Some store-bought mozzarella cheese has a distinctively sour-to-pungent smell, almost chemical, which many associates with vinegar.
It doesn’t necessarily mean the cheese is spoiled or gone bad. This is likely due to the natural sourings added to the cheese to help coagulate the cheese by neutralizing the negative charges on the protein surface. It will make the cheese melt and stretch better.
Some of the common acids are citric acids, vinegar, lemon, and lime juice. They serve as an acidity regulator to adjust the right pH environment but will also create a sour and tangy flavor in mozzarella cheese. Not all store-bought mozzarella cheeses contain these souring agents and are labelled as such. For example, Galbani mozzarella is using citric acid; Kraft fresh mozzarella is using vinegar. And they will smell differently depending on the kind of acid used.
You can first check the ingredient label on the packaging, if that’s not the taste you are expecting for your cheese, try to find one with other souring agents. A little bit of sourness or acidic taste is fine in fresh mozzarella.
Bad mozzarella will also smell sour but stronger (not light aroma) and closer to ammonia-like — that’s to say, if you did not notice clear signs of spoilage in your mozzarella, as we discussed here.
Another reason is when the cheese ages, byproducts of fermentation form and create an acidic scent naturally which smells somewhat vinegary. But it’s less likely to happen since mozzarella is usually a fresh cheese with no aging needed, if at all. Some cultured ‘aged’ mozzarella will use lactic acid bacteria (LAB) culture, just like the normal aged cheeses, and lightly ripe the mozzarella for 2-3 weeks to produce a much more flavorful mozzarella. Streptococcus thermophilus is a common strain of LAB bacteria used in culturing mozzarella.