4-Hydroxybenzyl isothiocyanate
Also written p-hydroxybenzyl isothiocyanate
Quick answer
The pungent compound of white and yellow mustard seed, released from sinalbin when the seed is wetted and crushed: an oily, mildly irritant heat that stays on the tongue, and that water destroys within minutes at a neutral pH.
What it does
Where black mustard’s glucosinolate, sinigrin, releases the aggressive, volatile allyl isothiocyanate, white mustard’s sinalbin releases the mildly irritant, oily 4-hydroxybenzyl isothiocyanate. The difference in volatility is the difference a cook notices: the heat of yellow mustard is felt in the mouth and does not rise into the nose as black or brown mustard’s does.
How it behaves when cooked
Four properties of the molecule, and what they mean at the stove.
- Volatility
- low
- Fat solubility
- moderate
- Water solubility
- moderate
- Heat stability
- low
Like all mustard pungency it does not exist until the enzyme myrosinase meets its glucosinolate in the presence of water, so dry ground seed has none until it is mixed. Once formed it is short-lived in water: in a study of white mustard seed meal its half-life was 321 minutes at pH 3.0 and only 6 minutes at pH 6.5, breaking down to ionic thiocyanate, with more alkaline conditions faster still. Those figures, from seed meal rather than prepared mustard, are consistent with the custom of mixing yellow mustard with an acid such as vinegar where its heat is meant to keep.
Not present until the plant is damaged
Sinalbin (4-hydroxybenzyl glucosinolate) + Myrosinase
The seed keeps sinalbin and myrosinase apart until it is damaged in the presence of water. White mustard seed carries about 140 µmol of glucosinolate per gram, and the isothiocyanate it releases is the seed’s pungency.
Spices that carry it
Recorded on the compound rather than on each spice, so the two cannot disagree.
- Mustard seedMajor component
The pungency of white or yellow mustard seed (Sinapis alba). Brown and black mustard carry allyl isothiocyanate instead, which is why the two kinds of seed are not interchangeable where the sensation matters.
Not to be confused with
Compounds a reader is likely to take for this one, and why they differ.
Allyl isothiocyanate
Both are isothiocyanates released by myrosinase. Allyl isothiocyanate, from black and brown mustard, is volatile and reaches the nose; the 4-hydroxybenzyl compound of yellow mustard is oily, milder and stays in the mouth.
Questions this page answers
- why is yellow mustard milder than brown
- what is 4-hydroxybenzyl isothiocyanate