What Makes Onions Spicy? The Science Behind Onion Burn

Onions aren't technically spicy like chili peppers. Their characteristic burn comes from sulfur-based compounds called syn-propanethial-S-oxide and lachrymatory factor, released when onion cells are damaged during cutting. These volatile compounds react with moisture in your eyes and mucous membranes, creating a temporary burning sensation.

When you slice into an onion, you're triggering a fascinating biochemical defense mechanism. Onions contain amino acid sulfoxides that transform into volatile sulfur compounds when their cells are ruptured. The primary irritant, syn-propanethial-S-oxide, is a lachrymatory agent that specifically targets the TRPA1 receptors in your eyes and nasal passages—receptors responsible for detecting potential irritants.

The Biochemical Reaction Behind Onion Burn

Here's the precise sequence of events that creates that familiar onion sting:

1. Intact onion cells store flavor precursors called S-alk(en)yl-L-cysteine sulfoxides
2. When cut, the enzyme alliinase is released from vacuoles
3. Alliinase converts these precursors into sulfenic acids
4. Sulfenic acids rapidly rearrange into syn-propanethial-S-oxide (the lachrymatory factor)
5. This volatile compound evaporates and reaches your eyes
6. It reacts with tear film to form sulfuric acid, triggering nerve endings
7. Your body responds by producing tears to wash away the irritant

This entire process happens within seconds of cutting an onion. Interestingly, different onion varieties contain varying concentrations of these compounds, which explains why some onions like red onions deliver a sharper bite than milder varieties like Vidalias.

Onion "Spiciness" vs. Chili Pepper Heat: A Scientific Comparison

Though both create burning sensations, onions and chili peppers work through completely different mechanisms:

Characteristic	Onions	Chili Peppers
Primary Compound	Syn-propanethial-S-oxide	Capsaicin
Receptor Targeted	TRPA1 (pain/cold receptor)	TRPV1 (heat receptor)
Sensation Duration	Temporary (seconds-minutes)	Prolonged (minutes-hours)
Water Solubility	Highly volatile, evaporates quickly	Oil-soluble, lingers on surfaces
Biological Purpose	Deterrent against microorganisms	Deterrent against mammals

This distinction explains why water helps alleviate onion burn (washing away the water-soluble compounds) but makes chili heat worse (spreading the oil-based capsaicin). The TRPA1 receptor activation from onions creates a sharp, immediate sting that dissipates quickly, while capsaicin's TRPV1 activation produces a lingering burn that builds over time.

Factors Influencing Onion Pungency Levels

Several elements determine how "spicy" an onion will be:

• Variety: Yellow onions typically have higher sulfur content than sweet varieties like Walla Walla
• Growing conditions: Soil sulfur levels directly impact pungency—more sulfur means sharper onions
• Maturity: Older onions develop stronger flavors as compounds concentrate
• Storage: Properly stored onions become milder over time as enzymes break down
• Preparation method: Cutting technique significantly affects compound release

Research shows that chilling onions before cutting slows enzyme activity, reducing the amount of lachrymatory factor produced. Similarly, cutting near running water or using a sharp knife (which causes less cell damage) can minimize the burning sensation when handling onions.

Practical Applications: Managing Onion Pungency

Understanding what makes onions spicy allows you to control their intensity:

• For milder flavor: Soak sliced onions in cold water for 15-30 minutes to dissolve sulfur compounds
• To reduce eye irritation: Chill onions before cutting or use a fan to direct vapors away
• For cooking: Cooking denatures the enzymes, eliminating the raw bite
• For salads: Pair onions with acidic ingredients like vinegar which can neutralize some compounds

Interestingly, the same compounds that make onions irritating also provide their health benefits. These sulfur compounds have demonstrated antioxidant and anti-inflammatory properties in numerous studies, showing how nature's defense mechanisms often translate to human health advantages.

Why do onions make your eyes burn but not garlic?

Onions produce significantly more syn-propanethial-S-oxide, the specific lachrymatory compound that targets eye receptors. Garlic creates different sulfur compounds that don't evaporate as readily, which is why it doesn't make you cry despite having a strong flavor.

Are red onions spicier than white onions?

Generally yes. Red onions contain higher concentrations of the sulfur compounds that create that sharp bite. However, individual onion pungency varies more by growing conditions than color alone. Some white onion varieties can be sharper than certain red onion types.

Does cooking eliminate onion spiciness?

Yes, cooking denatures the alliinase enzyme responsible for creating the volatile sulfur compounds. As onions cook, their sharp bite transforms into sweet caramelized flavors through the Maillard reaction, completely changing their chemical profile.

Why do some people tolerate onions better than others?

Genetic variations in TRPA1 receptor sensitivity explain why some people experience more intense burning from onions. Additionally, repeated exposure can build mild tolerance, similar to how people adapt to spicy foods over time.

Can you reduce onion spiciness without losing flavor?

Yes, chilling onions before cutting slows enzyme activity, reducing irritant production while preserving flavor compounds. Using a very sharp knife causes less cell damage, and cutting under running water can help without significantly altering taste.