How Is Cheese Made?

The process is remarkably consistent across styles. Milk is acidified with bacteria, then set with an enzyme so it forms a gel. That gel is cut, the liquid whey drains away, and what remains is salted, shaped and often aged.

Everything that distinguishes one cheese from another is a choice within those steps: which milk, which bacteria, how finely the curd is cut, how much moisture is removed, how long it ages and what is encouraged to grow on it.

Milk contains casein proteins that normally float apart. Adding rennet, an enzyme traditionally from calf stomachs and now often produced by fermentation, clips off part of the protein so the casein molecules link into a network that traps fat and water.

Cutting that gel releases whey. The size of the cut is critical: small pieces lose more moisture and give hard cheeses like parmesan, while large gentle cuts retain moisture and give soft cheeses like brie. Heating and stirring drive out more whey again.

Starter cultures convert lactose, the sugar in milk, into lactic acid. This lowers the pH, which helps the curd set properly and makes the cheese hostile to spoilage organisms.

Different bacteria then shape the character. Some produce carbon dioxide, which is where the holes in Swiss style cheese come from. Others break down proteins and fats slowly during ageing, producing the compounds responsible for sharp, nutty or savoury flavours.

A few variables account for most of the world's cheese styles:

• •Milk source, since sheep, goat and cow milk differ in fat and protein
• •Moisture, which separates fresh, soft, semi hard and hard styles
• •Surface treatment, such as washing the rind or introducing mould
• •Ageing time, from days for mozzarella to years for aged gouda

Blue cheeses are inoculated with a mould, usually Penicillium roqueforti, and then pierced with needles so air can reach the interior. The mould needs oxygen to grow, which is why the blue veins follow the channels.

Brie and camembert use a different mould applied to the surface, forming the white rind. It works inwards over time, breaking down the interior into that soft, oozing texture. In both cases the mould is deliberately chosen and controlled, which is why it is safe to eat while random fridge mould is not.

Lactose is the sugar in milk, and it is water soluble, so most of it leaves with the whey during draining. What remains is then consumed by the bacteria during ageing.

This is why hard aged cheeses such as parmesan, cheddar and gouda contain almost no lactose and are usually tolerated well, while fresh cheeses like ricotta and cottage cheese retain considerably more. It is a good illustration of how a preservation process incidentally changes the nutritional profile of a food.

Not every cheese uses rennet. Some are set by acid and heat alone. Paneer, common in South Asian cooking, is made by heating milk and adding an acid such as lemon juice or vinegar, which makes the casein clump into curds that are pressed into a block. Because the protein network forms in a different way, paneer and similar cheeses soften when heated but do not melt and stretch, which is why they hold their shape in a curry or on a grill.

Ricotta shows another route. Its name means recooked in Italian, because it is traditionally made from the whey left over after making other cheeses. Whey still contains proteins that rennet does not capture. Heating the whey, often with a little acid, makes these proteins clump into soft, fine curds that are scooped out and drained. This turns a by product into food, and explains why ricotta has such a light, grainy texture compared with rennet set cheeses.

Cheese is older than writing, so the evidence comes from chemistry. In Poland, archaeologists found pieces of pottery pierced with many small holes, about 7,000 years old. Some suspected they were cheese strainers, used to separate curds from whey. Researchers then analysed fat residues absorbed into the clay and found the chemical signature of milk fats, strongly supporting that idea. Much later evidence comes from burials in the Tarim Basin in western China, where lumps of cheese around 3,600 years old were found preserved with the dead.

These finds connect to human biology. Most early farmers in Europe could not digest much lactose as adults, because the enzyme that breaks it down usually switched off after childhood. Fresh milk would have caused discomfort, but cheese and other fermented dairy foods contain far less lactose. Turning milk into cheese therefore let early farming communities use milk safely and store it for leaner months. The genetic change that keeps the lactase enzyme working into adulthood became common in some populations only later, which suggests cheese-making came before most people could comfortably drink milk.

How a cheese melts depends on the casein network inside it, which is held together partly by calcium. Mozzarella is made by the pasta filata method. The curd is left to grow more acidic until enough calcium has been drawn out of the network for it to soften in hot water. Cheesemakers then knead and stretch the hot curd, lining the proteins up into long fibres, which is why good mozzarella pulls into strings. Timing is critical. Curd stretched too early breaks instead of stretching, while curd left too long turns soft and pasty.

An aged cheese heated too quickly behaves differently. Its protein network tightens, squeezes out fat and clumps into greasy strands, which is the classic failure of a home cheese sauce. Processed cheese, developed in the early twentieth century, solves this with emulsifying salts such as sodium citrate. These bind calcium, loosening the casein so that it coats fat droplets and keeps them evenly spread, which is why a processed slice melts smoothly. At home, tossing grated cheese in a little cornflour and stirring it into a warm, not boiling, sauce a handful at a time helps in a similar way.

Cheddar takes its name from a village in Somerset, but cheddaring now describes a specific step in making it. After the whey is drained, the curd knits into a mat that is cut into slabs. These are stacked, turned and restacked for an hour or more, so their own weight presses out whey while the starter bacteria keep producing acid. The curd gradually becomes dense and fibrous, a texture often compared to cooked chicken breast. The slabs are then milled into chips and salted before pressing, which gives cheddar its close, firm body rather than the open texture of cheeses pressed straight from loose curd.

Salt does far more than season. It draws further moisture out of the curd, slows the starter bacteria so acid stops building, and helps decide which microbes can survive on the surface. Cheesemakers apply it in different ways. Cheddar is dry salted as chips, many cheeses are rubbed with salt on the outside, and wheels of gouda and parmesan are floated in brine baths, parmesan for weeks. Feta is stored in brine, which partly explains its salty bite. Because so much water leaves as whey, a hard cheese typically needs roughly ten litres of milk for each kilogram of finished cheese.

Cheese is made by acidifying milk, setting it with rennet, cutting and draining the curd, then salting and ageing it, and every difference between cheeses comes down to choices made within those same four steps.