How Cheese Works

Cheesemaking Basics

All cheese starts with milk. In the United States, most cheese is made with pasteurized milk. Some cheese connoisseurs argue that raw-milk cheese tastes better, and some small dairies produce raw-milk cheese (although to be legal in the United States, the cheese has to be aged for 60 days). But in addition to being considered safer, using pasteurized milk to make cheese is also easier because its behavior is predictable.

Cheese making
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Milk is separated into curds and whey.

Large cheesemakers get their milk in tanker trucks, which have to be spotlessly clean and keep the milk at about 42 degrees Fahrenheit (5.6 degrees Celsius). Small dairies may use milk from their own herds. Once the milk is collected, it is put into a huge container and warmed.

First, the milk must separate into curds (solid) and whey (liquid). To start this process, the lactose, or milk sugar, needs to become lactic acid. After warming the milk, cheesemakers add a starter culture that contains one or more types of bacteria, including Streptococcus thermophilus and Lactobacillus helveticus. These bacteria are also known as lactic acid bacteria (LAB) because they produce lactic acid as they metabolize. The specific mix of bacteria depends on the type of cheese being produced.

cheese curds
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Cheese curds

It's All in the Curdle
Only the most traditional artisan cheesemakers continue to use actual pieces of calf (or lamb, or kid) stomach to make cheese. Many cheesemakers now use a genetically modified rennet. Calf genes are added to a bacterium or fungus, which makes it produce chymosin, the main enzyme in rennet. Some bacteria and fungi can make a rennet-like coagulant, known as microbial rennet, on their own through fermentation. Vegetable "rennet" can also be made from plants like fig leaves and thistle. People who want to avoid cheese made with animal rennet for moral or religious reasons have to read labels carefully to be sure.

Once the acidity level in the milk rises, the casein (one of the proteins in milk; whey is the other) can curdle. This requires the addition of rennet, which is a group of enzymes extracted from the stomach lining of a young cow, sheep or goat. In the stomach, rennet allows the animal to digest its mother's milk. When added to milk, it makes the casein turn into curds.

After settling for up to two hours, the curdled milk has the appearance and texture of custard or pudding. The temperature of the cheese at this point depends on the type of cheese being made. Generally, higher temperatures produce firmer cheeses. Next, the curd is cut using a tool called a harp, which releases the whey. The size of the curds will determine the type of cheese -- soft cheeses come from large curds, while harder ones come from very fine curd. The whey is drained and used as an additive in processed foods and in animal feed.

The next steps in the cheesemaking process depend on the type of cheese. We'll look at the possibilities in the next section.