Understanding Onion Chemistry: What Chemicals Are in Onions?

October 19, 2025 •

5 min read

Onions are one of the richest dietary sources of flavonoids, particularly quercetin, which is highly concentrated in their outer layers. When you slice or chop this common vegetable, a remarkable chemical transformation occurs, releasing a cascade of powerful compounds responsible for its distinctive flavor, pungent aroma, and surprising health benefits. The exact composition of what chemicals are in onions? varies by cultivar and cooking method, influencing not only their taste but also their nutritional impact.

Quick Summary

Onions contain two primary groups of beneficial chemicals: organosulfur compounds, which create their characteristic flavor and pungency upon cutting, and flavonoids, potent antioxidants like quercetin. These compounds contribute to the vegetable's anti-inflammatory, antioxidant, and cardiovascular-protective properties. Their concentration is affected by factors like onion color, layer, and preparation methods.

Key Points

Organosulfur Compounds: Responsible for the pungent smell, flavor, and tear-inducing effect when onions are cut, forming from precursors like isoalliin.
Flavonoids (Quercetin): Onions are a top source of the antioxidant flavonoid quercetin, which supports heart health and combats oxidative stress.
Anthocyanins: Red onions contain anthocyanins, powerful antioxidants that give them their distinctive reddish color.
Prebiotic Fiber (Fructans): Onions contain fructans, a fiber that promotes gut health by feeding beneficial bacteria.
Cooking Affects Compounds: How onions are cooked impacts their chemical content; boiling can cause nutrient loss, while sautéing or frying may increase bioavailability.
Outer Layers are More Potent: The outer, edible layers of an onion often contain the highest concentration of beneficial antioxidant compounds.

The Primary Chemical Compounds in Onions

Onions, and other members of the Allium genus like garlic and chives, are celebrated not only for their culinary versatility but also for their potent health-promoting compounds. These phytochemicals fall primarily into two major categories: organosulfur compounds and flavonoids.

Organosulfur Compounds: The Source of Pungency

Organosulfur compounds (OSCs) are responsible for the sharp, pungent taste and tear-inducing effects associated with cutting onions. These volatile substances are not present in the intact onion but are formed when the vegetable's cells are damaged.

Here’s how the process works:

Enzyme Action: When an onion is cut, the enzyme alliinase is released from its compartment within the cell and reacts with sulfur-containing precursor molecules called S-alk(en)yl-L-cysteine sulfoxides (ACSOs).
Flavor and Tears: One of these ACSOs, isoalliin, is rapidly converted into 1-propenesulfenic acid. This, in turn, is acted upon by another enzyme called lachrymatory factor synthase (LFS), which creates syn-propanethial-S-oxide—the volatile chemical that irritates the eye and makes us cry.
Thiosulfinates and Sulfides: The initial thiosulfinates formed, like allicin, are highly unstable and quickly rearrange into more stable compounds such as diallyl disulfide (DADS), diallyl trisulfide (DATS), and other polysulfides. These compounds have been linked to numerous health benefits, including cardiovascular protection by helping to break down blood clots and reduce cholesterol.

Flavonoids: Antioxidants with Anti-inflammatory Power

Onions are particularly rich in flavonoids, a class of polyphenolic antioxidants. The most abundant flavonoid in onions is quercetin, which is found in higher concentrations in the outer layers and in colored varieties like red and yellow onions.

Quercetin: This powerful flavonoid is known for its anti-inflammatory effects and ability to fight oxidative stress by scavenging harmful free radicals. It may also help support heart health by assisting with blood pressure regulation. While quercetin is present in many foods, onions contain a concentration that is 5 to 10 times higher than in many other commonly consumed fruits and vegetables.
Anthocyanins: These are the flavonoid pigments that give red and purple onions their vibrant color. Anthocyanins are potent antioxidants that are associated with a lower risk of heart disease.

Other Important Bioactives

Beyond OSCs and flavonoids, onions contain other beneficial chemicals:

Fructans: These are soluble prebiotic fibers that serve as food for the beneficial bacteria in your gut. A healthy gut microbiome is linked to improved digestion and better nutrient absorption. However, fructans are also a type of FODMAP and may cause digestive distress in sensitive individuals.
Vitamins and Minerals: Onions are a good source of vitamin C, vitamin B6, and folate. They also provide essential minerals like potassium and manganese.

Comparing Onion Varieties and Preparation Methods

The chemical composition of an onion can be influenced by factors such as its color, which part of the bulb is consumed, and how it is prepared. The following table illustrates some of these differences, drawing from scientific research.


