Pizza

Leftover pizza keeps 3–4 days refrigerated and about 1–2 months frozen. Get it in the fridge within two hours of it arriving or coming out of the oven.

Do not refrigerate the box. Cardboard is a poor conductor and a good insulator, so a boxed pizza cools slowly and the box takes up half your fridge. Stack the slices with parchment or wax paper between them in an airtight container, or wrap them individually.

Freezing works well. Wrap slices individually, then bag them. Reheat from frozen in a hot oven or a dry skillet — a skillet with a lid and a few drops of water gives the best crust of any method, and the microwave gives the worst.

Pizza left out overnight is not safe, whatever the internet consensus. It is a cooked food with meat, cheese, and sauce that has spent eight hours in the bacterial growth range.

Pizza has an undeserved reputation for being safe left out, and the reasoning behind it is worth dismantling because it is a common one.

The intuition is that pizza looks dry, is fairly acidic from the tomato, and is often cured with pepperoni — so it seems preserved. But the cheese is a high-moisture dairy product, the toppings are frequently cooked meat, and the crust is a cooked starch carrying the same Bacillus cereus risk as any other. None of those tolerate eight hours at room temperature.

Refrigerated within two hours, pizza keeps the standard leftover window of 3–4 days. The failure modes are ordinary: Staphylococcus aureus from handling, which produces a heat-stable toxin that reheating will not destroy, and general bacterial growth in the cheese and toppings.

The staling you see over those days is separate and harmless. Bread goes stale through starch retrogradation, a physical rearrangement of the starch molecules rather than any microbial process, which is why day-three pizza is tough rather than dangerous and why reheating with a little steam restores much of the texture.

Discard on visible mold — common on refrigerated pizza after several days — a sour smell, or a slimy surface on the cheese or toppings.