Curds, Whey, and Rennet

Once milk has the necessary level of lactic acids, and spores have been added, it is ready to separate into curds and whey through the use of rennet.

Curds and Whey

The basic properties of milk are water, protein, fat, sugar, vitamins, and minerals. Curds and whey are the result of separating the water and sugar, or whey, from the protein and fat, or curd. The separation happens by introducing an enzyme, commonly known as rennet, which acts on the milk protein, or casein molecules. These molecules are naturally attracted to water, but when rennet is introduced they lose their hold on water and instead accumulate around fat. The result is a gelatinous, coagulated curd, surrounded my milky water or whey, all of which will eventually become cheese.

Whey is rich in water, sugar, vitamins, and minerals. Some cheese makers use it to make ricotta, others use it to bathe the curd while it's being worked, and still others use it as feed for animals. Pigs are especially fond of whey, and pig farmers often buy whey from cheese makers.


As mentioned previously, rennet is a term used by cheese makers to describe the enzyme responsible for separating curds and whey and causing the curd to coagulate. In fact, cheese makers have several different rennets to choose from. Rennet can be the same as the the enzyme chymosin, which occurs naturally in the stomachs of animal ruminants, or can be a derivative of vegetable enzymes, or can be an enzyme bioengineered from chymosin. Lemon juice and vinegar can also be used as rennet.

Chymosin or Animal Rennet

It's most likely cheese was discovered in ancient times when people stored fresh milk in pouches created from animal stomachs. This milk would have encountered the enzyme chymosin and curdled within a matter of minutes or hours, depending on the strength of the rennet and temperature of the day. The same people must have also realized that milk stored in other containers soured before it curdled, and must have come to the conclusion that something within the stomach lining was responsible for curdling milk in a way that created cheese.

These ancient people must have also figured out that not just any stomach lining would do to make cheese, and that in fact, the fourth stomach of an unweaned calf, kid, or lamb produced the best results. The enzyme chymosin was not identified until modern times, but regardless, once other containers were available to make cheese, cheese makers knew they still had to dry and scrape stomach linings to produce a powder that curdled milk. Today, animal rennet can also be obtained by extraction from animals.

Bioengineered Rennet

In 1989, the U.S. FDA granted “generally recognized as safe” (GRAS) status to genetically engineered rennet, which is created by inserting a calf's prochymosin gene into a microorganism. The results are difficult to distinguish from pure animal rennet, and bioengineered rennet can be used to successfully make both soft and hard cheeses.

Vegetable Rennets

Vegetable rennets are derived from extractions of plants such as stinging nettle and thistle, and from fungus fermentation. These rennets are successful in producing soft and semisoft cheeses, and are commonly used to produce vegetarian cheeses.

Lemon Juice and Vinegar

Lemon juice and vinegar can be used to create curds for cheeses that do not need to age such as queso fresco, fromage blanc, and cottage cheese (also referred to as “rennetless cheeses”). This technique has also been around for thousands of years.

Why are cows called ruminants?

Ruminants are animals that chew their food, and then after it's been swallowed, it is essentially chewed again by the action and enzymes of several stomachs. To accomplish this feat they have three to four stomach chambers. While their food passes through each chamber, it is digested and chewed. Cows, goats, and sheep are all ruminants, as are deer, camels, water buffalo, and a host of other plant-eating animals.

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