Vol. VI · Proteins and Heat Plate 2 of 11coagulation, gel, protein network, syneresis, custard, scrambled eggs, overcookingMF

Coagulation.

A set egg is a net of proteins holding water. Heat it further and the net pulls tight, wringing the water out.

Unfolded proteins expose sticky parts that were hidden inside the fold, and they bond to each other. If the proteins were dilute, as in a custard or an egg white, the result is a three-dimensional net that traps the water around it: a gel. A little heat makes a loose, delicate net; more heat adds more bonds and pulls it tighter.

Too tight, and the net squeezes out the water it was holding — syneresis. Overcooked scrambled eggs sit in a puddle, an overcooked custard weeps and turns grainy, an overcooked omelette is rubbery, and overcooked meat is dry for the same reason (VI.5). The workshop heats an egg custard and shows how firm it is and how much water it has expelled.

Below: the firmness of a baked egg custard and the water it squeezes out against the highest temperature it reached (a teaching model built on the egg-protein kinetics).

What you will learn
Workshop
Teaching model: firmness grows with the set fraction of the egg proteins (course kinetics, 10 minutes at each temperature); water expelled grows steeply once the custard passes about 85 °C, where the network tightens.

The window between set and weeping

A sweetened custard in this model sets enough to hold a slice once it passes about 67 °C, and begins to weep and curdle above about 87 °C; without sugar the window moves down to 63–83 °C. That gap of a dozen degrees is what a water bath protects (XVII.5): the custard cannot rise far above the bath, while dry oven heat would overshoot at the edges.

The same physics runs through every protein food. Scrambled eggs taken off the heat while still a little loose finish in their own warmth and stay creamy (XVII.4). Tofu pressed too hard or boiled too long turns spongy. Meat squeezes out juice as its collagen and actin tighten above 60–70 °C (VI.5). In each case the cook's job is to stop inside the window.

At the stove

Bake four small ramekins of the same custard in a water bath and take one out each time the centre reaches 75, 80, 85 and 90 °C on a probe. Cool and cut them: the first trembles, the second is silky, the third firm, the fourth grainy with liquid at the bottom.

Chinese steamed egg (蒸水蛋) relies on the same window: cooks lay a plate or film over the bowl and steam at a gentle simmer, because a full rolling boil makes it honeycombed and watery.

Terms

Coagulation
Unfolded proteins bonding into a network.
Gel
A network of molecules that holds a liquid in its spaces.
Syneresis
A gel contracting and squeezing out liquid.
Curdling
Proteins clumping into visible lumps, separating from liquid.
Set proteins form a net that holds water; overheated, the net tightens and squeezes it out. Stop inside the window.

Sources

  1. Egg proteins: ovotransferrin sets near 62 °C, ovalbumin near 80 °C, the yolk thickens near 65 °C and sets near 70 °C (H. McGee, On Food and Cooking, 2004, ch. 2; P. Barham, The Science of Cooking, 2001).
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.