Rennet is an enzyme that causes proteins to coagulate. It is available in two forms:

• Animal rennet comes from the stomach of a calf or lamb fed on its mother’s milk.
• Vegetarian rennet, also known as microbial rennet, contains no animal protein.

Curd is the firmer, thicker form of milk obtained after adding rennet. It contains most of the proteins present in milk, as well as fat.

By noting how long the rennet takes to start coagulation, we can estimate the time needed for the milk to set: approximately 2.5 times as long. Here are two ways to determine whether coagulation has begun:

• Place a sterile, clean toothpick on the surface of the milk. When coagulation starts, the toothpick stops moving.
• Drop a little water onto the milk. If it rests on the surface or makes a small indentation, coagulation has begun.