🔄 Curdling

Coagulation is the conversion of liquid milk into a firm curd from which cheese is made. The Codex Alimentarius standard defines cheese in just these terms: milk protein is coagulated with rennet or another suitable coagulating agent, and the whey is partly drained off. There are two main methods — enzymatic and acid — and they are often combined.

Rennet is a mixture of enzymes, the chief one being chymosin. It 'snips' a small piece off the protein kappa-casein, after which the casein micelles stop repelling each other and stick together into a network that also traps the fat. The result is a gel from which whey separates. At a cheese dairy, milk is usually brought to about 30 °C, the enzyme is added, and a curd forms in about 30 minutes. It is cut into grains of 1 to 15 mm: the finer the grain, the drier the cheese. The whey is then drained, and the grains are moulded and pressed.

In acid coagulation, casein precipitates when the pH falls to about 4.6 — through added acid or through lactic acid bacteria. This is how lemon juice or vinegar added to hot milk produces paneer, chhena and other fresh cheeses. FAO notes that the method also determines the whey: after rennet cheeses it is sweet, after acid cheeses it is sour.

Curdling is not the same as milk simply going off: in cheesemaking it is a controlled process, and there are two kinds. Rennet works for about 30 minutes at around 30 °C and gives the elastic curd that most cheeses are made from, while acid (lemon juice, vinegar or lactic acid bacteria) precipitates casein at a pH of about 4.6 and gives fresh cheeses such as paneer. According to FAO, the method also decides the whey: sweet after rennet cheeses, sour after acid ones. Cutting the curd into grains of 1 to 15 mm then sets how dry the cheese will be.

On a cheese label, the ingredients may list “rennet”, “microbial rennet” or “lactic cultures”, which hints at how the milk was curdled. Fresh acid cheeses such as paneer usually sit in the chiller cabinet in sealed packs, and hard cheeses are cut from a block at the cheese counter. You will find them in supermarkets, delis and farmers' markets. The pack seldom names the curdling method, so the ingredients list is your main clue.

Rennet was once obtained only from calves' stomachs. Today chymosin produced by microorganisms is widely used, along with substitutes from fungi and plants. The speed of coagulation and the size of the grains determine what the cheese will be like — moist and tender or dry and hard.