Milk, curds and whey

Milk is mostly water holding fat, sugar (lactose) and proteins. The main protein, casein, floats in tiny clusters. Cheese making is the art of making those clusters join into a solid curd that traps fat, while the watery whey drains away.

Two ways to set a curd

• Acid set: heat milk and add an acid such as vinegar, lemon juice or citric acid. The casein clumps at once. Paneer, queso blanco and ricotta-style cheeses are made this way — quick and forgiving.
• Rennet set: rennet is an enzyme that makes casein form a firm, smooth gel at warm temperatures. Most cheeses, from mozzarella to cheddar, use rennet.

Cultures

Many cheeses add friendly bacteria — cultures — that slowly turn lactose into acid and build flavor. Mesophilic cultures work at moderate temperatures (cheddar, gouda, chèvre); thermophilic cultures like warmth (parmesan, Swiss, some mozzarella).

Choosing milk

• Whole milk gives the best yield and flavor.
• Avoid ultra-pasteurized (UP or UHT) milk — the high heat damages the proteins so it will not form a firm curd.
• Store-bought pasteurized and homogenized milk often benefits from a little calcium chloride to firm the curd.

Try it: open the free cheese calculator and see how much cheese a gallon of your milk should make.