Ask a Scientist

Courtesy of The University of Tennessee, Knoxville.

As a self-proclaimed fanatic of the jalapeno and all things hot pepper-related, being able to write a scientific ode to this plant was simply irresistible. Clearly, I’m not the only one that loves this particular ingredient, either; of the hot peppers eaten by US citizens, jalapenos are probably the most popular, and they are found in practically every grocery store in the country.

Found in everything from nachos and tacos to stews and stir-fries, jalapenos are extremely versatile and delicious. But beyond their subtle sweetness and irresistible aroma, the hallmark of all this hype is the “heat” content.

When we say heat content, we don’t mean temperature. But when you eat these peppers, it definitely feels like your mouth is on fire, even though there are no physical burns going on. What is going on here? 

Well, jalapenos, like all peppers, have a class of chemical in them called “capsaicinoids,” and they are responsible for this sensation. You’ve probably heard of at least one: The aptly named capsaicin.

Capsaicin is the most common and prevalent of the capsaicinoids, but all capsaicinoids mostly function the same way. These compounds bind to a specific group of neuron receptors in your mouth called sensory neurons. Of the sensory neurons that could make that burning sensation, only a few actually go off, like ones that have the “transient receptor potential cation channel subfamily V member 1,” which is a mouthful and is often abbreviated as “TRPV1.”

 Other neurons that don’t have these biological devices won’t fire, which is why touching the flesh of a jalapeno might not burn as much as eating it. Unfortunately — or fortunately, if you’re a sucker for hot food — it just so happens that our mouths have a lot of the right receptors.

We didn’t exactly get these receptors because we wanted to taste heat in hot peppers, either. These receptors normally activate when the tissue around them is physically damaged or inflamed. But, in the case of capsaicinoids for jalapenos, these receptors are chemically triggered anyway, creating this artificial sensation we’re so accustomed to. The more capsaicin that binds to those receptors, the more burning you feel. Quantities matter, though, and not every person has the same number of receptors. A person that has lots of these receptors will feel a lot more burning than a person with less of them when they eat the same pepper.

Seems simple enough. But how do people measure spice? Pepper lovers and hot sauce connoisseurs around the world use a metric called the “Scoville” scale, which is broken down into Scoville heat units, or SHUs for short. The higher on the scale a certain food is, the hotter it is. 

Some peppers, like red and green bell peppers, have no or very little capsaicin and thus are marked at zero SHUs. At the opposite end of the scale, the hottest pepper in the world — which goes by the ominous name of “Pepper X” —  clocks in at 3.18 million Scovilles

Pepper X was invented by a pepper cultivator named Ed Currie, who was actually responsible for making the previous record holder, the Carolina Reaper. His work has been featured in media like the online show “Hot Ones,” and the peppers have definitely gained buzz around the internet. Naturally, some people have taken it upon themselves to eat these ridiculously hot peppers and record the results. They usually involve tears, sweating and/or snot bubbles. While it's fun to watch, I imagine it’s extremely unpleasant and gross. Consume at your own risk.

So, the beloved jalapeno ... where does that fall on the scale? Well, despite a jalapeno definitely having some heat, it’s nowhere near the hottest pepper. According to the New Mexico State Pepper Institute, a jalapeno usually falls around 0-50,000 SHUs, and one study determined that the average was right around 21,700 SHUs. This means that Pepper X is about 64 times hotter than a jalapeno. Can you imagine packing all the heat of 64 jalapenos into a single pepper? That would be like eating close to two and a half pounds of jalapenos at once!

Now you might have noticed something. Up until now, we’d been pretty vague about the Scoville system. Nowhere did we define the Scoville scale. We just sort of casually said, “Aha! Here’s how we measure a pepper’s heat!” But it’s definitely different than other measurements. It’s not like weight, length or volume. When we think of those things, we can easily come up with tools to measure them, but we’re willing to bet that it wouldn’t be that easy to come up with something for SHUs. Which begs the question: Where did it come from, and how do you measure SHUs?

Well, the premise of the Scoville system is actually pretty straightforward. Take something that is hot, dilute it, and have people taste the diluted product. If it’s still hot, dilute it some more. Repeat this process until your test subjects can’t taste the spice anymore. This technique is called the “water dilution” technique, and it is really how this whole unit system got started.

You might be thinking that surely the Scoville system can’t be very accurate. We just said that everyone’s sensitivity to heat is different, after all. You’re right too, the Scoville unit system as it was first made is probably not very useful by today’s standards. So are they really using it anymore?

We reached out to the New Mexico Chile Pepper Institute, and it said that instead of using traditional methods, it can use modern chemistry to simply detect the concentrations of individual heat-causing chemicals. As long as they know what they’re looking for, scientists can find what causes the burn using high performance liquid chromatography, a technique that separates solutions into their individual compounds and can record those compounds’ concentrations.

Despite new scientific tools, most pepper cultivars and hot sauce makers still end up reporting their products' heats using Scoville units. Chances are that this is most likely because it’s easier and because it's tradition. Maybe if you’re really into chemistry and chili peppers like us, you might enjoy knowing the exact chemical makeup of your jalapenos. But for most people, a single scale that measures heat level gives plenty of information.

So there you are: A chili-head’s explanation of what makes jalapenos hot. Compounds such as capsaicin trigger certain receptors in our mouths. For some people, it burns a little too much, but others of us aren't even sure we ate the right pepper. The amount of capsaicinoids can vary pretty considerably from pepper to pepper, creating different heat profiles. Some sneak up on you, and others get hot very quickly, like the jalapeno. No matter the profile, it looks like our love for all things spicy isn’t going anywhere anytime soon.

Have a question for Ask a Scientist or want to join our organization? Contact us by email at or tweet us at @AskAScientistUT! Check us out on VOLink for sources used in this article and upcoming events we’ll be hosting.

UT Sponsored Content