Are Onions Full of Sulfur? Unpacking the Science of This Pungent Vegetable

November 13, 2025 •

4 min read

The characteristic pungent aroma and flavor of an onion come directly from its natural sulfur compounds. So, are onions full of sulfur? Yes, they are, and these compounds are responsible for much more than just their strong taste, playing a key role in their health benefits and why they make you tear up.

Quick Summary

Onions are rich in various organic sulfur compounds responsible for their distinctive taste, smell, and health benefits. These natural compounds are also the cause of the eye irritation experienced when an onion is cut.

Key Points

Onions are Rich in Sulfur Compounds: The vegetable contains organic sulfur compounds, not elemental sulfur, which are responsible for its distinctive smell and flavor.
Crying is a Chemical Reaction: The irritating gas that makes you cry when chopping onions is syn-propanethial-S-oxide, formed when an enzyme reacts with the onion's sulfur compounds.
Sulfur Compounds Provide Health Benefits: The organosulfur content is linked to heart health, blood sugar regulation, and potential anti-cancer effects.
Content Varies by Onion Type: The amount and type of sulfur compounds can differ based on the onion's variety, with red and yellow types often having more antioxidants.
Cooking Alters Compounds: Heat processing can change the concentration of volatile sulfur compounds, generally reducing them compared to raw onions.
Digestive Sensitivities Exist: For some people, particularly those with IBS, the fructans and raw sulfur in onions can cause digestive upset like gas and bloating.

The Pungent Chemistry: What Makes Onions "Full of Sulfur"?

While not composed purely of the element, onions are notably rich in various organic sulfur compounds, which are a key component of their unique flavor profile. This is a defining trait of the Allium genus, which also includes garlic, chives, and leeks. In their intact state, these vegetables are relatively odorless. However, once the plant's cell walls are damaged—through cutting, crushing, or chewing—a cascade of enzymatic reactions begins.

This process converts the non-volatile S-alk(en)yl cysteine sulfoxides (ACSOs), which act as storage compounds, into a range of unstable and volatile sulfur compounds. These include:

Thiosulfinates: Responsible for inhibiting harmful microorganisms.
Sulfides and polysulfides: The primary source of the characteristic onion flavor and smell.
Propylthiol: A volatile compound with a strong, irritating effect.
Syn-propanethial-S-oxide: The chemical responsible for causing eye irritation.

Why Onions Make You Cry: The Lachrymatory Factor

The phenomenon of crying while cutting onions is a direct result of the sulfur chemistry at play. When you slice into an onion, you rupture its cells, releasing an enzyme called lachrymatory-factor synthase. This enzyme reacts with the onion's amino acid sulfoxides, specifically isoalliin, converting it into sulfenic acid. The sulfenic acid then quickly rearranges itself into the highly unstable gas, syn-propanethial-S-oxide, which is the lachrymatory factor itself. When this gas reaches your eyes, it reacts with the moisture to form a mild sulfuric acid. This irritates your lachrymal glands, which then produce tears to flush out the irritant. Despite the discomfort, this is a temporary reaction and poses no long-term harm to your eyes.

The Health Benefits of Onion's Sulfur Compounds

The sulfur compounds in onions offer a wide range of potential health benefits, contributing to the vegetable's status as a superfood. These benefits are not only linked to the sulfur itself but also to other antioxidants, like quercetin, that are abundant in onions.

Heart Health: Organic sulfur compounds may help reduce cholesterol levels and break down blood clots, thereby lowering the risk of heart disease and stroke.
Blood Sugar Control: Studies suggest that the sulfur compounds and quercetin in onions can boost insulin production and help regulate blood sugar levels, which is particularly beneficial for individuals with diabetes.
Anticancer Properties: Observational studies have linked higher consumption of allium vegetables, including onions, to a reduced risk of certain cancers, such as colorectal and stomach cancer.
Bone Health: Research has indicated that regular onion consumption may help improve bone density, especially in postmenopausal women.
Antibacterial Action: In lab settings, onion compounds, such as quercetin, have been shown to disrupt the growth of bacteria like E. coli and Staphylococcus aureus.