Feature	Red Onions	Yellow Onions	White Onions
Flavonoid Content	Highest levels, containing both flavonols (quercetin) and anthocyanins.	High quercetin content, sometimes exceeding red varieties in certain layers.	Lowest levels of flavonoids and antioxidants overall.
Sulfur Compounds	Moderate levels of thiosulfinates and OSCs, varying by cultivar.	Tend to have higher levels of thiosulfinates and overall OSCs compared to other types.	Lower levels of pungent OSCs, resulting in a milder flavor.
Nutrient Distribution	Most antioxidants are concentrated in the outer, often-discarded layers and skins.	Highest antioxidant concentration is found in the outer layers.	Similarly, outer layers hold the highest levels of beneficial compounds.
Effect of Cooking	Moderate heating can enhance some phenolic compounds, but boiling can cause significant losses.	Frying and microwave cooking can increase antioxidant levels by concentrating the compounds.	Cooking methods will still alter chemical composition, with water-based cooking causing loss.

The Impact of Cutting and Cooking

Cutting and Crushing: The act of chopping an onion is crucial for unlocking its full chemical potential. This ruptures cell walls, allowing the enzymes and precursors to mix and generate the flavorful and pungent OSCs. This process also releases the lachrymatory factor that irritates the eyes. Waiting a few minutes after cutting before adding heat allows for maximum chemical formation.
Cooking Effects: As the table indicates, the way an onion is cooked significantly affects its chemical profile. Boiling onions can cause a loss of water-soluble compounds like quercetin into the cooking water. In contrast, cooking methods like sautéing, grilling, or air-frying can concentrate these compounds by evaporating water, sometimes increasing the bioavailability of phenols. Moderate heat can also inactivate the enzymes that produce pungent compounds, resulting in a sweeter, milder flavor.
Raw Consumption: Eating raw onion, such as on a sandwich or in a salad, ensures the highest concentration of volatile OSCs and maximum nutritional potency of the initial compounds, before heat degradation. This delivers the most potent dose of health-promoting phytochemicals.

A Valuable Part of a Healthy Diet

The diverse chemical profile of onions, featuring both pungent OSCs and potent flavonoids, makes them a valuable addition to a nutritious diet. From supporting cardiovascular health to providing prebiotic fiber for gut health, the compounds within this humble vegetable offer impressive health benefits. By understanding the chemistry behind the onion's flavor and function, you can make informed choices about how to best prepare and consume them to maximize their nutritional value.

For more in-depth information on the bioactive compounds in onions and their health implications, research from institutions like the National Institutes of Health (NIH) is a great resource.

Conclusion

Onions are a powerhouse of valuable chemicals, including organosulfur compounds that cause their characteristic aroma and potent flavonoid antioxidants, particularly quercetin. These compounds are responsible for the vegetable's significant health benefits, from supporting cardiovascular health and managing blood sugar to combating oxidative stress and inflammation. The concentration and profile of these chemicals are dynamic, influenced by the onion variety, the part of the bulb consumed, and the cooking method used. For example, red onions are richer in certain flavonoids than white varieties, and the outer layers of all onions contain the highest concentration of beneficial antioxidants. Furthermore, preparation methods like sautéing can increase some compounds, while boiling can lead to their loss. By appreciating the intricate chemistry of onions, we can enhance their role in a health-conscious diet.

Frequently Asked Questions

The chemical in onions that makes you cry is syn-propanethial-S-oxide. It is produced when the enzyme alliinase reacts with sulfur-containing precursors after the onion's cells are broken through cutting or crushing.

Cooking can alter the chemical compounds in onions. Water-based cooking methods like boiling can cause a loss of water-soluble compounds such as quercetin. However, sautéing, frying, or air-frying can concentrate these compounds by evaporating moisture and may increase the bioavailability of others.

Red and yellow onions generally contain more flavonoids and antioxidants than white onions. The colorful outer layers of red onions are particularly rich in antioxidants like quercetin and anthocyanins.

Yes, organosulfur compounds in onions are considered beneficial for health. Research suggests they have antioxidant, anti-inflammatory, and cardioprotective properties, potentially helping to reduce cholesterol and protect against heart disease.

Fructans are soluble prebiotic fibers found in onions that feed the beneficial bacteria in your gut. This supports digestive health and nutrient absorption. However, they are also a type of FODMAP and can cause digestive issues for some people.

The highest concentration of beneficial phytochemicals, particularly antioxidants, is typically found in the outermost, edible layers of the onion bulb. These are often peeled and discarded by consumers, leading to a loss of valuable nutrients.

To maximize the health benefits of onions, eat them raw or use cooking methods that don't involve a lot of water, like sautéing or air-frying. Also, try to use more of the outer, edible layers, as they contain a higher concentration of potent antioxidants like quercetin.