Vol. XVII · Eggs and Dairy Plate 7 of 10milk, casein, whey proteins, lactose, skin, boiling over, scalding, scorching, homogenisation, heating milk, 牛奶MT

Milk.

Milk boils over where water would not: its proteins stabilise the foam. The skin on hot milk is the same proteins drying at the surface.

Cow's milk is about 87 % water, with 3–4 % fat in small globules (X.1), about 2.6 % casein in clusters called micelles, 0.6 % whey proteins dissolved in the water, and 4.8 % lactose, the milk sugar. Casein is remarkably heat-stable — milk can be boiled without curdling — but it is sensitive to acid (XIV.5) and to rennet (XIV.6). The whey proteins are the reverse: indifferent to acid but unfolded by heat above about 70–75 °C.

That explains milk on the stove. Unfolded whey proteins and casein collect at the hot surface as water evaporates and form a skin; stirring or a lid prevents it. The same proteins stabilise bubbles, so boiling milk foams up and over in a moment where water would just boil. And on a hot pan bottom, proteins and lactose stick, brown (VIII.1) and scorch, giving a burnt taste to the whole pot — so milk is heated gently, stirred, in a heavy pan. The workshop heats milk.

Below: the share of whey protein unfolded in milk held five minutes at the temperature you choose (a teaching curve centred near 78 °C), with the ranges for warm, scalded and boiling milk and what each brings: skin, cooked flavour, boil-over.

What you will learn
Workshop
Composition from USDA FoodData Central (whole milk). Whey unfolding: a teaching logistic curve centred at 78 °C with a width of 3 °C after five minutes, shaped on published β-lactoglobulin denaturation; casein is far more heat-stable.

Gentle heat, a heavy pan, a whisk

In this teaching curve about 6 % of the whey protein has unfolded after five minutes at 70 °C and 91 % at 85 °C. Above that, milk tastes "cooked" (sulphur compounds released by the unfolded proteins), forms a skin readily and foams easily. Recipes that say "scald the milk" — heat it to just below the boil — originally aimed to kill microbes and enzymes in raw milk; with pasteurised milk it is mostly about speeding up infusions and helping some doughs, where unfolded whey proteins would otherwise weaken gluten.

Practical rules. Heat milk in a heavy pan over moderate heat, stirring, or in the microwave in short bursts; take it off as soon as bubbles form at the edge; a lid or a whisk stops a skin forming. Do not leave it at a full boil: a pot of milk can rise and overflow in seconds. If it scorches, pour it into a clean pan at once without scraping the bottom. In Chinese and Indian kitchens, milk is boiled down deliberately for desserts (双皮奶 uses the skin itself; Indian khoa is milk reduced to a paste).

At the stove

Heat two cups of milk side by side to just below the boil, one covered and one uncovered. After cooling for five minutes, compare the surfaces: a skin forms only on the uncovered one.

双皮奶, "double-skin milk", from Shunde in Guangdong, is made by boiling milk, pouring it into a bowl so a skin forms, pouring the milk out from under the skin, mixing it with egg white and sugar, pouring it back under the first skin and steaming it so a second skin forms.

Terms

Casein
Milk's main protein, in heat-stable clusters (micelles).
Whey proteins
Milk proteins dissolved in its water, unfolded by heat above about 70 °C.
Scalding
Heating milk to just below the boil.
Lactose
Milk sugar; it browns and scorches on hot metal.
Milk is water, fat, heat-stable casein, heat-sensitive whey proteins and lactose. Heated, it forms a skin, boils over and scorches: heat it gently and stir.

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.