After the milk is collected, the cheesemaking process begins by adding starter cultures that change lactose (milk sugar) into lactic acid. This process changes the acidity level of the milk and starts the process of thickening the milk into a solid. Next, rennet is added to encourage even more thickening so that curds form and separate from the whey (liquid).
Rennet is derived from one of four sources:
- The stomach lining of a calf, ewe or kid (baby goat). The enzyme rennin is found in the stomach lining of animals because it aids in the digestion of their mother's milk. This type of rennet is usually called “traditional rennet” on cheese labels.
- Plants, typically thistle
- Microbes in fungus and yeast. This type of rennet is called "microbial rennet" on cheese labels
- Genetically engineered rennet that imitates animal rennet
Rennet does not directly affect the flavor of cheese, with one exception: thistle rennet. Cheese made from thistle rennet has a distinctly vegetal flavor. Many types of cheese from Portugal are made using thistle rennet. Some examples are Torta del Casar, Azeitao, Serra da Estrela and Serena.
Most vegetarians want to avoid traditional rennet derived from animals and prefer to eat cheese made from microbial rennet or rennet derived from plants. There are many types of cheese that are made with vegetarian rennet. Ask at your local cheese shop or check out this partial list of cheeses made with vegetarian rennet.
Not just hard cheeses have rennet in them. Most types of soft cheese and fresh cheeses are made using rennet as well. There are some exceptions, however, such as ricotta and quark, which usually do not contain rennet. You can also make several types of cheese at home without using rennet: