Yes, cheese is a fermented food. Nearly every variety, from cheddar to camembert, relies on bacteria converting milk sugar (lactose) into lactic acid. This process thickens the milk, preserves it, and creates the tangy flavors we associate with cheese. A few fresh cheeses skip bacterial fermentation entirely, but the vast majority depend on it.
How Milk Becomes Cheese
Cheesemaking begins when starter cultures of lactic acid bacteria are added to milk. These bacteria feed on lactose and convert at least 85% of it into lactic acid. The acid lowers the milk’s pH, causing proteins to clump together into curds while the liquid whey separates out. This is fermentation in the same sense that yeast ferments grape juice into wine: microorganisms consume sugars and produce new compounds that change the food’s flavor, texture, and shelf life.
There are two broad categories of these bacteria. Homofermentative strains focus almost exclusively on producing lactic acid. Heterofermentative strains also produce acetic acid, ethanol, and carbon dioxide, which contribute subtle fizzy, sharp, or fruity notes depending on the cheese style. The cheesemaker’s choice of bacterial strains is one of the earliest decisions that shapes how the final cheese will taste.
What Happens During Aging
For aged cheeses, fermentation doesn’t stop once the curds form. It continues for weeks, months, or even years during a phase called ripening. This is where cheese develops most of its complex flavor. Enzymes from the bacteria (and from rennet, the coagulant) break down the two main components of cheese: proteins and fats.
Protein breakdown, or proteolysis, is most active in the first month. Large milk proteins called caseins are chopped into smaller fragments, eventually releasing individual amino acids. Some of those amino acids undergo further chemical reactions. Phenylalanine, for example, can transform into benzaldehyde, which gives certain cheeses a bitter almond note. Lactose metabolism produces acetaldehyde, responsible for the sharp, fruity character in some varieties.
Fat breakdown runs on a parallel track. Enzymes called lipases split milk fat into short-chain fatty acids like butanoic and hexanoic acid, which carry the pungent, savory aromas typical of aged cheese. These fatty acids don’t just accumulate. They get converted into ketones, alcohols, aldehydes, and esters. Ester levels, for instance, are nearly zero in fresh curd but climb steadily throughout ripening, adding fruity and floral layers to the flavor profile. The result is a constantly shifting balance: new compounds form while others transform into something else.
Blue Cheese and Mold-Ripened Varieties
Some cheeses undergo an additional layer of fermentation driven by mold rather than bacteria. Blue cheeses like Roquefort, Gorgonzola, Stilton, and Cabrales rely on the mold Penicillium roqueforti, which has been used as a ripening agent for centuries. The mold colonizes the interior of the cheese through small air channels and produces its own proteolytic and lipolytic enzymes, breaking down proteins and fats more aggressively than bacteria alone. This is what gives blue cheese its intense, spicy flavor and distinctive veined appearance.
Surface-ripened cheeses like Brie and Camembert use a different species, Penicillium camemberti, which grows as a white rind on the outside and works inward. The mold raises the pH near the surface, softening the paste from the outside in. That’s why a ripe Brie is runny just beneath its rind but firmer in the center.
Cheeses That Skip Fermentation
A small group of fresh cheeses can be made without bacterial fermentation at all. Cottage cheese, cream cheese, queso blanco, and quarg are sometimes produced through direct acidification, where an acid like vinegar or lemon juice is added to hot milk to curdle it. These are called acid-set or heat/acid-coagulated cheeses. They reach a pH below 5.0, which is acidic enough to form curds, but the acid comes from outside rather than from bacterial activity.
In practice, many commercial versions of these cheeses still use starter cultures for flavor consistency, so even “fresh” cheeses are often lightly fermented. Paneer, the Indian cheese, is the clearest example of a truly non-fermented cheese: it’s made by adding lemon juice or vinegar to boiling milk, straining the curds, and pressing them. No bacteria involved.
Why Fermentation Makes Cheese Easier to Digest
One practical consequence of fermentation is that aged cheeses contain far less lactose than the milk they started with. Bacteria consume lactose as fuel during both initial fermentation and ripening, so the longer a cheese ages, the less lactose remains. Swiss cheese, Brie, and Camembert contain less than 1 gram of lactose per 100 grams. Cheddar, Gouda, and Parmesan typically range from 0 to 3 grams per 100 grams. For comparison, a glass of whole milk contains roughly 12 grams of lactose per cup. Many people with lactose intolerance can eat well-aged cheeses without digestive trouble precisely because the bacteria have already done the work of breaking down the sugar.
Live Bacteria in Finished Cheese
Fermentation also means that many cheeses still contain living bacteria when you eat them. Research on cheddar cheese aged for 270 days found that probiotic lactobacilli added during production not only survived but increased 10- to 100-fold over the aging period. Even bacteria that weren’t intentionally added, called nonstarter lactobacilli, thrived during ripening. The dense, low-moisture, acidic environment of cheese turns out to be a surprisingly hospitable place for beneficial bacteria to persist, which is why aged cheese is sometimes discussed alongside yogurt and kefir as a source of live cultures.
Pasteurized processed cheeses are the exception. The heating and emulsification steps kill off most microorganisms, so a slice of American cheese doesn’t carry the same live bacterial population as a wedge of aged Gruyère.