Cheese is made from just a few core ingredients: milk, something to curdle it, salt, and time. The process involves separating milk into solid curds and liquid whey, then pressing, salting, and often aging those curds into the final product. While the basics are simple, the specific choices at each step are what create the hundreds of cheese varieties you see at the store.
It Starts With Milk
Milk is the foundation of all cheese, and its composition determines everything that follows. The two components that matter most are casein (the main protein) and fat. Casein is what actually forms the solid structure of cheese, while fat contributes to flavor, texture, and richness. Cheesemakers often standardize their milk to a specific ratio of casein to fat before they begin, adjusting with skim milk or cream depending on the cheese they’re making.
Cow’s milk is the most common starting point, but goat, sheep, buffalo, and even camel milk all produce cheese with distinct flavors and textures. Sheep’s milk, for instance, has roughly twice the fat content of cow’s milk, which is why cheeses like Pecorino Romano and Manchego taste so rich. The animal’s breed, diet, and even the season all influence the milk’s chemistry and the final cheese.
In the U.S., if a cheese is labeled as made from pasteurized milk, the milk must be heated to at least 161°F for no less than 15 seconds. This kills harmful bacteria. Some cheeses are made from raw (unpasteurized) milk, but federal regulations require those cheeses to be aged at least 60 days before sale to ensure safety.
How Milk Becomes Solid
The central event in cheesemaking is coagulation: turning liquid milk into a wobbly, gel-like mass. There are two main ways to do this, and each produces a different kind of cheese.
Enzyme Coagulation (Rennet)
Most cheeses you’re familiar with, from cheddar to Gruyère, use an enzyme called chymosin (the active ingredient in rennet) to curdle the milk. Here’s what happens: milk proteins called casein float around in tiny clusters called micelles. These micelles normally repel each other because their outer layer carries a negative electrical charge, keeping everything liquid. Chymosin snips a very specific bond on the protective outer protein of each micelle, stripping away the layer that keeps them apart. With that protection gone, the micelles lose their charge, become stickier, and clump together into a solid gel, trapping fat and water inside.
Rennet traditionally comes from the stomachs of young calves, where it naturally helps them digest their mother’s milk. Today, most commercial rennet is produced by microorganisms that have been engineered to make the same enzyme, making the vast majority of cheese suitable for vegetarians.
Acid Coagulation
The other path skips enzymes entirely and uses acid. When milk’s pH drops to about 4.6, casein proteins reach their isoelectric point, meaning they lose their electrical charge and can no longer repel each other. They collapse into a dense, compact gel. This pH drop can come from adding vinegar, lemon juice, or citric acid directly, or from bacteria that slowly produce lactic acid over time. Fresh, soft cheeses like ricotta, paneer, queso fresco, and cream cheese are typically made this way. The curds tend to be more fragile and crumbly than rennet-set curds, which is why acid-set cheeses are almost always eaten fresh rather than aged.
The Role of Bacteria
Starter cultures are specific strains of bacteria added to milk early in the process. Their primary job is converting lactose (milk sugar) into lactic acid, which lowers the pH, develops flavor, and helps preserve the cheese. But their influence goes far beyond that, shaping the taste, texture, and aroma of the finished product in ways that vary dramatically by strain.
Cheesemakers choose between two broad categories. Mesophilic cultures thrive at around 86°F and are used for cheeses made at lower temperatures, like cheddar, Gouda, and Brie. These include species in the Lactococcus and Leuconostoc families. Thermophilic cultures prefer hotter conditions, between 104°F and 113°F, and are essential for cheeses like mozzarella, Parmesan, and Swiss. These typically include Lactobacillus and Streptococcus species.
Some of these bacteria are homofermentative, meaning they produce almost exclusively lactic acid. Others are heterofermentative, generating lactic acid along with carbon dioxide, ethanol, and other byproducts. That carbon dioxide is what creates the characteristic holes (eyes) in Swiss cheese.
Cutting, Cooking, and Draining the Curds
Once the milk has set into a solid gel, the cheesemaker cuts it into smaller pieces. This step, called syneresis, is where whey (the liquid) separates from the curds (the solids). The size of the cut matters: smaller pieces expose more surface area, releasing more whey and producing a drier, harder cheese. Larger pieces retain more moisture, resulting in softer cheese. Stirring speed also plays a role. Stirring too fast can shatter larger curd pieces, causing losses of fat and protein into the whey.
Many cheeses involve a cooking step, where the curds are gently heated while being stirred. Higher temperatures cause the curds to contract and expel even more moisture. A soft Brie might barely be heated at all, while the curds for Parmesan are cooked to around 131°F. After cooking, the whey is drained off, and the curds are pressed into molds to form their final shape.
What Holds Cheese Together
The structural glue inside cheese is colloidal calcium phosphate, a mineral compound that makes up about 7% of the dry weight of casein micelles. It works by cross-linking protein molecules, neutralizing their negative charges and allowing them to bond tightly through hydrophobic (water-repelling) interactions. Without enough calcium phosphate, the protein network falls apart, which is why calcium levels in the milk directly affect how firm or crumbly the final cheese turns out.
This is also why acid matters structurally, not just for flavor. As pH drops, calcium phosphate dissolves out of the protein network. A cheese with a very low pH (high acidity) will have less internal calcium and a more crumbly, brittle texture. A cheese with moderate acidity retains more calcium and holds together in a smooth, sliceable block.
Salt Does More Than Add Flavor
Salt is added after the curds are formed, either by rubbing it on the surface, soaking the shaped cheese in brine, or mixing it directly into the curds. It serves three purposes: it slows bacterial growth (acting as a preservative), it draws out additional moisture, and it enhances flavor. The amount of salt and how it’s applied also influences which microorganisms survive during aging, steering the development of the rind and interior.
How Aging Creates Flavor
A fresh cheese and an aged cheese made from the same milk taste completely different, and that transformation happens through three overlapping biochemical processes during ripening.
Proteolysis is the breakdown of casein proteins by residual enzymes from the rennet, natural enzymes already present in the milk (like plasmin), and enzymes produced by bacteria. This process releases free amino acids, which contribute savory, complex flavors. Amino acids are further converted into volatile flavor compounds. Leucine, for example, breaks down into 3-methyl butanal, a compound associated with malty, fruity notes. Phenylalanine converts to benzaldehyde, which gives an almond-like aroma.
Lipolysis is the breakdown of milk fat by enzymes called lipases and esterases. This releases free fatty acids, esters, lactones, and ketones, all of which contribute sharp, tangy, and sometimes pungent flavors. The characteristic bite of aged Parmesan or the sharpness of blue cheese comes largely from lipolysis.
Glycolysis is the simplest of the three: bacteria continue to ferment any remaining lactose into lactic acid, along with small amounts of ethanol and acetaldehyde. This is why aged cheese contains almost no lactose and is often tolerated by people who are lactose intolerant.
One study cataloging the volatile compounds in a ripening cheese identified acids as the most abundant category (10 distinct compounds), followed by esters (9), alcohols (7), cyclic aromatic compounds (6), ketones (5), and aldehydes (4). The specific balance of these dozens of compounds is what makes a Gruyère smell different from a Camembert, even though both started as milk, bacteria, rennet, and salt.