How is cheese made?

1. Milk is mostly water, but it's full of tiny balls of protein called casein, plus little drops of fat. Fuzzy hairs on the protein balls keep them apart.
2. The cheesemaker warms the milk and adds good bacteria called a culture. They eat the milk sugar and make lactic acid, which makes the milk slightly sour.
3. Next comes rennet. It snips the fuzzy hairs off the protein balls, so they stick together into a web. In about half an hour, the milk sets into a soft, wobbly curd.
4. The curd is cut into cubes. Watery liquid called whey seeps out. Smaller cubes and gentle warming squeeze out more whey, which makes a firmer cheese.
5. For fresh cheese, the curds are just drained and eaten soon. For firm cheese, the curds are salted, packed into a mold and pressed to squeeze out even more whey.
6. Firm cheeses age in cool, damp rooms for months or even years. Tiny helpers slowly change them, building strong flavors and a hard outside called the rind.

Follow the milk through each step. Watch for the moment curds and whey separate.

• Warm the milk and add culture. The milk is warmed until it is a little cooler than your body, and a culture of good bacteria is stirred in. They start making lactic acid.
• Add rennet. A small amount of rennet is stirred in, then the milk is left completely still so the curd can form.
• The curd sets. After about half an hour, the milk has set into a soft gel. The cheesemaker checks for a clean break: the curd splits neatly, with clear whey in the crack.
• Cut the curd. Long knives cut the curd into cubes. Small cubes make hard cheese; big cubes make soft cheese.
• Cook and stir. The curds are stirred and gently warmed. They shrink and get bouncy as more whey comes out.
• Drain the whey. The whey is drained away. It's mostly water with milk sugar, some protein and minerals. Some is even used to make another cheese, ricotta!
• Salt, mold and press. The curds are salted, packed into a mold and pressed. Salt adds flavor and keeps bad microbes away. Pressing joins the curds into one solid cheese.
• Age. The cheese ages in a cool, damp room and is turned often. Over months the flavor grows stronger, the inside gets firmer and a rind forms.

Change how warm the milk is and how long you wait after adding rennet. Watch the protein balls in the magnifier. When the curd is firm, cut it.

Experiment: How does the milk's temperature change how fast curds form?

1. Wait one hour with cold milk. Does it set?
2. Now try warm milk, then very hot milk.
3. When the curd is firm, press Cut the curd.

What we found: Rennet works best when the milk is warm, a bit cooler than your body. Cold milk barely changes, and very hot milk stops the rennet working, so no curd forms.

Turn on JavaScript to try it yourself.

Make three cheeses: fresh and soft, medium and springy, and hard and sharp. Choose the cut, the press and how long to age each one.

Milk looks like a smooth white liquid, but it is full of tiny balls of protein called casein. Each ball has fuzzy hairs on the outside that keep it from sticking to the others, so the milk stays runny.

Cheesemakers add good bacteria and a helper called rennet. The bacteria make the milk a little sour, and the rennet snips the fuzzy hairs off. Now the protein balls stick together into a web that traps the fat and water: a soft gel called curd.

Next the cheesemaker decides what kind of cheese to make. Cutting the curd small, warming it and pressing it hard squeezes out the watery whey, so the cheese ends up firm. Leaving the curds big and pressing gently keeps the whey in, so the cheese is soft.

Then comes waiting. Fresh cheeses are eaten within days and taste mild and milky. Others age for months or years while tiny helpers break down proteins and fats into strong flavors. Same milk, very different cheeses!

Key words

Curd

The solid part when milk proteins clump together; it becomes the cheese.

Whey

The watery liquid left after curds form.

Rennet

A helper (an enzyme) that makes milk set into curd.

Casein

The main protein in milk, found in tiny balls.

Culture

Good bacteria added to milk that turn milk sugar into lactic acid.

Ripening

Aging cheese so its flavor and texture develop.

Check your thinking

1. What two parts does milk separate into when making cheese?

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   Curds and whey. Curds are the solid part that becomes cheese; whey is the watery part.

2. What does rennet do to the protein balls in milk?

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   Snips their fuzzy hairs so they stick together. Without their hairs, the protein balls stick together into a web: the curd.

3. Which makes a harder cheese?

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   Small curds pressed hard. Small curds and pressing squeeze out more whey, so the cheese is drier and firmer.

4. Why do aged cheeses taste stronger than fresh ones?

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   Tiny helpers slowly break down proteins and fats into new flavors. Over months, enzymes and microbes turn proteins and fats into tasty new molecules.

5. Why does very hot milk (about 70 °C) fail to set with rennet?

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   The enzyme is denatured by the heat. Like most enzymes, chymosin loses its shape and activity at high temperature, so κ-casein isn't cleaved.