The Enzyme That Makes You Cry

An onion's tear-gas is not an accident of cutting. A second, dedicated enzyme — lachrymatory-factor synthase, discovered only in 2002 — builds it from a fragment the first enzyme makes. Silence that one enzyme and the onion keeps every bit of its flavour and stops your tears entirely. Cut the onion below, watch the cascade fire, then test every kitchen remedy against the chemistry: which ones interrupt a real step, and which are pure folklore.

The cascade — cut it and watch

Tear-gas reaching your eye

100%

Flavour compounds (in the food)

60%

Cells ruptured: 80% · onion 25 °C · still air · eyes open · LFS gene ON

Cells ruptured 80% A clean cut with a sharp knife severs cells along one plane; crushing or grating ruptures the whole tissue, mixing far more enzyme with substrate.

Onion temperature 25 °C Cold slows the enzymes (Arrhenius) and keeps the gas from evaporating (Clausius–Clapeyron). Both shrink the airborne dose during the minute you're cutting.

The flavour gauge tracks the pungent sulfur compounds that end up in your food — note that switching the LFS gene OFF kills the tears but the flavour stays (the sulfur is simply routed elsewhere). Chilling, by contrast, mutes both during the cut.

The same story, as a reaction

This is the path your knife sets off. The fork at the bottom is the whole point: with the second enzyme present you cry; without it the very same sulfur becomes flavour and nothing else.

0 · intact cell
Substrate (isoalliin) sits locked in the vacuole; the enzyme alliinase waits in the cytoplasm. Apart, nothing happens — which is why a whole onion never stings.

▼ the knife ruptures the cell, mixing them

1 · alliinase fires
isoalliin → 1-propenylsulfenic acid + 2-aminoacrylate (which becomes the onion's familiar smell precursors, pyruvate + ammonia).

▼ a fork — and here is the surprise

2a · LFS present → TEARS
Lachrymatory-factor synthase isomerises the sulfenic acid into syn-propanethial-S-oxide — a small, volatile sulfine. It evaporates, drifts up, and reaches your eye.

2b · LFS absent → FLAVOUR
With no LFS, the sulfenic acids instead condense into thiosulfinates — the savoury, oniony flavour molecules. Same sulfur, no tear-gas.

▼ the gas reaches the eye's surface

3 · the eye reacts
The sulfine activates irritant-sensing channels (TRPA1 / TRPV1) on the trigeminal nerve endings in your cornea. Your brain answers the only way it can: flush it out with tears.

The remedy bench — folklore vs. chemistry

Every trick people swear by is really a claim about which step it interrupts. Click one and watch the gauges above: a remedy that targets a real step in the cascade drops the tear-gas; a remedy that targets nothing does nothing.

Pick a remedy to see whether it has a mechanism — and which step it acts on.

What the internet gets wrong

"The gas just forms on its own once you cut." For decades that was the textbook story. The 2002 discovery of lachrymatory-factor synthase overturned it: with alliinase alone and no LFS, the lachrymatory factor is not produced at all. Crying needs a second, dedicated enzyme — which is exactly why a "tearless" onion (LFS silenced, or low-pungency lines) can keep its flavour.

"It turns into sulfuric acid on your eyeball." An over-simplification you'll see widely. The intact sulfine is itself the irritant: it is an electrophile that covalently tweaks cysteine residues on the TRPA1/TRPV1 ion channels in your eye's sensory nerves. That chemical handshake — not a bath of acid — is what makes you tear up.

The check

Every reaction step below balances atom-for-atom, and the molecule's identity and the chill-it estimates are recomputed by the verifier in research/why-onions-make-you-cry/. Numbers shown here are printed by that file.

Lachrymatory factor	C3H6OS
Its molar mass	90.14 g/mol
LFS discovered	2002 (Nature)
LFS silenced (RNAi) by up to	1,544×
Enzyme rate, 4 °C vs 25 °C (Arrhenius, E_a=50 kJ/mol, illustrative)	0.217×
Vapour pressure, 4 °C vs 25 °C (Clausius–Clapeyron, ΔH_vap=40 kJ/mol, illustrative)	0.294×

The two temperature ratios use generic constants, not onion-measured ones; they establish the direction and rough size of the chilling effect, no more. Everything else is exact bookkeeping. The on-page tear/flavour gauges are a transparent relative model (each factor maps to one step of the cascade), not a measured dose.