Why are onions spicy?

A bite of raw onion will instantly have your tongue burning and your eyes watering. But onions aren’t the “same kind” of spicy as chili peppers — so what makes them so hot?

Do onions have capsaicin?

Onions do not have any capsaicin. In fact, there are very few plants or vegetables other than chili peppers that contain capsaicin, and onions don’t make the list.

What makes onions so spicy?

Rather than capsaicin, onions are made spicy by sulfurous compounds. Raw onions contain propyl propanethiosulfonate, propenyl propanethiosulfonate thiopropanal S-oxide, and propyl methanethiosulfonatem², which contribute to their flavor and “spiciness”.

Two factors contributing to just how spicy an onion becomes are the sulfur content of the soil it’s grown in and how long it is grown, which impacts the level of the chemicals in the bulb itself. Spring onions are harvested very early, and so they tend to be sweeter and less spicy than onions that are harvested in the fall for storage. Similarly, onions grown in soil with especially high sulfur content will be spicier than those grown in lower-sulfur environments.

Why do onions make your eyes water?

Sulfurous compounds are also to blame for the tears you shed when you slice onions. When you cut into an onion, a chemical reaction occurs, and propanthial S-oxide is generated¹. This is the specific compound that causes you to tear up.

Measuring the spiciness of onions

Because onions do not have any capsaicin, they have a Scoville rating (SHU) of 0 — since the Scoville scale measures the quantity of capsaicin in different spicy foods.

However, there is a different scale that can measure the spiciness of onions and garlic called the Pyruvate scale, which measures enzymatically produced pyruvate³. The pyruvate scale runs from 1-10, based on the number of micromoles per gram fresh weight (µmol/gfw). Onions that are less spicy, like a sweet onion or a fresh spring onion rank around 1.5-3 on the scale, while spicier cured white or yellow onions might rank between 6-8.

How to make onions less spicy

While I love spicy chili peppers and wasabi, I don’t find the spiciness of onions to be pleasant or fun — chances are you are the same!

To make onions less spicy (and burn your eyes less), you can quickly slice them and put them into a mixture of 1 tablespoon of baking soda and 1 cup of water and soak them for 15 minutes. The thought behind it is that the alkalinity of the baking soda will neutralize the acidic sulfurous compounds, toning down the spiciness.

References

1. Imai, S., Tsuge, N., Tomotake, M. et al. An onion enzyme that makes the eyes water. Nature 419, 685 (2002). https://doi.org/10.1038/419685a
2. McGorrin, R. J. (2011). The significance of volatile sulfur compounds in food flavors. ACS Symposium Series, 3–31. https://doi.org/10.1021/bk-2011-1068.ch001
3. Wall, M. M., & Corgan, J. N. (1992). Relationship between Pyruvate Analysis and Flavor Perception for Onion Pungency Determination, HortScience HortSci, 27(9), 1029-1030. Retrieved Sep 22, 2022, from https://journals.ashs.org/hortsci/view/journals/hortsci/27/9/article-p1029.xml