Varieties of Onions and Their Sulfur Content

Not all onions are created equal when it comes to sulfur content and health benefits. The variety of onion, the growing conditions (including soil sulfur levels), and the season can all play a role in its chemical composition.

Red and Yellow Onions: These varieties generally contain higher levels of antioxidants, including quercetin, compared to white onions. For example, yellow onions can have almost 11 times more antioxidants than their white counterparts.
Sweet Onions: Varieties like Vidalias tend to have a lower concentration of the sulfur compounds that cause tearing, making them a milder option.
Soil and Fertilization: The amount of sulfur available in the soil during cultivation can directly influence the pungency and sulfur content of the final product. A higher application of sulfur fertilizer has been shown to increase the bulb's sulfur content.

Raw vs. Cooked: The Impact of Heat on Onion Sulfur Compounds

The preparation of onions significantly impacts the concentration and availability of their beneficial sulfur and antioxidant compounds. The table below outlines the general effects of heat processing.


Feature	Raw Onions	Cooked Onions
Sulfur Compounds	Highest concentration of organic sulfur compounds.	Levels of some heat-sensitive sulfur compounds may decrease.
Tear-inducing Gas	Releases the irritant syn-propanethial-S-oxide when cut.	Cooking neutralizes the enzyme responsible, eliminating the tear-inducing effect.
Antioxidant Activity	High antioxidant activity, but can be affected by variety.	Can increase antioxidant activity through heat-induced reactions, though this can be inhibited by very high sulfur content.
Digestive Tolerance	Higher potential for causing digestive issues (gas, bloating) for sensitive individuals due to higher fructan and raw sulfur content.	Generally easier to digest than raw varieties for those with sensitivities.

Considerations and Potential Side Effects

While healthy for most, some individuals may experience adverse effects from consuming onions, especially in large, raw quantities. The fructans, a type of fermentable carbohydrate in onions, can cause bloating and gas in people with irritable bowel syndrome (IBS). Furthermore, raw onions can trigger heartburn in susceptible individuals.

It is also crucial to note that while onions are healthy for humans, the sulfur compounds they contain can be toxic to some animals, including dogs and cats, and can cause Heinz body anemia. Pet owners should exercise caution to prevent their animals from ingesting onions.

Conclusion

In summary, the notion that onions are full of sulfur is scientifically sound in that they are rich in organic sulfur compounds. These compounds are not just responsible for their signature pungent flavor but also offer a range of health benefits, from heart health to potential anticancer properties. However, the exact composition varies by variety and growing conditions, and cooking can alter the concentration of these volatile compounds. For those who enjoy them, onions are a valuable and flavorful addition to a healthy diet, though individuals with digestive sensitivities should be mindful of their intake.

For more in-depth information on the nutritional aspects of onions, including their antioxidant profile, you can visit Healthline's detailed guide: Onions 101: Nutrition Facts and Health Effects.

Frequently Asked Questions

The strong smell is caused by volatile organosulfur compounds released when the onion's cells are damaged through cutting or crushing. These compounds are a natural defense mechanism for the plant.

The chemical that causes tearing is syn-propanethial-S-oxide, which is created when an enzyme in the onion reacts with its sulfur compounds after being cut.

Raw onions contain the highest concentration of organic sulfur compounds, which can be beneficial for heart health and blood sugar control. However, cooking neutralizes some of the compounds and can increase antioxidant availability.

The concentration of sulfur compounds varies by variety and can be influenced by soil conditions. However, red and yellow onions are known to contain significantly more antioxidants, like quercetin, than white onions.

For most people, the sulfur compounds in onions are beneficial. However, some individuals with an intolerance or sensitive digestive system may experience discomfort like gas, bloating, or heartburn, especially from raw onions.

Onions, garlic, leeks, and chives are all part of the Allium genus. A common characteristic of this family is their use of sulfur as a key component for creating the distinct flavors and aromas that define them.

The sulfur in the onion is an organic compound. When the onion is cut, an enzyme is released which breaks down the sulfur compound into a volatile, irritating gas called syn-propanethial-S-oxide. This gas causes the tearing response, not the pure sulfur itself.