This animated video shows how the molecular structure of milk changes at various stages of cheese making.
Milk in the liquid phase contains suspended fat globules and casein proteins. Lactose, whey proteins and some minerals are dissolved in the liquid.
Casein is present as molecular clusters called micelles. It is believed that hair-like strands protruding from kappa casein on the micelles’ surface are hydrophilic. They repel other casein micelles, keeping them suspended in the milk.
When you add rennet, the chymosin enzyme binds to the micelles, releasing the hair-like strands. This alters the charge balance on the micelles’ surface from hydrophilic to hydrophobic.
The micelles then bond together, forming a gel and trapping liquid whey and fat globules.
Cutting and stirring breaks the gel into small curds, releasing some of the whey.
Heat also shrinks the curds, removing more whey.
When the curd reaches the desired texture, the whey is drained off.
Pressing the curds removes more whey, forming a solid cheese.