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.