Cooking science
What cutting does
A whole onion barely smells. Cut one and it is overwhelming within seconds. The pungency did not exist in the intact vegetable — cutting created it, by letting two things meet that the plant kept apart.
Two compartments, kept apart
Alliums — onion, garlic, leek, shallot, chive — store a harmless precursor compound in one part of the cell and an enzyme in another. Intact, nothing happens.
Break the cell and the two mix. The enzyme converts the precursor into the pungent, volatile, eye-irritating compounds within seconds. That is why the smell arrives the moment the knife goes in and not before.
The same principle operates in mustard and horseradish, where crushing the seed or root generates the heat, and in brassicas, where cutting produces the sulphurous compounds that make cabbage smell of cabbage.
How much you cut decides how strong it is
- A whole clove, roasted
- Almost no cells broken. Sweet, mild, nutty.
- Sliced garlic
- Some cells broken. Moderate, and it browns in distinct pieces.
- Chopped garlic
- More cells. Noticeably sharper.
- Crushed or grated garlic
- Most cells. Fiercest, and it burns fastest in a pan.
- The same is true of onion
- Grated onion in a marinade is far more aggressive than sliced onion in a stew.
- And of herbs
- Bruised mint releases more than torn mint, which releases more than whole leaves.
Practical consequences
- Choose the cut to suit the strength you want. This is a real seasoning decision and most recipes leave it implicit.
- A sharp knife makes food taste different. It severs cells cleanly rather than crushing swathes of them, so there is less pungency and less browning.
- Cut alliums last if you want maximum punch, or early if you want it to mellow.
- Cutting starts a clock. Garlic left chopped on a board for twenty minutes is harsher than garlic used immediately.
- Heat stops it. Cooking destroys the enzyme, so garlic added to a hot pan generates far less pungency than garlic sitting raw.
- Water carries it away. Rinsing chopped onion genuinely reduces its sharpness.
- Cutting also starts browning in apples, potatoes and avocados, by the same logic of exposure.
Cutting onions without crying
Several things help and none of them is a myth-free zone.
- Use a genuinely sharp knife. It ruptures fewer cells than a blunt one and releases less.
- Chill the onion for twenty minutes first. Cold slows the enzyme and reduces how much becomes airborne.
- Cut near an extractor fan or an open window, which moves the compounds away from your face.
- Leave the root end intact until last. It holds a high concentration of the precursor.
- Do not lean over the board. Distance genuinely helps.
- A candle, wetting the knife and holding a spoon in your mouth are all folklore of varying quality.
- Goggles work perfectly and look ridiculous.
Problems it can cause
Common questions
Why do onions make me cry?
Cutting mixes an enzyme with a precursor and produces a volatile irritant within seconds. It does not exist in the intact onion.
Why is crushed garlic stronger than sliced?
More cells broken means more of the pungent compound generated.
Does a sharp knife really change the flavour?
Yes. It severs cells rather than crushing them, so less pungency is released.
Does chilling the onion help?
It slows the enzyme and reduces how much becomes airborne. Twenty minutes is enough.
Why is roasted whole garlic sweet?
Very few cells were broken, so almost none of the pungent compound was ever created.