Vol. VI · Proteins and Heat Plate 9 of 11milk proteins, casein, whey, curdling, paneer, ricotta, milk skin, acidMF

Casein and Whey.

Milk is stable through boiling, until you add acid. Then its casein clumps — slowly when cold, almost at once when hot.

About four-fifths of milk's protein is casein, gathered in microscopic clusters (micelles) that stay suspended because they carry a negative charge and a hairy outer layer. Remarkably, heat alone hardly disturbs them: milk can boil without curdling. Acid does: as the pH falls towards 4.6 the charge disappears and the micelles clump into curds. Rennet, an enzyme, clips the hairy layer and does the same thing at milk's own pH — the start of most cheeses (XIV.6).

The other fifth is whey protein, which behaves like egg white: it sets with heat above about 70 °C. That is the skin on hot milk and the curds of ricotta, made by heating the whey left from cheese. The workshop adds lemon juice to milk at the temperature you choose and shows when it curdles — the way paneer is made.

Below: the pH of a litre of milk as lemon juice is added (a teaching buffering model) and the pH at which it curdles at your temperature (lower when cold, higher when hot).

What you will learn
Workshop
Teaching model: milk starts at pH 6.7 and falls about 0.04 per mL of lemon juice per litre; it curdles at pH 4.6 when cold, and at a higher pH the hotter it is (about +0.012 per °C above 20 °C).

Acid, heat and rennet

Hot milk at 90 °C curdles in this model once its pH falls to about 5.44, which takes about 32 mL of lemon juice per litre; cold milk needs the pH down near 4.6, about 57 mL. That is why paneer and Latin American queso fresco are made by adding acid to milk just off the boil: the curds form at once and can be strained and pressed. It is also why a sauce with milk and wine or tomato can curdle when it boils, and why cream, with less protein for its fat, survives where milk splits (XVII.8).

Whey proteins need heat, not acid: the skin that forms on hot milk is whey protein (and fat) set at the surface where the milk dries a little; stirring or covering prevents it. Ricotta — "re-cooked" — is made by heating the whey left after cheese making until its proteins set into fine curds. Yogurt combines both: bacteria lower the pH slowly, so the casein forms a soft, continuous gel instead of curds (XIV.5).

At the stove

Bring a litre of whole milk just to the boil, take it off the heat and stir in two tablespoons of lemon juice. Within a minute the milk separates into white curds and greenish whey. Strain through a cloth, rinse, and press under a weight for an hour: paneer.

Paneer, chhena and the curd cheeses of South Asia are acid-set; most European cheeses are rennet-set; Chinese 奶豆腐 and Mongolian dried curds come from soured milk — three routes to the same casein clump.

Terms

Casein
The main milk protein, in clusters that curdle with acid or rennet.
Whey protein
Milk protein that stays in solution after curdling and sets with heat.
Micelle
A tiny cluster of casein molecules suspended in milk.
Rennet
An enzyme that clips casein micelles so they clump, used in cheese making.
Casein curdles with acid or rennet (sooner when hot); whey protein sets with heat.

Sources

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