Onions are loaded with sulfur. The element arrives through the roots from the soil and gets built into a family of organic molecules called cysteine sulfoxides, which sit quietly inside onion cells until you break them open with a knife. At that point, a surprisingly elaborate chain reaction converts those sulfur compounds into the volatile chemical that stings your eyes and triggers tears. The story behind that reaction turns out to be more interesting than scientists assumed for most of the twentieth century, involving an enzyme that was not even discovered until 2002.
Where the Sulfur Comes From
Onions, like all plants in the genus Allium (which also includes garlic, leeks, shallots, and chives), are unusually good at pulling sulfate from the soil and incorporating it into complex organic molecules. Once absorbed, the sulfur is channeled into the amino acid cysteine, which then becomes the backbone of the flavor precursors that give alliums their pungent character. In onions specifically, the key precursor is a compound commonly abbreviated as PRENCSO (1-propenyl-L-cysteine sulfoxide). The biosynthetic pathway builds these precursors by attaching sulfur-containing groups to cysteine, either through an intermediate involving glutathione or through a more direct chemical route, followed by an oxidation step that creates the sulfoxide bond.
1Journal of Experimental Botany. Biosynthesis of the flavour precursors of onion and garlicThese sulfur-rich precursors are the chemical reason onions smell and taste the way they do. Without them, an onion would be a bland, mildly sweet vegetable with the texture of an apple. But the precursors themselves are not volatile and do not sting your eyes. They need to be broken apart first, and that only happens when onion cells are damaged.
The Compartment Trick That Keeps the Onion Stable
If onion cells contain both the sulfur precursors and the enzyme capable of breaking them down, why doesn’t the reaction happen all the time inside the intact bulb? The answer is physical separation at the cellular level. The enzyme responsible for the first step of the breakdown, called alliinase, is locked inside the cell’s vacuole, a storage compartment sealed off from the rest of the cell interior. The cysteine sulfoxide precursors, meanwhile, float around in the surrounding cytoplasm. As long as the cell membrane is intact, the two never meet.
2Plant Science Letters. Presence of alliinase in isolated vacuoles and of alkyl cysteine sulphoxides in the cytoplasm of bulbs of onion (Allium cepa)The moment a knife blade ruptures onion cells, that barrier collapses. Alliinase floods out of the vacuole and comes into contact with PRENCSO for the first time. The reaction begins immediately, and it happens fast. This is why the pungency of an onion intensifies right after cutting and why a whole, uncut onion sitting on your counter produces no smell or irritation at all.
The Enzyme Nobody Knew About
For decades, the standard explanation for onion tears went like this: alliinase breaks down PRENCSO, and one of the spontaneous by-products is a volatile sulfur compound called the lachrymatory factor (from the Latin lacrima, meaning “tear”) that drifts up into your eyes. The story was tidy and seemed complete. Then in 2002, a Japanese research team led by Shinsuke Imai showed it was wrong.
Their work revealed that the lachrymatory factor is not a random by-product of the alliinase reaction. Instead, a second, previously unknown enzyme takes the sulfenic acid produced by alliinase and specifically converts it into propanthial S-oxide, the actual tear-inducing molecule. They named this enzyme lachrymatory factor synthase, or LFS.
3Nature. An onion enzyme that makes the eyes waterThis was a significant finding because it meant the onion is not passively leaking irritants when damaged. It is actively manufacturing a specific chemical weapon through a dedicated enzyme. Later structural studies solved the crystal structure of LFS and examined how it converts the unstable sulfenic acid substrate into the lachrymatory factor, confirming it as a distinct catalytic step rather than a spontaneous rearrangement.
4PubMed Central. Enzyme That Makes You Cry-Crystal Structure of Lachrymatory Factor Synthase from Allium cepaThe discovery also immediately suggested a practical application: if LFS could be suppressed without touching alliinase, you could potentially create an onion that still has full flavor but produces no tears. That idea has been pursued seriously, as we will see below.
How Propanthial S-Oxide Actually Makes You Cry
Once propanthial S-oxide becomes airborne, it rises from the cutting board and reaches the surface of your eyes. There, it dissolves into the thin film of moisture on the cornea and activates sensory nerve endings. The specific receptor involved is TRPA1, an ion channel on the surface of pain-sensing neurons that functions as a broad detector for reactive, potentially harmful chemicals. When TRPA1 fires, it sends a signal along the trigeminal nerve to the brainstem, which interprets the signal as a chemical irritant threat.
5PubMed Central. Transient receptor potential ankyrin 1 antagonists block the noxious effects of toxic industrial isocyanates and tear gasesThe brainstem responds reflexively. It activates the parasympathetic and sympathetic nerves that control the lacrimal gland, the small gland tucked behind the upper outer corner of each eye socket. The lacrimal gland ramps up tear production to flush the irritant off the corneal surface.
