While mold is the most obvious indication that mozzarella cheese is old, there are other recognizable indications that the cheese is no longer good to eat. Discard the cheese if you aren't sure. For healthy people, foodborne illness may be limited to unpleasant gastrointestinal symptoms. However, some types of bacteria are dangerous, especially for pregnant women, infants, the elderly and people with weak immune systems. Tightly packaged, properly refrigerated, mozzarella cheese lasts as long as six months, but once opened, storage time drops to about one month.
Check the date on the package. The use-by date is not an indication of safety and properly stored mozzarella often keeps beyond the date. However, it tells you how long the cheese is expected to maintain optimum quality and freshness.
Smell the mozzarella and discard the cheese if it smells bad. Although many cheeses have a strong odor, mozzarella has a light, mild aroma. If mozzarella has an off smell, or if it smells like sour milk, it's a sign that the cheese has gone bad.
Taste the cheese, then discard it if it tastes bad. Tasting a tiny amount of old mozzarella will be unpleasant, but it isn't likely to make you sick. If mozzarella cheese tastes fine, it is safe to eat.