Propanethial S-oxide
Also written syn-propanethial-S-oxide, (Z)-propanethial S-oxide, thiopropanal S-oxide, onion lachrymatory factor
Quick answer
The small volatile sulphur compound that makes the eyes water when an onion is cut: absent from the whole bulb, and made within moments of cutting by two enzymes working in sequence.
What it does
Propanethial S-oxide is the lachrymatory factor of the onion, the cause of the eye irritation that goes with chopping one. It is a small, volatile molecule of an unusual kind: thioaldehyde S-oxides are very rare in nature, with only four known, all from plants. It is felt, not smelled or tasted; the sources read name no receptor for it. That it is the cause is shown by taking it away. Onions in which the gene for the enzyme that makes it was silenced, cutting that enzyme’s activity by up to 1,544-fold, produced far less of it when wounded and did not irritate the eyes.
How it behaves when cooked
Four properties of the molecule, and what they mean at the stove.
- Volatility
- high
- Fat solubility
- moderate
- Water solubility
- moderate
- Heat stability
- low
There is none of it in a whole onion. It forms only where cells are broken, so how much is made follows how much tissue is cut, and because it is volatile it leaves the cut surface as a vapour and reaches the eyes of whoever is cutting. The chemists who study it describe it as both volatile and labile, unstable enough to make the reaction that forms it hard to follow in the laboratory, which is why it is rated here as lost quickly to air and to heat. Making it also uses up material that would otherwise become flavour: in onions with the enzyme silenced, more of the same intermediate went into thiosulphinates and on into sulphur volatiles that are otherwise found in an onion only in traces. No kitchen remedy for onion tears was tested in anything read for this profile, and none is offered. The two solubility levels rest on a computed value that puts the molecule almost evenly between water and fat; neither was measured.
Not present until the plant is damaged
Isoalliin, by way of 1-propenesulfenic acid + Alliinase, then lachrymatory factor synthase
Cutting lets alliinase cleave isoalliin to 1-propenesulfenic acid. Left to itself that acid condenses into thiosulphinates; in the onion a second enzyme, lachrymatory factor synthase, rearranges it into propanethial S-oxide. The rearrangement was once thought spontaneous, but its energy barrier is too high at room temperature and the evidence is that the enzyme is required.
Spices that carry it
Recorded on the compound rather than on each spice, so the two cannot disagree.
- OnionContributing component
Formed only when the fresh bulb is cut. It is what makes cutting an onion unpleasant; nothing read here measures what it adds to the flavour of the onion once eaten.
Not to be confused with
Compounds a reader is likely to take for this one, and why they differ.
Allicin
Garlic’s answer to the same injury. Both plants use alliinase to turn a stored sulphoxide into a reactive sulfenic acid. In garlic the acid condenses into allicin, a thiosulphinate that stays in the clove and is pungent in the mouth; in onion a second enzyme turns much of it into a vapour that reaches the eyes.
Isoalliin
What it is made from. Isoalliin is the odourless amino acid stored in the intact bulb; the tear factor is the volatile made from it after cutting.
Questions this page answers
- why do onions make you cry
- what is the onion lachrymatory factor