GARLIC
Crushing makes the bite
Oregon State University's Linus Pauling Institute says crushing or chopping raw garlic releases the enzyme alliinase, which makes allicin within 10 to 60 seconds.
Check a cutKITCHEN CHEMISTRY
The bite starts
with the cut.
A whole garlic clove holds no allicin. Crush it and the reaction starts.
Check a cut
10–60 sfor garlic's bite to form
Onions and garlic keep their sharp flavours locked away until their cells are broken. Crushing, chopping or chewing lets an enzyme loose, and within seconds it makes the compounds that sting your eyes or bite your tongue.
At a glance
- Cells break, enzymes start
- Onions make a tear gas
- Heat stops the enzyme
- Sulphur chemistry on the chopping board
10–60 seconds
Time for allicin and related compounds to form after crushing
The reaction is very rapid: thiosulfinates have been found to be fully formed within 10 to 60 seconds of crushing garlic.
FORMATION · LINUS PAULING INSTITUTE
2.5 days
Half-life of allicin in crushed garlic at 23°C
Allicin is unstable and breaks down into a family of other sulphur compounds, which is why crushed garlic changes as it sits.
HALF-LIFE · LINUS PAULING INSTITUTE
TWO NUMBERS WORTH KNOWING
Fast to form,
slow to fade.
One figure for how quickly crushed garlic reacts,
one for how long its key compound lingers.
Garlic cloves yield about 2.5 to 4.5 mg of allicin per gram of fresh weight when crushed, and one fresh clove weighs 2 to 4 g, the Institute says.
A QUICK CUT CHECK
How much of the
sharp stuff?
Pick what happens to the onion or garlic. The bar shows roughly how much of the pungent sulphur reaction the step sets off.
Based on the Linus Pauling Institute's garlic article and an Aston University explainer on onions in The Conversation.
THE SHORT ANSWER
Break the cells,
start the reaction.
Onion and garlic flavours are a defence the plant sets off when damaged.
Three things that follow from it.
ONIONS
Why you cry
A cut onion makes a volatile chemical that drifts into your eyes and forms a small amount of irritating acid, an Aston University lecturer explains.
Why garlic doesn't
STORING
Cool, dry and airy
The RHS says dried garlic keeps in a cool, dry place at 5–10°C. Onions, too, prefer somewhere cool, dark and airy rather than the damp fridge.
Kitchen tips
FOUR THINGS TO TRY
Fewer tears,
more control.
You can work with the chemistry rather than against it. These ideas come straight from how the enzymes behave.
Chill the onion first
A short spell in the fridge or freezer slows the enzymes before you chop. Do not keep onions in the fridge long term.
Chop under the hood
Drawing the vapour away, with a cooker hood or near running water, keeps less of it from reaching your eyes.
Crush for bite, keep whole for mild
Crushing raw garlic sets off the full reaction; cooking cloves whole before cutting leaves very little allicin formed.
Store dry and cool
Keep dried garlic at 5–10°C in a dry place, and onions somewhere cool, dark and airy, not damp.
WHY GARLIC DOES NOT MAKE YOU CRY
Two cousins,
two chemicals.
Onion cells hold a sulphur compound that, once cut, is turned by an enzyme and then by a second one, lachrymatory-factor synthase, into propanethial S-oxide: the gas that stings. That second enzyme was described in Nature in 2002.
Garlic holds a slightly different compound, alliin, which does not break down into the eye-stinging gas. Instead it becomes allicin, the source of raw garlic's sharp bite.
Questions
Questions people ask
Why do some onions make me cry more than others?
There is debate. One plausible explanation is the amount of sulphur the onion absorbed from the soil. Sweeter onions tend to have fewer of the sulphur compounds that make the tear-causing gas.
Is the onion gas harmful to my eyes?
The Aston University explainer says the amount of acid formed in the eye is so small that its effect is only irritating, not harmful.
Does it matter if I crush or slice garlic?
Both break cells and release alliinase, so both start the reaction that makes allicin. An intact clove contains none, which is why the bite only appears once garlic is cut.
Why does garlic smell linger on the breath?
Allicin breaks down into volatile sulphur compounds. The Linus Pauling Institute notes that some of their products, such as allyl methyl sulfide, have been detected in human breath after eating garlic.
How long does home-grown garlic keep?
The RHS says hardneck varieties store well for four to six months, and softneck varieties for longer, potentially up to a year, once dried and kept cool.
Sources
Where the facts come from
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