What Chemical Is Present in Onions?

December 16, 2025 •

4 min read

When you cut into an onion, a series of complex enzymatic reactions occur, releasing a volatile compound that causes your eyes to water. This reaction is a protective mechanism for the plant, but for humans, it's a well-known culinary phenomenon. The specific chemical responsible for this irritant effect is called syn-propanethial-S-oxide, and its formation is only one part of the rich chemical profile present in onions.

Quick Summary

An onion's unique flavor, pungent aroma, and eye-irritating properties stem from organosulfur compounds, like syn-propanethial-S-oxide. Onions also contain beneficial flavonoids, such as quercetin, which have antioxidant effects. Cooking alters these chemical compounds, changing the vegetable's taste and overall profile.

Key Points

Tear-Inducing Chemical: When an onion is cut, the enzyme lachrymatory-factor synthase converts sulfur compounds into the volatile irritant syn-propanethial-S-oxide, which causes tears.
Flavor Compounds: The pungent aroma and flavor come from various volatile organosulfur compounds, including sulfides and polysulfides, formed when the onion's cells are broken.
Beneficial Antioxidant: Onions are a rich source of the flavonoid quercetin, a powerful antioxidant that offers anti-inflammatory and other health benefits.
Cooking's Effect: Heat deactivates the enzymes that create pungent compounds, allowing the onion's natural sugars to caramelize and produce a sweeter, milder flavor.
Variety Matters: Different onion varieties have different concentrations of chemical compounds, affecting their pungency and flavonoid levels; for example, red onions have high levels of anthocyanins.
Health Benefits: The sulfur compounds and antioxidants in onions have been linked to potential health benefits, including lower blood pressure and reduced cancer risk.
Gut Health: Onions contain prebiotics called fructans that feed beneficial gut bacteria and support digestive health.

Onions are an integral part of cuisines worldwide, but their culinary function is a direct result of their rich and complex chemistry. The compounds responsible range from those that cause tears to potent antioxidants with potential health benefits. Understanding these chemicals provides insight into why onions taste and behave the way they do.

The Lachrymatory Factor: Why Onions Make You Cry

The immediate and most startling chemical reaction associated with onions happens when their cells are broken. This is a defensive strategy evolved to deter pests. When you slice or dice an onion, the cell walls rupture, causing enzymes and other molecules to mix. Specifically, an enzyme called lachrymatory-factor synthase (LFS), previously separated from amino acid sulfoxides, converts these sulfoxides into unstable sulfenic acids. These sulfenic acids then rearrange into a volatile gas known as syn-propanethial-S-oxide. As this gas floats up and makes contact with the water covering your eyes' surface, it converts into a weak sulfuric acid. This acid irritates the lacrimal glands, triggering a tear response to wash away the irritant. The amount of this chemical irritant can vary between onion varieties, with sweeter onions having less of the sulfurous compounds.

Organosulfur Compounds and Flavor

Beyond the eye-stinging effect, other organosulfur compounds are primarily responsible for the characteristic taste and aroma of onions, both raw and cooked. These compounds are formed from the enzymatic breakdown of the sulfenic acids. The volatile thiosulfinates that are created quickly break down into a wide array of other sulfur-containing molecules, including sulfides, disulfides, and trisulfides. The specific composition and concentration of these compounds determine the overall flavor profile. For instance, the pungency of an onion can be measured by its pyruvate content, which is a byproduct of the enzymatic reaction. Raw onions have a sharp, pungent flavor due to these active sulfur compounds, while cooking transforms them. The heat breaks down the enzymes, prevents the formation of the harsh sulfur compounds, and promotes the creation of sweeter, milder flavors through other reactions.

The Role of Flavonoids: Antioxidants in Onions

Onions are also a significant source of flavonoids, which are plant pigments with powerful antioxidant properties. The most prominent flavonoid found in onions is quercetin. Red and yellow onions, in particular, are rich in quercetin and other flavonoids, including anthocyanins, which give red onions their color. These compounds do not contribute to the pungent flavor but offer numerous health benefits, such as fighting inflammation, protecting against cell damage, and supporting cardiovascular health. Quercetin has been specifically linked to potentially lowering blood pressure. The concentration of these beneficial flavonoids can differ depending on the onion variety and the way it is processed. Research suggests that some health benefits may be more pronounced when consuming raw onions, but cooking can also enhance the bioavailability of certain compounds, so a combination of preparation methods is beneficial.

How Cooking Alters an Onion's Chemistry

Heat plays a significant role in altering the chemical composition of an onion. The intense, pungent flavors found in raw onions are largely eliminated during cooking as heat deactivates the enzymes responsible for creating the volatile sulfur compounds. This allows the natural sugars within the onion to come forward, resulting in a milder, sweeter flavor. Caramelization, for example, is a process where the sugars in the onion undergo a non-enzymatic browning reaction, producing new flavor compounds. Frying or sautéing can also introduce new aldehydes and ketones formed from the interaction of the onion's components with added fats and heat. This chemical transformation is why a raw onion and a sautéed onion can taste like completely different ingredients.

Comparison of Key Onion Chemicals


Chemical Compound	Primary Effect	Volatility	Health Implications
syn-Propanethial-S-oxide	Causes eye irritation and tearing.	Highly volatile.	Acts as an irritant; no known health benefits.
Quercetin	Provides powerful antioxidant properties.	Non-volatile.	Anti-inflammatory, potential blood pressure-lowering effects, reduced cancer risk.
Sulfides & Trisulfides	Contribute to pungent flavor and aroma.	Volatile.	May have antimicrobial properties and anti-cancer effects.
Fructans (Fiber)	Feeds beneficial gut bacteria.	Non-volatile.	Promotes digestive health, acts as a prebiotic.

Conclusion

The humble onion is a fascinating package of complex chemistry. From the tear-inducing syn-propanethial-S-oxide created when it's cut to the powerful antioxidant quercetin that offers numerous health benefits, the chemical makeup of onions dictates their function in both the kitchen and our bodies. The interaction of sulfur compounds, flavonoids, and other molecules gives onions their unique sensory profile and potent health-promoting properties. Whether eaten raw for its pungent kick or cooked to bring out its natural sweetness, the onion's chemical reactions provide a diverse range of flavors and benefits.

Visit a resource for more information on the chemistry of Allium vegetables.

Frequently Asked Questions

The main chemical is syn-propanethial-S-oxide. When you cut an onion, enzymes convert sulfur-containing amino acids into this volatile gas, which irritates your eyes and triggers tear production.

Yes, cooking an onion significantly alters its chemical makeup. Heat deactivates the enzymes responsible for creating the pungent sulfur compounds. This process softens the flavor, bringing out the onion's natural sweetness through caramelization.

Yes, many compounds in onions, particularly flavonoids like quercetin, are considered very healthy. They have antioxidant and anti-inflammatory effects that can protect against cell damage and reduce the risk of certain diseases.

Studies show that red and yellow onions are generally richer in antioxidants, such as quercetin, than white onions. Red onions also contain anthocyanins, which are potent antioxidants.

The flavor is derived from a variety of organosulfur compounds, including thiosulfinates, sulfides, and trisulfides. These are formed in a series of reactions that begin when the onion's tissue is damaged.

You can try several methods to minimize tearing. Using a sharp knife or chilling the onion in the refrigerator for 30 minutes can slow down the enzyme reactions. Cutting the onion near a running fan or under a vent can also help disperse the volatile gas.

Yes, the method of cutting affects the intensity of the flavor. The more finely an onion is chopped, the more cells are broken and the more sulfur compounds are released, resulting in a stronger, more pungent flavor.