6PubMed Central. Neural regulation of lacrimal gland secretory processes: relevance in dry eye diseasesThe entire loop, from knife stroke to tear, takes only seconds. And it is not a learned response or an overreaction. TRPA1 responds to all kinds of electrophilic compounds, including industrial tear gases, mustard oil, and the allyl isothiocyanate in wasabi. Propanthial S-oxide just happens to be the version an onion produces. Your eyes treat it the same way they would treat exposure to pepper spray, just at a far lower intensity.
Why the Onion Bothers Making Tear Gas
From the plant’s perspective, the sulfur chemistry is not about flavor or cooking. It is a defense system. Sulfur-containing compounds in plants form a broad chemical barrier against herbivores, insects, and microbial pathogens.
7Natural Product Communications. Sulfur and Sulfur Compounds in Plant DefenceThe compartmentalization strategy is key to why it works so well. By keeping the enzyme and its substrate physically separated until damage occurs, the onion creates what amounts to a binary chemical weapon. Nothing happens while the bulb is intact and underground. But the moment a burrowing insect, a grazing animal, or a fungal infection breaches the cell walls, the reaction fires instantly. The volatile sulfur products irritate mucous membranes, repel pests, and in some cases have direct antimicrobial activity. It is a remarkably elegant system: the plant pays no metabolic cost for defense until the moment defense is actually needed.
Why Garlic Does Not Make You Cry
Garlic is an allium, and it is famous for its pungent sulfur chemistry. Yet crushing or slicing garlic cloves produces no tears. The difference comes down to which cysteine sulfoxide predominates and whether the plant has lachrymatory factor synthase. Garlic’s primary precursor is alliin (allyl cysteine sulfoxide) rather than PRENCSO. When garlic’s alliinase acts on alliin, it produces allicin, the compound responsible for raw garlic’s sharp bite and many of its reputed health properties. Allicin is pungent, but it is not volatile enough at room temperature to drift into your eyes and trigger TRPA1 at meaningful concentrations.
Onions, by contrast, produce PRENCSO, and they have the additional LFS enzyme to convert the intermediate sulfenic acid into the highly volatile propanthial S-oxide. Garlic does not have LFS, so its sulfenic acid intermediates condense into allicin and related compounds instead of forming a lachrymatory agent. The two plants start from similar sulfur biochemistry but diverge at a critical branch point, and that divergence is the entire difference between watering eyes and watering mouths.
Can You Actually Grow a Tearless Onion?
The discovery of LFS raised an obvious question: what happens if you turn the gene off? In 2008, researchers in New Zealand accomplished exactly that, using RNA interference to silence the LFS gene in onion plants. They reduced LFS activity by up to 1,544-fold. The modified onions produced dramatically less lachrymatory factor when cut.
8PubMed Central. Silencing Onion Lachrymatory Factor Synthase Causes a Significant Change in the Sulfur Secondary Metabolite ProfileThe interesting wrinkle was what happened to the rest of the sulfur chemistry. Shutting down LFS did not simply eliminate one compound and leave everything else unchanged. It redirected the sulfur metabolite profile. Without LFS intercepting the sulfenic acid, more of it was shunted into thiosulfinates and other flavor-active compounds. In principle, this meant the onion might actually taste different, possibly more intensely flavored in some respects, rather than just less irritating.
Despite the success of the lab demonstration, tearless onions have not become a grocery-store staple. Onion is notoriously difficult to genetically transform, consumer wariness of genetically modified produce remains a factor in many markets, and conventional breeding toward lower-pungency varieties (like sweet Vidalia onions) has partially filled the demand gap. Some companies have pursued non-GMO tearless varieties through traditional breeding and mutagenesis, with a few reaching limited commercial availability. But a true no-tear, full-flavor onion grown at commodity scale is still more aspiration than reality.
Does Soil Sulfur Make Onions More Pungent?
Since onion pungency is driven by sulfur compounds, it seems logical that onions grown in sulfur-rich soil should be hotter and more tear-inducing. This is a widespread belief in the food world, and it contains a grain of truth, but the relationship is not as straightforward as “more soil sulfur equals more pungent onions.”
Field studies comparing onions grown with and without supplemental sulfur fertilization in commercial soils have found that adding extra sulfur to soil that already contains adequate levels does not increase pungency, total bulb sulfur, or related quality measurements. In one trial, researchers grew the mild ‘Texas Grano 1015Y’ variety in two different field soils (clay and loam) with and without additional sulfur at commercial application rates. They found no significant effect on any pungency parameter from the extra sulfur treatment.
9Scientia Horticulturae. Changes in pungency of onions by soil type, sulphur nutrition and bulb maturityA separate study on short-day onions grown in high-sulfur soils came to a similar conclusion: additional sulfur application up to 26 kg per hectare did not increase pungency or related quality components.
