Onion Organosulfur Compounds and the Tear Factor

Table of Contents

1. The Onion Is a Loaded Spring
2. The Precursors: What Is Stored in the Cell
3. Alliinase and the First Two Seconds
4. Why You Cry — and Why the Textbook Was Wrong
5. How to Stop Crying, Ranked by What Actually Works
6. Onion Sulfur Is Not Garlic Sulfur
7. What Cooking Does — and Where the Sweetness Comes From
8. The Antimicrobial Evidence, Honestly
9. Platelets, Blood and the Circulation
10. Allium Vegetables and Cancer Epidemiology
11. Bone Density and Other Scattered Findings
12. Getting the Most Sulfur Chemistry Out of an Onion
13. Key Research Papers
14. Connections
15. Featured Videos

The Onion Is a Loaded Spring

Onion cells store their sulfur compounds as stable, odourless, water-soluble precursors dissolved in the cytoplasm. In a separate compartment — the vacuole — sits the enzyme that acts on them. As long as the cell wall is intact, nothing happens. The two never meet.

Damage the cell and they meet immediately. A knife, a tooth, an insect's mandible, a fungal hypha — anything that ruptures the membrane triggers the reaction. This is a defence system, and the design is deliberate: the plant carries no toxin at all until something starts eating it, at which point the toxin is manufactured on the spot, in the wound, in under a second.

Everything downstream follows from that single fact. Whole onions do not smell. Cut onions do. Cooked onions smell different again, because heat destroys the enzyme before it can do much work. And a blender full of onion is far more pungent than a diced one, because you have ruptured every cell at once.

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The Precursors: What Is Stored in the Cell

The stored compounds are S-alk(en)yl-L-cysteine sulfoxides — a mouthful that simply means cysteine, the sulfur-containing amino acid, with an extra sulfur-bearing side group attached. Lancaster and Shaw worked out the biosynthetic route through gamma-glutamyl peptide intermediates, which is how the plant builds and stores them without poisoning itself.

Which sulfoxide dominates is what separates one allium from another:

1. Onion is dominated by isoalliin (S-1-propenyl-L-cysteine sulfoxide). This is the compound that leads, uniquely, to the tear factor.
2. Garlic is dominated by alliin (S-allyl-L-cysteine sulfoxide), which leads to allicin. Garlic contains essentially no isoalliin, which is why chopping garlic does not make you cry.
3. Leeks, chives, shallots and spring onions carry mixtures, and their flavours sit where their precursor ratios sit. Shallots lean toward the onion pattern; chives carry a distinctive profile of their own.

Freeman and Whenham demonstrated the logic of this directly in 1976 by synthesising individual sulfoxides, letting alliinase cleave them, and showing that the resulting products reproduced identifiable allium flavour components — the precursor determines the flavour.

Onions also accumulate selenium into analogous selenium-containing compounds when grown in selenium-rich soil. Arnault and Auger surveyed these seleno-compounds in garlic and onion; the amounts depend heavily on the soil, so an onion is a variable rather than a reliable selenium source. The site's Selenium page covers dependable sources.

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Alliinase and the First Two Seconds

Alliinase is a pyridoxal-phosphate-dependent enzyme, and it is fast. Within a second or two of cell rupture it cleaves the sulfoxide precursors, releasing pyruvate, ammonia, and a highly reactive sulfenic acid.

Sulfenic acids are unstable. In garlic, two of them combine to form allicin, the compound responsible for most of garlic's activity. In onion, most of them likewise condense into thiosulfinates, then rearrange further into a large family of downstream products — thiosulfonates, mono- di- and trisulfides, and the cyclic compounds called zwiebelanes (from Zwiebel, German for onion). Lanzotti's review of the analytical chemistry of onion and garlic is the standard map of this territory, and it is worth noting how many distinct compounds it contains: onion sulfur chemistry is not one molecule, it is a shifting mixture whose composition depends on time, temperature, pH and what else is in the pan.

