🔄 Curdling

Curdling turns liquid milk into a firm curd that cheese is made from. Most often, milk is curdled with rennet. Its main part is chymosin, an enzyme from the stomachs of calves. It “snips off” a small piece of the protein kappa-casein. Without this piece, the protein particles in milk stop pushing each other away and stick together into a mesh along with the fat. The result is a gel that whey seeps out of.

In practice, milk is warmed or cooled to about 30–33 °C and the enzyme is added. After 5–7 minutes the milk must be completely still, and after about half an hour a curd forms. It is cut into grains from 1 to 15 mm. The smaller the grains, the drier and harder the cheese will be. Then the whey is drained off, and the grains are shaped and pressed.

There is also acid curdling. Lemon juice, vinegar or sour whey is added to hot milk. This is how paneer, chhena and some fresh cheeses are made. Rennet used to come only from calves' stomachs. Since 1990, chymosin made with the help of microbes has been on sale. By 2017, it made up about 90% of the world market. There are also substitutes from fungi and plants.