10Scientia Horticulturae. Application of extra sulfur to high-sulfur soils does not increase pungency and related compounds in shortday onionsThe takeaway is that genetics and maturity matter far more than soil sulfur levels in most commercial growing conditions. The famous mildness of Vidalia onions has more to do with the variety grown and the timing of harvest than with some magical property of Georgia soil. Sweet onion varieties are bred to produce less PRENCSO in the first place. Soil sulfur only becomes a limiting factor when it is genuinely deficient, which is uncommon in irrigated agricultural settings.
Kitchen Strategies That Actually Reduce Tears
People have invented an enormous number of folk remedies for onion tears: hold a piece of bread in your mouth, light a candle, chew gum, whistle while you chop. Most of these do nothing, because they do not address the actual mechanism. But a few approaches are grounded in the chemistry and physics of the problem.
Chilling the onion works. The volatility of propanthial S-oxide is temperature-dependent. A cold onion releases less of the compound into the air, so less reaches your eyes. Thirty minutes in the refrigerator before cutting makes a noticeable difference.
Blade sharpness and cutting speed also matter. A 2025 study systematically varied blade sharpness and cutting speed and found that faster or blunter blades significantly increased both the number and energy of droplets ejected from the cut onion surface.
11PubMed Central. Droplet outbursts from onion cuttingA dull blade crushes and tears cells rather than slicing cleanly through them, which ruptures more vacuoles and releases more alliinase to react with more PRENCSO. It also physically sprays more liquid into the air. A sharp knife making a smooth, even cut minimizes the mechanical damage footprint. Cutting speed matters for the same reason: a faster chop compresses cells more violently.
Ventilation is the simplest intervention. Chopping under a kitchen vent hood, near an open window, or even with a small fan blowing across the cutting board disperses the volatile compound before it accumulates near your face. Contact lens wearers sometimes report fewer tears than people without lenses, likely because the lens acts as a partial physical barrier over the cornea.
What Cooking Does to Onion Sulfur Compounds
The sulfur chemistry that produces tears in a raw onion is almost entirely transformed by heat. Cooking triggers a series of thermal reactions that break down the cysteine sulfoxide precursors and their derivatives into a different set of sulfur compounds. These cooked-onion products include various disulfides, trisulfides, and other molecules that are responsible for the sweet, caramelized, savory character of sautéed or roasted onions.
12Woodhead Publishing. Flavour Development, Analysis and Perception in Food and BeveragesHeat also deactivates alliinase and LFS. Once an onion has been cooked, the enzymatic machinery that produces the lachrymatory factor is destroyed, which is why cooked onions never make you cry no matter how long you handle them. The sulfur is still there, just in a different chemical form. This also explains why dried onion powder or dehydrated onion flakes are less pungent than fresh raw onion. Processing methods that involve high temperatures or pretreatments that reduce enzyme activity can preserve certain beneficial sulfur compounds while eliminating the volatile irritants.
13PubMed. Effects of post-harvest processing techniques on the content of sulfur-containing compounds in fruit and vegetable produce and product flavor: a reviewSulfur Compounds and Health
The same sulfur chemistry that makes onions inconvenient to prepare has attracted considerable interest from nutrition researchers. Onions and other alliums are rich in organosulfur compounds, and these have shown a range of biological activities in laboratory and epidemiological studies, including anti-inflammatory, antioxidant, and antimicrobial properties.
14PubMed Central. Therapeutic Role of Functional Components in Alliums for Preventive Chronic Disease in Human BeingIt is worth being cautious about how far to extrapolate from this. Many of the most striking health claims for allium sulfur compounds come from cell-culture experiments or animal studies where purified compounds are administered at concentrations far higher than you would get from eating onions. The leap from “this compound kills cancer cells in a petri dish” to “eating onions prevents cancer” is enormous and not well supported by clinical trial evidence. What the epidemiological data does suggest is that diets rich in allium vegetables are associated with modestly lower rates of certain cancers and cardiovascular disease, but disentangling the effect of onions specifically from the general pattern of eating more vegetables is difficult.
Onion Breath and Sulfur Metabolism
The sulfur compounds in onions do not stay confined to your mouth after you eat them. Some are absorbed through the gut, metabolized in the liver, and then released through the lungs, which is why onion breath can persist long after you have brushed your teeth. The volatile sulfur compounds responsible for bad breath after eating alliums are produced both by direct release of sulfur-containing molecules in the mouth and by systemic metabolism that brings sulfur compounds back into exhaled air.
15PubMed. Dietary Sulfur Compounds and Halitosis: Bridging Food Science, Microbial Metabolism, and Oral Health: A Comprehensive ReviewDietary sulfur compounds also influence the oral microbiome by affecting substrate availability and the balance of sulfur-metabolizing bacteria. This is why the intensity and duration of onion breath varies between people: individual differences in gut absorption, liver enzyme activity, and oral bacterial populations all play a role. The folk remedies (parsley, milk, apples) that people swear by for onion breath work to varying degrees by either physically trapping sulfur compounds or providing enzymes that break them down in the mouth, but none of them address the systemic component. If you can still smell onions on your breath hours later, that sulfur is coming from your bloodstream, not from residue stuck between your teeth.