The measurable consequence is that pyruvate concentration is the industry's pungency test. Because one pyruvate is released for every precursor cleaved, measuring pyruvate in onion juice gives a number for how pungent that onion is. Sweet onion cultivars are bred and certified on it.

The other consequence, useful in a kitchen: the reaction needs time and it needs water. A dry, fast slice releases less than a slow crushing does; a blend releases the most of all. Waiting a few minutes after chopping before adding the onion to heat lets the enzyme finish its work — the same trick that is more famously recommended for garlic.

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Why You Cry — and Why the Textbook Was Wrong

For decades the standard explanation was that alliinase produces the tear gas directly — that the unstable sulfenic acid from isoalliin simply rearranges on its own into the volatile lachrymator, syn-propanethial S-oxide, and drifts up into your eyes.

In 2002 Imai and colleagues published a one-page paper in Nature titled "An onion enzyme that makes the eyes water," and the textbook explanation turned out to be incomplete. There is a second enzyme — lachrymatory factor synthase (LFS) — that takes the sulfenic acid produced by alliinase and converts it specifically into the tear compound. Without LFS, the sulfenic acid does something else: it condenses into thiosulfinates, the flavour compounds, instead.

This was not a technicality. It meant the tear factor was a dedicated product, made by an enzyme whose only job is to make it — and therefore that it could be removed without dismantling the rest of the onion. Eady and colleagues did exactly that in 2008, silencing LFS in transgenic onions. The result confirmed the mechanism precisely: the modified onions produced far less tear factor, and the sulfur that would have gone into it was diverted into the thiosulfinate pool instead, measurably changing the secondary metabolite profile. In other words, a tearless onion is not a blander onion — it is an onion whose sulfur has been routed into flavour rather than into your eyes. (Whether such onions ever reach shops is a regulatory and commercial question, not a scientific one; conventionally bred low-pungency onions remain what is on the shelf.)

What happens in your eye is straightforward once the compound is airborne. Propanethial S-oxide is volatile and reaches the cornea, where it activates the sensory ion channel TRPA1 on trigeminal nerve endings — the same irritant-sensing channel activated by mustard oil, wasabi and tear gas. The nerve reports pain and irritation, the lacrimal glands flood the eye to wash the irritant away, and you cry. Nothing is being damaged. The response is protective and it resolves in a minute or two.

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How to Stop Crying, Ranked by What Actually Works

Because the mechanism is now well understood, the folk remedies sort cleanly into ones that follow from the chemistry and ones that do not.

1. Chill the onion first — works. Thirty minutes in the refrigerator, or ten in the freezer, slows both the enzymes and the volatility of the product. Less gas is generated and less of it reaches your face. This is the single most effective ordinary intervention.
2. Use a genuinely sharp knife — works. A sharp blade severs cells; a dull one crushes and ruptures a much larger number of them. Fewer broken cells means less precursor released.
3. Cut under running water or submerged — works, at a cost. Water absorbs the volatile compound before it reaches the air. It also washes away flavour and quercetin, and it is awkward. Fine for a large batch you are about to boil anyway.
4. Ventilation — works. Cutting under an extractor hood, beside an open window, or with a fan blowing across the board and away from your face removes the gas before it climbs.
5. Goggles — works, obviously. Sealed swimming goggles are unglamorous and completely effective. Loose safety glasses are not.
6. Leave the root end intact until last — helps a little. The basal plate is denser in sulfur compounds than the rest of the bulb, so cutting it last delays the worst of the release.
7. Breathe through your mouth, hold a spoon in your teeth, chew gum — no. The compound reaches the eye through the air, not through the nose or mouth. These do nothing except occupy your attention.
8. A candle or lit match nearby — no. There is no plausible mechanism at kitchen scale, and it is a fire risk beside a distracted cook.
9. Sweet onions — works, by definition. Low pungency means low precursor content means little tear factor. It also means less of the chemistry this page is about.

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Onion Sulfur Is Not Garlic Sulfur

This distinction is worth stating plainly because a great deal of onion health writing simply borrows garlic's evidence base.

