7.3 Proteins set: coagulation

The change often starts at about 38°C (100°F) and is finished by 71–82°C (160–180°F). In the bakeshop, three groups of protein do this work: egg proteins, dairy and soy proteins, and the proteins in flour.

Eggs

An egg holds many proteins. The white, called the albumen, has about 40 of them. The largest share is ovalbumin, at 54%, then ovotransferrin, at 12%. The yolk is mostly fat (lipids), with some lipoproteins.

Each protein sets at its own temperature. That is how you can cook an egg with a firm white and a soft yolk:

Milk and soy

Cheese is built on casein, the semi-solid part of milk that forms when milk coagulates. To set milk for cheese, cheesemakers use rennet. Rennet is taken from the lining of the stomach in cattle, sheep and goats, and there is also a plant-based kind. The enzyme that does the work is chymosin, also called rennin. It curdles the milk, which then splits into solid curds and liquid whey.

Acid also sets milk:

• lemon juice (citric acid) or vinegar, for fresh ricotta
  ▶ Watch
• tartaric acid, for mascarpone
• lactic acid, which builds up as milk sours, so sour milk curdles by itself
• bacteria that make acid, added on purpose to make yogurt or crème fraîche (a thick, slightly sour cream)
  ▶ Watch

Tofu is the soy version. Soybean milk is set with salt, with acid, or with an enzyme.

Flour

Wheat flour carries two main proteins, glutenin and gliadin. Other grains have smaller amounts. Mix them with liquid and they join into gluten, which stretches. In the oven, heat firms the gluten, and it hardens into the solid frame of the bread. That firming is coagulation too.