Why and How Do Onions Give Off Sulfur?

November 21, 2025 •

3 min read

While it may not release pure sulfur gas, the answer to 'do onions give off sulfur?' is a definite yes in a more complex chemical sense. When an onion's cells are ruptured, such as during chopping, a rapid enzymatic reaction occurs that transforms sulfur compounds into volatile irritants. This process explains the sharp odor and the famous tear-inducing effect that kitchen-goers have experienced for centuries.

Quick Summary

Onions contain sulfur compounds that, when cut, undergo a chemical reaction releasing a volatile gas called syn-propanethial-S-oxide. This gas irritates the eyes, triggering a defense mechanism that produces tears. The specific enzymes and precursor molecules are what facilitate this pungent, tear-jerking process.

Key Points

Onions do give off sulfur, but indirectly: They contain sulfur compounds that are converted into tear-inducing gases only when the onion is cut.
The 'crying' chemical is syn-propanethial-S-oxide: This volatile compound is formed when the enzymes and amino acids in the onion's ruptured cells mix, irritating your tear ducts.
Two enzymes are involved: The process begins with alliinase acting on amino acid sulfoxides, followed by lachrymatory factor synthase (LFS) creating the irritant.
The sharpness of your knife matters: A dull knife crushes more cells, releasing more gas. A sharp knife causes less cell damage, reducing the irritant.
Chilling or soaking can help: Lowering the onion's temperature or submerging it in water slows down the chemical reaction, minimizing the release of the tear-inducing gas.
Garlic has a different chemical pathway: Though a relative, garlic uses a different precursor molecule (alliin) and enzyme (alliinase) reaction, which does not produce the same tear-inducing gas as onions.

The Surprising Truth About Onion Chemistry

The tear-jerking experience of chopping an onion is a prime example of fascinating food science in action. It's a common misconception that the tears are caused by raw sulfur gas. In reality, the process is much more intricate, involving a specific chain of enzymatic reactions that produce sulfur-containing chemical irritants. These organosulfur compounds are what give onions their characteristic pungent aroma and flavor.

The Enzymatic Reaction Explained

When an onion is sliced, its cells are damaged, allowing two previously separated components to mix: the amino acid sulfoxides and the enzyme alliinase. The alliinase quickly acts on the amino acid sulfoxides, converting them into sulfenic acids. At this point, a different enzyme, the lachrymatory factor synthase (LFS), comes into play and converts the unstable sulfenic acids into a more volatile compound called syn-propanethial-S-oxide. This volatile liquid is the real culprit behind the tears, as it evaporates and diffuses through the air to reach your eyes, where it reacts with the moisture to form a mild sulfuric acid. Your body's natural defense is to produce tears to wash away the irritant.

Factors Influencing the Tear-Inducing Effect

Not all onions are created equal when it comes to causing tears. The intensity of the reaction depends on several factors, including the onion variety and growing conditions. For example, some sweet onions are bred to have lower levels of the tear-inducing compounds. Furthermore, environmental factors can influence the concentration of sulfur-based precursors. A 2022 study showed that fresh-cut onion pieces stored at higher temperatures (20-25 °C) initially contained lower total sulfur compounds than those stored at lower temperatures (4 °C). This suggests that temperature and processing methods significantly impact the release of these volatile compounds.

How to Minimize Crying When Cutting Onions

Use a sharp knife: A dull knife crushes more onion cells, releasing more enzymes and thus more tear-inducing gas. A sharp knife makes cleaner cuts, minimizing cellular damage.
Chill the onion: Placing the onion in the freezer for 10-15 minutes before chopping slows down the chemical reaction, reducing the amount of syn-propanethial-S-oxide produced.
Leave the root intact: The root end of the onion has a higher concentration of sulfur compounds. By leaving it whole and cutting towards it last, you can reduce the amount of irritant released.
Increase ventilation: Cutting onions under a range hood, near a fan, or even under running water can help disperse the volatile gas away from your face.
Create a decoy: Place a wet paper towel near your cutting board to attract the water-seeking irritant away from your eyes.

Sulfur-Containing Compounds: Onions vs. Garlic

While onions and garlic both belong to the Allium genus and contain organosulfur compounds, their chemical pathways differ, resulting in distinct flavors and effects. This table highlights some key differences:


Feature	Onions (Allium cepa)	Garlic (Allium sativum)
Primary Precursor	Isoalliin (S-1-propenyl-L-cysteine sulfoxide)	Alliin (S-allyl-L-cysteine sulfoxide)
Tear-Inducing Compound	Syn-propanethial-S-oxide, formed by LFS	Allicin, but does not produce the same lachrymatory factor
Key Enzyme(s)	Alliinase and Lachrymatory Factor Synthase (LFS)	Alliinase
Bioactive Compound	Isoallicin (di-1-propenyl thiosulfinate)	Allicin (diallylthiosulfinate)
Primary Aroma/Flavor	Pungent, slightly sweet (varies by type)	Strong, spicy, and often lingering

Conclusion

Ultimately, the answer to "do onions give off sulfur?" is found in the complexity of plant biochemistry. Onions do not emit pure sulfur gas, but a series of enzyme-driven reactions converts their sulfur-containing precursor molecules into volatile compounds like syn-propanethial-S-oxide. This chemical is responsible for the stinging sensation in your eyes, triggering tears as a protective response. Understanding this process allows us to not only appreciate the science behind our cooking but also employ clever kitchen techniques to mitigate the tearful experience. The unique interplay of enzymes and organosulfur compounds is a perfect example of nature's ingenious—and sometimes inconvenient—chemical defenses.

For more in-depth information on the specific enzymes and biochemical pathways, consider exploring research published in scientific journals. For example, research on alliinase and LFS activity can provide further insight into the molecular mechanism at play.

Frequently Asked Questions

The chemical compound in onions that makes you cry is syn-propanethial-S-oxide. It is a volatile gas that is released when the onion's cells are damaged and reacts with the moisture in your eyes.

The sulfur-containing compounds released by onions in the form of syn-propanethial-S-oxide are not considered harmful in the quantities experienced during cooking. The eye irritation and tearing are temporary defensive reactions by your body to flush out the mild irritant.

The amount of the tear-inducing irritant can vary based on the onion variety and freshness. Some sweet onions are bred to have lower concentrations of the precursor compounds, while older onions may produce more of the volatile gas.

You can minimize tears by using a sharp knife, chilling the onion before cutting, or cutting under running water or near a fan. Some people also find success with a wet paper towel placed near the cutting board to attract the gas.

Allicin is the potent sulfur-containing compound found predominantly in garlic, not onions. The analogous compound in onions is isoallicin, and the tear-inducing gas, syn-propanethial-S-oxide, is produced by a different enzymatic pathway.

Cooking an onion, particularly applying heat, can inactivate the enzyme alliinase, which halts the production of the irritant. This is why cooked onions have a different, often sweeter, flavor profile without the pungent, tear-jerking effect.

Green onions, or scallions, are part of the Allium family and contain sulfur compounds. However, their flavor is milder, and they have a lower concentration of the lachrymatory factor precursor, so they are much less likely to make you cry.