Garlic's dominant reactive product is allicin, and allicin has been studied intensively in its own right — Borlinghaus and colleagues' review is a good summary of its chemistry and biological properties. Allicin is a potent, broad-spectrum antimicrobial in the test tube and reacts readily with thiol groups in proteins, which is the basis of most of its activity.

Onion produces a different and more heterogeneous mixture, dominated by 1-propenyl and propyl thiosulfinates rather than allyl ones, plus the zwiebelanes and the tear factor that garlic does not make at all. The compounds are chemically related but they are not the same molecules, they are not present in the same amounts, and evidence for one does not transfer to the other. Where a claim about onions is really a claim about allicin, this page says so. The site's Garlic page covers garlic's own evidence.

Gram for gram, garlic carries considerably more sulfur precursor than onion. Onion's counterweight is that people eat far more of it — a household may get through a kilogram of onions a week and a bulb of garlic in the same period.

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What Cooking Does — and Where the Sweetness Comes From

Heat destroys alliinase. Drop onion into a hot pan and the enzyme is denatured quickly; whatever thiosulfinates were formed during chopping are what you get, and they themselves break down further with heat into simpler sulfides. Those sulfides are what a long-cooked onion smells of — mellow, savoury, faintly sweet, nothing like the raw bite.

Two things happen in parallel during a long, slow cook, and they are both worth understanding:

1. The pungent chemistry is dismantled. Sharp thiosulfinates degrade into milder sulfides. The harshness goes.
2. The sweetness appears — but not from added sugar. A caramelised onion tastes sweet partly because its fructans (the long chains discussed on the prebiotic fructans page) hydrolyse under prolonged heat into fructose and glucose, and partly because those sugars then brown through Maillard and caramelisation reactions. The onion contained the sugar all along, locked into chains you could not taste. Long cooking unlocks it.

That second point has a practical consequence people find surprising: caramelising an onion makes it sweeter without making it lower in FODMAPs. The fructan chain has been shortened, not removed, and shorter fructo-oligosaccharides are, if anything, fermented faster. Long-cooked onion is not a safe onion for a fructan-sensitive gut, however gentle it tastes.

For the sulfur chemistry specifically, the hierarchy is:

1. Raw — the full thiosulfinate profile, at the cost of pungency.
2. Chopped, rested a few minutes, then briefly cooked — the enzyme finishes its work before heat arrives, and more of the product survives a short cook. This is the best compromise.
3. Straight into hot fat — the enzyme is killed early, less is formed.
4. Long slow caramelisation — superb flavour, most of the sulfur chemistry gone.
5. Boiled and drained — the least of everything, since both the sulfur compounds and the flavonols are water-soluble.

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The Antimicrobial Evidence, Honestly

Onion extracts inhibit bacteria and fungi in a Petri dish. This is well documented, it is not controversial, and it is the basis of a very large amount of overstated writing.

What is true: thiosulfinates react with thiol groups, which are essential to many bacterial enzymes, and this produces genuine growth inhibition in laboratory culture across a wide range of organisms. Griffiths and colleagues' review of onions and health summarises the in-vitro literature fairly.

What does not follow:

1. Eating onions does not sterilise anything. The concentrations that inhibit bacteria in a dish are far above anything achieved in blood or tissue after a meal, and the reactive compounds are consumed or metabolised rapidly.
2. An onion on the windowsill does not absorb illness from a room. This is a persistent folk belief with no mechanism and no evidence. Volatile sulfur compounds diffuse out of a cut onion; nothing diffuses in.
3. A cut onion left out is not uniquely dangerous either. The mirror-image myth — that a leftover cut onion becomes a "magnet for bacteria" and is poisonous the next day — is equally unfounded. A cut onion is a moist cut vegetable; refrigerate it in a covered container and use it within a few days, like any other.
4. Onion poultices are not a wound treatment. The wartime use of allium preparations on wounds is discussed on the history page, where the honest verdict is that there was a real antibacterial basis and the claims made for it were considerably larger than the evidence.

