Isoalliin
Also written trans-S-1-propenyl-L-cysteine sulfoxide, S-1-propenyl-L-cysteine sulphoxide, propenyl cysteine sulphoxide
Quick answer
The odourless sulphur amino acid an intact onion stores in place of a smell: the same atoms as garlic’s alliin, differently arranged, and the starting point of onion flavour, onion pungency and the tear factor alike.
What it does
Isoalliin has no smell of its own. It is the main cysteine sulphoxide of the onion: measured beside bunching onion and garlic, the bulb onion held isoalliin as its major sulphur precursor and no detectable alliin, where garlic was dominated by alliin and held only a little isoalliin. The handbooks describe the content of these sulphoxides as the most important character of the onion, the thing that gives rise to its flavour and pungency. When the bulb is bruised the enzyme alliinase breaks the sulphoxides down, releasing pyruvic acid and thiosulphinates that turn rapidly into sulphides and disulphides; in the onion part of the same breakdown is diverted into the volatile tear factor.
How it behaves when cooked
Four properties of the molecule, and what they mean at the stove.
- Volatility
- low
- Fat solubility
- low
- Water solubility
- high
- Heat stability
- moderate
Isoalliin is a precursor, so what a cook meets is what becomes of it. How much there is to begin with differs between onions: pungency is measured as the pyruvate released when the bulb is crushed, and the handbook gives a range of at least tenfold between a mild cultivar and a strong one, varying further with temperature, with the nitrogen and sulphur in the soil and with storage. What it turns into depends on the cooking. The handbook’s account is that boiling and frying each steer the breakdown of the sulphoxides differently, which is why the same onion tastes different raw, boiled and fried, and that the flavour of fried onion is sulphides together with caramelised sugars. The four levels given here follow from what the molecule is, an amino acid with a computed preference for water over fat; none was measured in anything read for this profile, and its stability to heat in particular is unmeasured here.
Not present until the plant is damaged
Made in the living plant; it is the precursor, not the product + Alliinase acts on it when the tissue is cut
Isoalliin does not form on damage: it is what damage consumes. Cutting or bruising lets alliinase cleave it to 1-propenesulfenic acid, which either condenses into thiosulphinates or is turned by a second enzyme into the tear factor.
Spices that carry it
Recorded on the compound rather than on each spice, so the two cannot disagree.
- OnionPrincipal component
The main sulphur precursor measured in the bulb, where alliin was not detected; onion flavour, pungency and the tear factor all begin with it.
Not to be confused with
Compounds a reader is likely to take for this one, and why they differ.
Alliin
Garlic’s precursor and onion’s have the same formula and differ in where one double bond sits: alliin carries an allyl group, isoalliin a 1-propenyl group. Garlic is dominated by the first and onion by the second, and that one difference is why crushed garlic makes allicin and a cut onion makes the compound that draws tears.