.
If you don’t have sophisticated lab equipment to measure capsaicin levels, you can crack open a pepper and waft the pungent aroma toward your nose — just don’t forget to wash your hands with soap afterward. The aroma comes from the level of volatile compounds, which can correlate with spiciness.
But simply looking at a pepper is not an accurate way to gauge spice level, Crosby said.
“Environment has a huge impact on both capsaicin and the volatile compounds in peppers,” Crosby said. “Greenhouse-grown peppers look very beautiful typically, but they don’t have nearly the amount of volatile compounds as a field-grown pepper.”
Know your pepper varieties
While looking at a pepper will not tell you exactly how hot it is, knowing the variety can provide some clues. Below are just a few common varieties of chile peppers, ranging from least spicy to spiciest.
Bell pepper
Heat level:  None! Different cultivars of bell peppers ripen from green to other colors such as yellow, orange or red. Bell peppers are chiles that are never spicy and can be sweet and fruity when ripe. They score a zero on the Scoville scale.
Poblano
Heat level:  A little spicy. In the grocery store, you’re likely to see poblanos of the deepest green. Just a little spicy or not at all, they are a key ingredient in chile relleno. When these peppers are picked red, they can be dried and sold as ancho peppers. These relatively mild peppers score less than 1,000 on the Scoville scale.
Hatch or Anaheim
Heat level:  A little spicy. Known as Hatch or Anaheim chiles, these long, smooth peppers are typically light green at the grocery store. They are generally similar in spiciness to poblanos, at around 1,000 SHUs. A few varieties, like the Rio Grande, are hotter — over 3,000 SHUs. Hatch chiles’ mild flavor has become more popular nationwide in recent years. And, in 2021, they became the first variety of chiles to be grown on the International Space Station.
Jalapeño or chipotle
Heat level:  Pretty spicy! Jalapeños, typically found green in the produce section, are closely related to bell peppers but can be quite hot at 2,500–10,000 SHUs. They taste a little like green bell peppers, but with some heat. A chipotle is a fully ripe, red jalapeño that’s smoked and dried, with a spiciness similar to that of a green jalapeño. A mild jalapeño bred by Crosby and team looks like the regular variety but is less spicy, scoring 1,000–1,500 SHUs.
Serrano
Heat level:  Likely to be hot! A serrano pepper looks like a sharper, more dangerous jalapeño. Although serranos and jalapeños taste similar and are closely related, serranos can score much higher on the Scoville scale, from jalapeños’ upper limit of 10,000 SHUs all the way up to 25,000 SHUs.
Habanero
Heat level:  Scorching! Shaped like little lanterns, habaneros are some of the world’s spiciest peppers. Don’t be deceived by their cute appearance! Habaneros can score 100,000–400,000 SHUs. They also have a sweet, fruity flavor aside from the spice. Crosby’s mild habanero scores only 1,000 SHUs. However, ghost peppers and Trinidad Moruga scorpions, at 750,000–1,500,000 SHUs, and Carolina reapers — which rival the pepper spray used by members of law enforcement at 1,500,000¬–3,500,000 SHUs — are all types of habaneros.
Chile peppers can grow well in gardens
While only a relatively small selection can be found in the grocery store, many more peppers can grow well at home, Crosby said.
“They are easy to grow and are a highly popular garden crop,” Crosby said. “You can even grow some dwarf types as a houseplant if you have enough light.”
As for your harvest, there are myriad uses for the peppers. Besides eating them fresh, cooked and pickled, some people use crushed chiles to repel garden pests. Capsaicin has no effect on birds, but many insects and mammals don’t like the spice. Some wild animals, however, are not bothered by it — just like some humans.