Mozzarella is traditionally a fermented cheese, but not all mozzarella on store shelves goes through fermentation. The difference comes down to how the milk is acidified before the curd can be stretched into that familiar soft, elastic texture. Traditional mozzarella relies on live bacteria to ferment the milk over several hours, while many mass-produced versions skip this step entirely by adding food-grade acids like citric acid directly to the milk.
How Traditional Mozzarella Is Fermented
In traditional mozzarella production, the fermentation step is driven by thermophilic (heat-loving) lactic acid bacteria. The dominant species are Streptococcus thermophilus, Lactobacillus delbrueckii, and Lactobacillus helveticus. These bacteria consume lactose in the milk and convert it into lactic acid, gradually lowering the pH of the curd to somewhere between 5.0 and 5.4. That acidity is essential because the curd can only be stretched at the right pH level.
For authentic Mozzarella di Bufala Campana, the protected Italian version made from water buffalo milk, the process must use a natural whey starter. This is whey saved from a previous batch of cheese, teeming with the same native bacteria. Producers add roughly 2 to 4 liters of this starter per 100 liters of milk, and the bacteria go to work acidifying the curd. After the whey starter is added, Streptococcus thermophilus dominates the bacterial community, followed by Lactobacillus helveticus and Lactobacillus delbrueckii. Mesophilic bacteria that were present in the raw milk become far less abundant as these heat-loving species take over during manufacturing.
The Stretching Step and What It Does to Bacteria
Mozzarella belongs to a family called pasta filata cheeses, which get their stretch from being kneaded in hot water after the curd is acidified. The water temperature during stretching typically ranges from 76°C to 86°C (about 169°F to 187°F), and the internal temperature of the curd itself reaches 58°C to 68°C (136°F to 154°F) depending on the process. This heat is what gives mozzarella its smooth, pliable texture, but it also kills off a significant portion of the bacteria that carried out fermentation.
That said, the heat doesn’t wipe out all bacteria. Research on probiotic strains added to mozzarella found that at a core temperature of about 62.5°C with 10 minutes of stretching, bacterial losses were relatively modest. One study found that a probiotic strain of Lactobacillus rhamnosus retained viable counts well above the threshold considered effective for a probiotic food even after 15 days of refrigerated storage. The fermentation bacteria leave their mark on the cheese’s flavor and texture regardless of whether they survive the stretching in large numbers.
Direct Acidification: Mozzarella Without Fermentation
Much of the mozzarella produced commercially, particularly the low-moisture variety used on pizza, is made through direct acidification rather than bacterial fermentation. Instead of waiting for bacteria to slowly produce lactic acid, manufacturers lower the milk’s pH by adding a diluted acid solution, most commonly citric acid, though acetic acid and lactic acid are also used. The milk is typically brought to a pH of around 5.2 to 5.6, then rennet is added to form the curd. The entire process is faster and more predictable than fermentation because it doesn’t depend on the performance of live bacterial cultures.
The differences between the two methods show up in the finished cheese. Directly acidified mozzarella undergoes less protein breakdown during storage, which means it stays whiter when cooked. It also retains more residual sugars and has a much lower count of lactic acid bacteria. One comparison found that starter-culture mozzarella contained roughly 7 log CFU/g of lactic acid bacteria (about 10 million per gram), while directly acidified mozzarella had only about 3.5 log CFU/g (a few thousand per gram). The flavor profile tends to be milder and less complex in the acid-set version because the bacteria that produce subtle flavor compounds during fermentation were never there.
How to Tell Which Type You’re Buying
If you’re buying fresh mozzarella packed in liquid, especially if it’s imported from Italy or made by a small-scale producer, it was almost certainly fermented with live cultures. Mozzarella di Bufala Campana with PDO certification is required to use natural whey starters, so fermentation is guaranteed. Artisan-style fresh mozzarella from domestic producers also typically uses starter cultures.
Low-moisture mozzarella sold in blocks or pre-shredded bags for pizza and cooking is more likely to have been made by direct acidification, though some brands still use cultures. The ingredient list can offer clues: if you see “cultures” or “starter culture” listed, the cheese went through fermentation. If you only see an acid like citric acid and rennet alongside milk, it was directly acidified. Many products use a combination of both, adding a small amount of culture alongside acid to get some of the flavor benefits of fermentation with the speed and consistency of direct acidification.
Does Mozzarella Count as a Probiotic Food?
Even traditionally fermented mozzarella is not typically considered a probiotic food in the way yogurt or kefir is. The hot-water stretching step significantly reduces the live bacterial population, and the specific strains used in mozzarella production were selected for their acidifying ability, not for probiotic properties. Some live bacteria do survive, but the counts are lower and the strains are different from those in foods marketed as probiotic.
Researchers have experimented with adding recognized probiotic strains to mozzarella and found it’s possible to maintain viable probiotic levels through the stretching process if the temperature and timing are carefully controlled. But this remains a specialty product, not the standard. If your goal is to eat fermented foods for gut health, mozzarella can be part of that picture, but fermented dairy products that aren’t heat-treated after culturing will deliver far more live organisms per serving.