Parmesan

Opened parmesan keeps 3–4 weeks refrigerated and about six months unopened — and in practice a well-wrapped wedge often keeps considerably longer, because it is among the driest cheeses made.

Wrap it in cheese paper or wax paper, then loosely in a container, and store it in the crisper. Parmesan's enemy is dehydration rather than bacteria: an unwrapped wedge turns to a brick.

Save the rinds. A parmesan rind is not waste — dropped into a simmering soup, stock, or tomato sauce it releases enormous savory depth. Rinds keep for months in the freezer in a bag.

Grated parmesan behaves quite differently from a wedge. The surface area is vastly greater, so it dries and stales quickly and is contaminated throughout if mold arrives. Grate from the block when you can.

Parmesan freezes for six months and grates straight from frozen, which is one of the more useful freezer tricks in a kitchen.

Parmigiano-Reggiano and its relatives are the most thoroughly preserved cheeses in ordinary use. Aged a minimum of twelve months and often two or three years, they are pressed hard, brined heavily, and reduced to roughly 30% moisture — low enough that bacterial growth is barely possible.

This is why a whole wheel can sit in a cave for years and why a wedge in your fridge is forgiving of mistreatment. Its refrigerator window is limited more by drying out than by spoiling.

The white crystalline crunch in aged parmesan is often mistaken for salt or mold. It is tyrosine, an amino acid that precipitates out as proteins break down during long aging. It is a sign the cheese has been aged properly — the opposite of a defect.

The one-inch rule applies fully here: on a hard block, cut an inch around and below any mold and use the rest. Grated parmesan should be discarded at the first sign of mold.

Be aware that shelf-stable grated parmesan in a shaker is a substantially different product, often containing cellulose and preservatives, and follows its own package dating.

Real problems: pink or black mold, an ammoniac smell, and a slimy surface. A dry, hard, darkened exterior is simply dehydration — trim it.