Yes, Swiss cheese is fermented. It actually goes through two distinct rounds of fermentation, which is more than most cheeses. The first converts milk sugars into lactic acid, and the second produces the carbon dioxide gas responsible for those signature holes. This double fermentation also creates Swiss cheese’s nutty, slightly sweet flavor and gives it a surprisingly rich nutritional profile.
How the First Fermentation Works
Swiss cheese production begins the same way most cheeses do. A bacterial culture, primarily Streptococcus thermophilus, is added to warm milk. These bacteria consume lactose (milk sugar) and convert it into lactic acid within the first 24 hours. This acidification thickens the milk, helps form the curd, and creates the foundation for later flavor development.
During this stage, the lactose in the original milk is largely broken down. By the time Swiss cheese has finished aging, very little lactose remains, which is why many people with lactose sensitivity can eat aged Swiss cheese without issues.
The Second Fermentation Creates the Holes
What makes Swiss cheese unique is what happens next. A second type of bacteria, Propionibacterium freudenreichii, takes over after the initial lactic acid fermentation. These bacteria feed on the lactic acid already produced and convert it into propionic acid, acetic acid, and carbon dioxide gas.
The carbon dioxide has nowhere to escape inside the dense cheese wheel, so it forms bubbles that become the characteristic “eyes” you see when you slice into Swiss cheese. Propionic acid is what gives Swiss cheese its distinct nutty, slightly sharp taste, setting it apart from milder cheeses that rely only on lactic acid fermentation.
To encourage this second fermentation, Swiss cheese wheels are moved into a warm room held at about 22 to 23°C (roughly 72°F) for four to eight weeks. This temperature range is ideal for the propionic bacteria to thrive and produce gas. After the warm room stage, the cheese moves to cooler storage where it continues to age and develop flavor.
Baby Swiss vs. Traditional Swiss
Not all Swiss cheese ferments for the same length of time. Baby Swiss is aged for only about a month, while traditional Swiss (sometimes called Emmental, after the Swiss valley where it originated) ages for two months to several years. Baby Swiss is also made in smaller wheels, which limits how much gas can accumulate. The result is smaller, more uniform holes and a milder, creamier flavor compared to the bolder taste of a well-aged traditional Swiss.
Vitamin B12 and Nutritional Benefits
The propionic bacteria in Swiss cheese do something unusual: they actually synthesize vitamin B12 during fermentation. While most cheeses lose some of the B12 originally present in milk (Cheddar retains only about 60 to 70% of milk’s original B12), Swiss cheese ends up with more B12 than the milk it started with. Gram for gram, Swiss cheese contains roughly twice the B12 of Cheddar, about 32 micrograms per gram compared to 15. For a nutrient that many people struggle to get enough of, especially on plant-heavy diets, this makes Swiss cheese one of the richest dairy sources available.
Live Bacteria in Every Slice
One of the more surprising aspects of Swiss cheese fermentation is that the propionic bacteria don’t just do their job and die off. They remain alive and abundant in the finished cheese. Ripened Emmental-style Swiss contains over a billion live Propionibacterium freudenreichii per gram, and these bacteria stay viable even after months of aging. In France, where Emmental consumption averages about 30 grams per day, that translates to roughly 10 billion live bacteria consumed daily per person.
These bacteria are classified as safe for consumption and have been studied for probiotic properties, including effects on gut microbiota and intestinal inflammation. Research has shown that the cheese itself actually enhances the bacteria’s ability to survive the harsh acid and bile salt conditions of digestion. Bacteria grown in a cheese environment tolerate digestive stress significantly better than the same species delivered in dried supplement form. In other words, the cheese acts as a protective vehicle that helps the bacteria reach your gut alive.
Why Flavor Varies Between Brands
Swiss cheese makers can fine-tune flavor by adding specific strains of Lactobacillus bacteria alongside the primary cultures. These adjunct cultures don’t create the holes or the main flavor, but they influence the ripening process and modify the final taste. For example, strains like Lactobacillus casei can dominate the bacterial population within 30 days of aging and alter the concentration of certain flavor compounds. This is one reason why Swiss cheese from different producers can taste noticeably different, even when the basic production method is the same. The choice of adjunct cultures, combined with aging time and temperature, gives each cheesemaker a distinct product.