Vol. XX · Vegetables, Fruit and Legumes Plate 6 of 12onions, garlic, alliinase, allicin, tears, lachrymatory factor, chopping onions, raw garlic, cooked onions, sweet, leeks, scallions, 葱姜蒜MF

Onions and Garlic.

A whole onion has no smell. The moment a knife breaks its cells, an enzyme starts a reaction that makes you cry within seconds.

Onions, garlic, leeks, shallots and scallions store sulphur-containing amino acids in their cells and, separately, an enzyme, alliinase. Cutting, crushing or chewing breaks the cells and mixes the two. In garlic the result is allicin, the sharp, hot smell of raw crushed garlic, formed within seconds and slowly rearranging into other sulphur compounds. In onions a second enzyme diverts the reaction to a volatile gas, the lachrymatory factor, which reaches the eyes, reacts with their moisture and stings: the reason for tears.

So the more cells broken, the stronger the result: crushed or puréed garlic is far sharper than sliced, and a mashed onion stings more than a cleanly sliced one. Cold slows both the enzyme and the gas's evaporation; a sharp knife breaks fewer cells. Heat destroys the enzyme and turns the sulphur compounds into milder, sweeter ones, while the onion's sugars caramelise (VIII.5): the reason a long-cooked onion is sweet and garlic roasted whole is mellow and spreadable. The workshop cuts an onion.

Below: a teaching index of the tears from cutting an onion at the temperature you choose, relative to a room-temperature onion cut with a dull knife: the enzyme's rate and the gas's volatility both fall with cold, and a sharp knife breaks fewer cells.

What you will learn
Workshop
Teaching index: enzyme rate with an activation energy of 50 kJ/mol (about doubling per 10 °C) times the volatility of the lachrymatory factor with an enthalpy of vaporisation of about 45 kJ/mol (Clausius–Clapeyron), relative to 20 °C; a sharp knife taken as breaking 40 % fewer cells. Shaped on the chemistry, not measured tears.

Cold and sharp for the eyes, heat for sweetness

In this teaching index an onion chilled to 4 °C gives about 0.11 of the tears of a room-temperature one; with a sharp knife, 0.06; a sharp knife alone at room temperature gives 0.60. Chill onions for half an hour before cutting, use a sharp knife and good technique (IV.3), leave the root end (where the enzyme is richest) till last, and work near a draught or an extractor hood.

For garlic, choose the cut for the strength you want: whole cloves mild, sliced moderate, crushed or grated sharp. Letting crushed garlic sit a few minutes before cooking lets the reaction finish; frying it at once in hot oil kills the enzyme early and makes it milder, and burning it makes it bitter (VIII.8), so it goes in after harder ingredients or over moderate heat — in Chinese cooking 爆香 is seconds, not minutes. Onions cooked slowly for 30–60 minutes turn deep, jammy and sweet (VIII.5). Garlic and onion stored in oil must be refrigerated and used soon (XIII.8).

At the stove

Chill one onion for 30 minutes and leave another at room temperature. Chop both the same way with the same knife and note which makes you cry.

The tear-making molecule, syn-propanethial S-oxide, was found to need its own enzyme, lachrymatory-factor synthase, in 2002; Japanese researchers later bred a "tearless" onion in which that enzyme is suppressed.

Terms

Alliinase
The enzyme in alliums released when their cells are broken.
Allicin
The sharp compound of freshly crushed garlic.
Lachrymatory factor
The volatile onion compound that makes eyes water.
爆香
Frying aromatics briefly in hot oil to release their fragrance.
Cutting mixes alliinase with sulphur compounds: allicin in garlic, the tear gas in onions. Cold and a sharp knife cut tears; crushing sharpens garlic; heat turns them sweet.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.