The reasonable position is that onion's antimicrobial chemistry is real and mainly relevant to the plant's own defence and to food preservation traditions, not to treating human infection.

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Platelets, Blood and the Circulation

Onion extracts inhibit platelet aggregation in laboratory assays, and this has been shown to vary by cultivar — Goldman and Kopelberg compared four onion genotypes and found the antiplatelet activity differed between them, tracking pungency in the expected direction. Pungent onions are more active; mild ones less so.

The honest reading:

1. The effect is consistent in vitro and is a plausible part of why allium-rich dietary patterns look good in population studies.
2. There is no clinical trial showing that eating onions changes bleeding risk, clotting times, or cardiovascular events in humans. It is a laboratory finding with a plausible bridge, not a demonstrated human effect.
3. Nobody needs to avoid onions before surgery or while on an anticoagulant on the strength of this. Concentrated garlic supplements are a more reasonable thing to mention to a surgeon; ordinary dietary onion is not.

The better-supported cardiovascular story for onions runs through quercetin and blood pressure, which is covered on the quercetin page.

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Allium Vegetables and Cancer Epidemiology

The strongest human signal for allium vegetables is observational, and it is strongest for cancers of the digestive tract.

Turati and colleagues reported a case–control study together with a meta-analysis of allium vegetable intake and gastric cancer, finding an inverse association — higher allium intake, lower gastric cancer risk. Hsing and colleagues, working in a population-based study in China, reported an inverse association between allium vegetable intake and prostate cancer, with the association strongest in the highest-intake group.

Three qualifications that belong beside those findings:

1. These are observational. People who eat a lot of onions and garlic differ from those who eat few — in overall diet quality, in cooking from scratch, often in region and income. Statistical adjustment reduces confounding but never eliminates it.
2. Intake is measured by questionnaire, and recalled vegetable intake is one of the noisier things nutritional epidemiology measures.
3. There is no randomised trial. No study has assigned people to eat onions and followed them for cancer outcomes, and realistically none will.

What the evidence supports is that allium vegetables belong in the general category of vegetables associated with lower digestive cancer risk. It does not support onions as a cancer preventive in any specific or dose-dependent sense, and anyone presenting it that way is going well beyond the data.

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Bone Density and Other Scattered Findings

One frequently cited finding deserves an accurate description. Matheson and colleagues analysed national survey data and reported an association between onion consumption and bone density in perimenopausal and postmenopausal non-Hispanic white women aged 50 and over. It is a cross-sectional analysis of survey data, not a trial — it shows a correlation in one population subgroup at one point in time. It is interesting and it is genuinely published; it is not evidence that eating onions builds bone.

Other findings in this scattered category, listed with the same honesty:

1. Blood sugar. Onion has been studied for glycaemic effects, mostly in small trials with concentrated preparations. The results are mixed and the trials are small. Onions are a reasonable food for someone managing blood sugar because they are low in available carbohydrate and high in fibre-like fructans, which is a different and more defensible claim than saying they lower blood glucose.
2. Anti-inflammatory markers. Onion peel extract has moved inflammatory markers in small trials (see the quercetin page); this is a quercetin story more than a sulfur one.
3. Onion juice for hair loss. A small, frequently cited trial exists and the mechanism proposed is irritant-driven. The evidence base is thin and the smell is not.

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Getting the Most Sulfur Chemistry Out of an Onion

1. Choose pungent onions. Red and yellow storage onions carry more precursor than sweet or white ones. If it makes you cry, it is working.
2. Chop, then wait 5–10 minutes before heating. Alliinase needs time. This single habit preserves more of the chemistry than any other.
3. Eat some raw. Thinly sliced red onion soaked briefly in cold water or vinegar loses harshness while keeping most of the compounds.
4. Crush rather than slice when the dish allows — grated or blended onion in a marinade or curry base ruptures far more cells.
5. Do not boil and drain. If there is cooking water, use it.
6. Vary the alliums. Onion, garlic, leeks, shallots, spring onions and chives all carry different precursor ratios, so eating across the family gives a broader mix than eating one of them heavily.
7. Store onions dry, dark and cool but not cold. Refrigeration converts starches and softens texture over time; a ventilated basket in a dark cupboard is better for storage, and the fridge is for the half hour before you chop.

