Vol. VIII · Browning Plate 5 of 9onions, caramelised onions, browning, evaporation, water, stirring, sweating, soffritto, 炸葱油HT

Browning Onions.

Recipes say ten minutes. Cooks know it takes most of an hour. The difference is about 350 grams of water.

A kilogram of onions holds nearly 900 g of water. Deeply browned onions keep under a third of their weight, so most of that water has to be boiled away, and every gram of it costs 2.26 kJ (VIII.2). While it is going, the onion is pinned near 100 °C and browns only slowly. Once the pieces are mostly dry they climb past 110 °C and their sugars and amino acids brown by the Maillard reaction (VIII.4), helped by their fructose and glucose.

So the time is set by the burner's power, not the cook's wishes. More heat boils the water off faster but scorches the bottom layer, which dries before the rest; stirring, a lid at the start, and a splash of water to lift what sticks keep it even. The workshop works out the time for an amount of onion and a setting of a domestic gas burner.

Below: minutes spent boiling off water and minutes browning for 500 g of onions at four burner settings and yours, and the time for the setting and amount you choose (a teaching energy budget).

What you will learn
Workshop
Teaching model: onions 89 % water, cooked until 80 % of it has gone (about 29 % of the raw weight left); a 3 kW gas burner of which 40 % reaches the pan and 70 % of that goes into the onions; then browning to deep brown at a surface that rises from 100 °C to 130 °C with the setting, at the onion's pH with the course Maillard rate and a factor of ten for its free sugars.

The water comes first

Five hundred grams of onion must lose about 356 g of water on the way to deep brown, leaving 144 g. At half heat on a domestic gas burner that takes 32 min of boiling and then 27 min of browning: 59 min in all. At full heat the boiling still takes 16 min and the whole job 22 min — and only with constant stirring, because the bottom layer dries and burns long before the top. No setting gets 500 g of onions to deep brown in ten minutes.

What a recipe usually means by ten minutes is softened and lightly golden — fine for many dishes, but not the jammy brown of French onion soup or the dark 炸葱油 of Shanghai scallion oil noodles, which is fried in oil at a far higher temperature. For deep brown onions: use a wide pan so the steam leaves, start covered for ten minutes to soften, then uncover and stir every few minutes; when brown bits stick, add a spoonful of water and scrape (the fond, VIII.7). A pinch of baking soda shortens the browning but softens the onions to a purée.

At the stove

Weigh 500 g of sliced onion, cook it to deep brown and weigh it again. The difference is the water you boiled off; divide by the minutes it took to see how fast your burner really works.

Indian cooks brown onions in plenty of oil (birista) for biryani and kormas: frying in oil at 150 °C or more gets past the water and into browning much sooner than a dry pan, and the crisp brown onions keep for weeks.

Terms

Sweating
Cooking onions gently, often covered, until soft but not coloured.
Caramelised onions
Onions cooked slowly to deep brown, mostly by Maillard browning.
Birista
Onions fried crisp and brown in oil, used in Indian cooking.
炸葱油
Scallions fried slowly in oil until dark brown, for Shanghai scallion oil.
Onions must boil off most of their water before they brown; the time is set by the burner's power, so deep brown takes most of an hour.

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.