Mozzarella

Opened mozzarella keeps about one week refrigerated, and the two kinds on sale behave quite differently.

Fresh mozzarella — the soft white balls packed in liquid — should stay in that liquid, fully submerged, in a sealed container. The whey or light brine protects it from air and keeps it from drying. Fresh mozzarella out of its liquid dries and sours within a couple of days. If you use half a ball, return the rest to the liquid rather than wrapping it.

Low-moisture mozzarella — the firm block or shredded bag for pizza — is a different and much hardier product, closer to a semi-hard cheese. Wrap the block in wax paper and keep it in the crisper. It keeps considerably longer than fresh.

Low-moisture mozzarella freezes acceptably for a few months and is one of the better cheeses for it, since it is usually melted anyway. Fresh mozzarella does not freeze — it turns rubbery and weeps.

The two mozzarellas illustrate the moisture rule better than any other pair of cheeses sold under one name.

Fresh mozzarella is a pasta filata cheese, stretched in hot water and sold within days. It has had almost no water removed, is barely salted, and is not aged at all. Its water content can exceed 60%, which puts it among the most perishable cheeses there is. It is also close to neutral in pH. That combination means it spoils quickly and that any mold means discarding the whole thing.

Low-moisture mozzarella has had much of that water driven off and more salt added, giving it a denser body that resists mold penetration. On a firm block, the standard hard-cheese rule applies: cut an inch around and below a mold spot. Shredded mozzarella is a different matter — the enormous surface area means contamination is effectively everywhere, so discard the bag.

Shredded cheese also carries added cellulose or potato starch as an anti-caking agent, which absorbs moisture and slightly extends its life.

For fresh mozzarella, look for liquid that has turned cloudy, thick, or slimy, a yellowing surface, a sour rather than milky smell, and a slick film on the ball. Any of these is a discard.