Vol. XX · Vegetables, Fruit and Legumes Plate 9 of 12tofu, 豆腐, soy milk, 豆浆, 石膏, gypsum, 卤水, nigari, 内酯, GDL, silken tofu, firm tofu, 北豆腐, 南豆腐, pressing tofu, 麻婆豆腐, yieldMT

Tofu and Soy.

The same litre of soy milk makes a kilogram of silken tofu or under a quarter of a kilogram of firm. The difference is water.

Tofu begins as soy milk: soaked soybeans ground with water, boiled and strained (the solids left are 豆渣, okara). Boiling is essential, both to destroy the beans' trypsin inhibitors and beany-tasting enzymes and to unfold their proteins so they can set. A coagulant is then stirred into the hot milk. Divalent salts — calcium sulphate (石膏, gypsum) or magnesium chloride (卤水, nigari) — bridge the proteins, as calcium bridges pectin (XX.2) and casein (XIV.6), and curds separate from whey. Ladled into cloth-lined boxes and pressed, the curds become tofu: lightly pressed for soft 南豆腐, hard for firm 北豆腐 and pressed tofu (豆干).

Glucono-delta-lactone (GDL, 内酯) works differently: it turns slowly into gluconic acid, so it acidifies the whole milk evenly and sets it in the container without curds or pressing, like a yoghurt (XIV.5): silken 内酯豆腐, all the milk's water kept. Gypsum gives smooth, mild tofu; nigari gives a firmer, slightly sweet-bitter one. The workshop presses tofu.

Below: the tofu made from one litre of soy milk of the protein content you choose, for silken, soft and firm styles (a mass balance with USDA compositions: the protein that stays in the curd sets the weight), and the water each contains.

What you will learn
Workshop
Mass balance: soy milk of 1.03 kg per litre; silken tofu keeps the whole milk; soft and firm curd keep 85 % of the milk's protein (a teaching value; the rest leaves with the whey) and take the protein fraction of USDA FoodData Central regular and firm tofu (8.1 % and 17.3 %). Silken tofu's water taken as 100 % minus 2.4 times the milk's protein.

Press for firmness, set whole for silk

From a litre of soy milk with 3.5 % protein, this balance gives about 1 030 g of silken tofu, 379 g of soft and 177 g of firm: firm tofu has had most of its water pressed out, so it holds its shape in a stir-fry and takes on a chewy, meaty texture, while soft tofu is tender and silken tofu is a custard. Richer soy milk makes more and firmer tofu, which is why tofu makers control the bean-to-water ratio closely.

Cooking. Firm and pressed tofu: press out more water under a weight for 20 minutes, cut, and fry, braise or stir-fry; it browns well and soaks up sauce. Soft tofu for 麻婆豆腐: cut into cubes and blanch for a minute in lightly salted simmering water, which firms the surface and seasons it so the cubes survive the wok. Silken tofu: serve cold with soy and scallion, steam it, or blend it into sauces and desserts. Frozen and thawed firm tofu turns spongy as ice crystals open its structure (XIII.3) — prized in hot pots for how it drinks broth. Tofu is a perishable, high-moisture food: keep it covered in cold water in the fridge and use it within a few days.

At the stove

Make tofu: blend 150 g soaked soybeans with 1 litre of water, strain, boil the milk 10 minutes, take it off the heat to about 80 °C, stir in a teaspoon of gypsum dissolved in a little water, wait 15 minutes, ladle the curds into a cloth-lined sieve and press.

Tofu is traditionally credited to 刘安, the Han-dynasty Prince of Huainan, in the second century BC, though the first clear written records are from the Song dynasty; it reached Japan by the eighth century with Buddhist monks.

Terms

石膏 (gypsum)
Calcium sulphate, a coagulant giving smooth tofu.
卤水 (nigari)
Magnesium chloride from sea water, a coagulant giving firm tofu.
内酯 (GDL)
Glucono-delta-lactone, which slowly acidifies soy milk into silken tofu.
豆渣 (okara)
The pulp left after straining soy milk.
Tofu is soy milk curdled by calcium or magnesium salts and pressed, or set whole by a slow acid into silken tofu. Pressing sets texture and yield; firm tofu is mostly the same protein with less water.

Sources

  1. USDA FoodData Central, SR Legacy (April 2018), public domain.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  3. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.