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Key Research Papers

1. Imai S, Tsuge N, Tomotake M, et al. An onion enzyme that makes the eyes water. Nature. 2002;419(6908):685. — doi:10.1038/419685a
2. Eady CC, Kamoi T, Kato M, et al. Silencing onion lachrymatory factor synthase causes a significant change in the sulfur secondary metabolite profile. Plant Physiology. 2008;147(4):2096-2106. — doi:10.1104/pp.108.123273
3. Lanzotti V. The analysis of onion and garlic. Journal of Chromatography A. 2006;1112(1-2):3-22. — doi:10.1016/j.chroma.2005.12.016
4. Griffiths G, Trueman L, Crowther T, Thomas B, Smith B. Onions — a global benefit to health. Phytotherapy Research. 2002;16(7):603-615. — doi:10.1002/ptr.1222
5. Lancaster JE, Shaw ML. Gamma-glutamyl peptides in the biosynthesis of S-alk(en)yl-L-cysteine sulphoxides (flavour precursors) in Allium. Phytochemistry. 1989;28(2):455-460. — doi:10.1016/0031-9422(89)80031-7
6. Freeman GG, Whenham RJ. Synthetic S-alk(en)yl-L-cysteine sulphoxides — alliinase fission products: simulation of flavour components of Allium species. Phytochemistry. 1976;15(4):521-523. — doi:10.1016/S0031-9422(00)88962-1
7. Borlinghaus J, Albrecht F, Gruhlke MCH, Nwachukwu ID, Slusarenko AJ. Allicin: chemistry and biological properties. Molecules. 2014;19(8):12591-12618. — doi:10.3390/molecules190812591
8. Goldman IL, Kopelberg M. Comparative antiplatelet activity of four cultivated onion (Allium cepa) genotypes. Acta Horticulturae. 1996;(426):255-258. — doi:10.17660/ActaHortic.1996.426.30
9. Turati F, Pelucchi C, Guercio V, La Vecchia C, Galeone C. Allium vegetable intake and gastric cancer: a case–control study and meta-analysis. Molecular Nutrition & Food Research. 2015;59(1):171-179. — doi:10.1002/mnfr.201400496
10. Hsing AW, Chokkalingam AP, Gao YT, et al. Allium vegetables and risk of prostate cancer: a population-based study. Journal of the National Cancer Institute. 2002;94(21):1648-1651. — doi:10.1093/jnci/94.21.1648
11. Matheson EM, Mainous AG, Carnemolla MA. The association between onion consumption and bone density in perimenopausal and postmenopausal non-Hispanic white women 50 years and older. Menopause. 2009;16(4):756-759. — doi:10.1097/gme.0b013e31819581a5
12. Arnault I, Auger J. Seleno-compounds in garlic and onion. Journal of Chromatography A. 2006;1112(1-2):23-30. — doi:10.1016/j.chroma.2006.01.036
13. Block E. Garlic and Other Alliums: The Lore and the Science. Royal Society of Chemistry; 2009. — doi:10.1039/9781839168369
14. Slimestad R, Fossen T, Vågen IM. Onions: a source of unique dietary flavonoids. Journal of Agricultural and Food Chemistry. 2007;55(25):10067-10080. — doi:10.1021/jf0712503

Live PubMed Searches

1. PubMed: onion thiosulfinate organosulfur
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Connections

• Onions — the main topic page
• Onions — Benefits Deep Dive
• Onions: History and Origins
• Onion Quercetin and Heart Health
• Onion Prebiotic Fructans and Gut Bacteria
• Onions, FODMAPs, and IBS
• Garlic — the allicin side of the family
• Leeks
• Sulfur
• Selenium
• Sulforaphane — the brassica parallel
• Broccoli
• Quercetin
• All Food Topics

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