Do garlic and onions have sulfur?

January 9, 2026 •

2 min read

As major sources of dietary sulfur, garlic and onions contain a wealth of potent organosulfur compounds, which are responsible for their distinctive aromas and numerous health benefits. This unique composition is part of a complex defense mechanism that activates upon cutting or crushing, transforming mild precursors into powerful, active molecules.

Quick Summary

Garlic and onions are rich in organosulfur compounds, like allicin, released when the vegetables are cut, contributing their characteristic flavor and providing health benefits, including antioxidant and anti-inflammatory properties.

Key Points

Yes, they do: Both garlic and onions are rich in organosulfur compounds, which are key to their distinct flavor and health benefits.
Formation reaction: These active compounds are primarily formed by an enzymatic reaction involving alliinase when the vegetables are cut or crushed.
Garlic's allicin: Crushing garlic produces allicin, a potent but unstable compound responsible for its pungent aroma, which then breaks down into other beneficial sulfides.
Onion's tear factor: The act of cutting an onion releases a volatile sulfur compound called lachrymatory factor, which causes eye irritation and tearing.
Health benefits: Organosulfur compounds are linked to antioxidant, anti-inflammatory, and cardiovascular protective effects in the body.
Cooking considerations: Crushing garlic and waiting before cooking can maximize allicin formation, as heat can deactivate the necessary enzymes.
Specific compounds differ: Although both contain sulfur, the specific sulfur precursors and resulting compounds differ between garlic and onions, giving them unique properties.

What are organosulfur compounds?

Organosulfur compounds are organic chemical compounds containing sulfur. In plants like garlic and onions (Allium genus), these compounds exist as odorless precursors until the plant tissue is damaged. Cutting or crushing releases the enzyme alliinase, which reacts with these precursors to form volatile sulfur compounds responsible for their flavor, smell, and health benefits. This reaction is also a natural defense mechanism against pests.

The sulfur in garlic: Allicin and its derivatives

Garlic's main sulfur precursor is alliin, which is rapidly converted to allicin by alliinase when crushed. Allicin gives fresh garlic its characteristic odor but is unstable, breaking down into other beneficial organosulfur compounds such as diallyl sulfide (DAS), diallyl disulfide (DADS), diallyl trisulfide (DATS), ajoene, and vinyldithiins. The breakdown products are affected by factors like temperature and time. Aged garlic extract, for instance, contains stable compounds like S-allyl-cysteine (SAC) with health benefits and less odor.

The sulfur in onions: Lachrymatory factor and other sulfides

Onions contain precursors like isoalliin. When cut, alliinase acts similarly to garlic, but an additional enzyme, lachrymatory factor synthase (LFS), creates syn-propanethial-S-oxide, the compound causing tears. Other sulfur compounds like propan-1-thiol contribute to the onion's flavor.

Health benefits of organosulfur compounds

The sulfur compounds in garlic and onions offer numerous potential health benefits:

Antioxidant effects: They combat oxidative stress and cellular damage by scavenging free radicals, supporting defenses against aging and chronic diseases.
Cardiovascular health: Consumption is linked to reduced heart disease risk factors, including lower cholesterol and blood pressure.
Anti-inflammatory properties: Compounds like ajoene can help regulate inflammatory responses.
Antimicrobial activity: Historically used for this purpose, these compounds show antibacterial and antifungal effects.
Potential anti-cancer effects: Some studies suggest a link between Allium vegetable intake and reduced risk of certain cancers.

Garlic vs. Onions: A sulfur comparison


Feature	Garlic	Onions
Primary Precursor	Alliin	Isoalliin, propiin, methiin
Key Active Product	Allicin	Syn-propanethial-S-oxide (lachrymatory factor)
Primary Volatile Compounds	Allicin, Diallyl sulfides, ajoene	Propan-1-thiol, dipropyl disulfide
Odor/Flavor Profile	Intense, pungent, garlicky	Sharp, savory, varies by type
Notable Characteristic	Allicin is unstable	Causes eye irritation

How processing affects sulfur compounds

Heat can affect the enzymatic reactions that produce active sulfur compounds. Crushing garlic and waiting 10-15 minutes before cooking allows allicin to form before the alliinase enzyme is deactivated by heat. While heat can decrease certain sulfur compounds in onions, some methods like stir-frying may increase others. Consuming raw or minimally cooked forms is often recommended for maximum benefits.

Conclusion: The power of pungent plants

In conclusion, garlic and onions are rich sources of diverse organosulfur compounds, which are fundamental to their flavor, aroma, and extensive health benefits. These active molecules, from garlic's allicin to onion's lachrymatory factor, contribute antioxidant, anti-inflammatory, and cardioprotective effects. Understanding the role of the alliinase enzyme and the impact of preparation methods helps in maximizing the health potential of these pungent vegetables.

For more detailed information, resources from the National Institutes of Health provide further insight into the sulfur compounds in Allium vegetables.

Frequently Asked Questions

Garlic and onions contain organosulfur compounds, which are organic chemical compounds containing sulfur. In garlic, the precursor is alliin, which forms allicin upon crushing, while onions produce a different array of sulfur-containing volatiles.

Yes, cooking can destroy some of the beneficial, heat-sensitive sulfur compounds. The alliinase enzyme is deactivated by high heat, so crushing raw garlic and letting it sit for 10-15 minutes before cooking is recommended to maximize allicin formation.

Cutting an onion releases the alliinase enzyme, which reacts with sulfur precursors to produce syn-propanethial-S-oxide. This volatile gas then irritates the nerves in your eyes, causing them to water.

To get the most allicin-related benefits, crush or mince fresh garlic and let it rest for about 10-15 minutes before adding it to your cooking. This allows the alliinase enzyme to produce allicin before heat can destroy the enzyme.

The organosulfur compounds are linked to several health benefits, including antioxidant and anti-inflammatory effects, improved cardiovascular health by reducing cholesterol, and potential anti-cancer properties.

No, while both contain organosulfur compounds, the specific types differ. Garlic's pungent aroma is dominated by allicin and its derivatives, while onions are known for the lachrymatory factor and various propyl and methyl sulfides.

Yes, sulfur is an essential mineral that the body requires to build and repair DNA and protect cells. Dietary intake from sources like garlic and onions helps to provide these important sulfur compounds.