Pizza doesn’t last long around me! Seriously, though, there are several factors that influence how quickly food “goes bad.”

What actually makes food go to the dark side is that bacteria and fungi (like mold) start to grow on the food. If you put a slice in an airtight container, then heated it up so high that nothing could live in it, it would never rot or get moldy. This is how canned foods work. Unless the can is damaged or develops holes, canned foods are safe for a long time. They may lose some flavor and get all mushy, but nothing will grow on them. (I still don’t recommend eating canned goods older than you are because the plastic or metal in the can might seep into the food after a while. )

This is a good time to mention that when food has already started to go bad, heating won’t make it good again. The bacteria and mold will die if the temperature is high enough, but if they have made toxins (like botulism toxin), those toxins will still be there.

You’re probably not going to can pizza, so let’s look at the factors that make food go bad faster. They’re basically the things that bacteria and fungi need to live. They generally like to live in warm, moist places. Long before refrigeration, people would dry foods like vegetables, meat and fruit to keep them from spoiling. That’s because living things need water to grow. The trick is to get the drying to happen fast enough so that the bad stuff can’t grow much.

So before there were food driers, people would cut things very thin, then hang them in the hot sun, preferably when it was windy. Using smoke protects meat from bad stuff, including flies that might be looking for meat to lay their eggs on. You can also dry things when it’s below freezing. It takes longer, but there won’t be any bugs around, and the bacteria and fungi can’t grow in the cold. Adding a lot of salt or sugar also helps preserve food because the salt or sugar draw water out of cells through a process called osmosis. It’s another type of drying.

Let’s get back to your pizza. The environment is important; cold and dry keeps food safe longer. If you want it to last a long time, put it in the freezer. If you want to eat it later, you’d better put it in something air-tight so that it won’t dry out. It will keep pretty much forever, but will start to taste bad and lose its texture as it dries out. This is called “freezer burn.” This is a good time to mention that when food has already started to go bad, freezing won’t make it good again. The bacteria and mold won’t die, they’ll just go into a sort of suspended animation. When you thaw the food, they get right back to work. If they have made toxins, those will still be there.

Putting food in the refrigerator buys you more time than leaving it on the counter because the metabolism (a fancy name for the chemical reactions) of the bacteria and mold will slow down in the cold. This slows down their growth. Refrigerators also dry food out, which is bad for the bad stuff, but won’t help with flavor or texture. The sooner you get food into the cold, dry environment, the better.

The nasty stuff grows fast, so you want to get it cold fast before the population of the bad stuff gets too big. I saw a site that said the pizza is good for about 3-4 days in the refrigerator, but you can’t leave it on the counter overnight, then expect it to last 4 days in the fridge.

The type of food makes a difference too. Most of the nasty stuff grows best at a neutral pH. Pickling was invented to preserve foods by making their environment acidic. People also recruit good bacteria to make environments acidic. Yogurt and cheese rely on friendly bacteria. Mold and bacteria love a little sugar. Pizza sauce is wet and has some sugar in it, so they’ll probably go after that first. Pineapple is probably another big target. Mold loves cheese. Bread gets moldy fast, but the pizza crust is so dry that it will probably go bad later than the other stuff. Olives and anchovies will take longer because they’re salty. Smoked meats will last longer than hamburger. Another factor is whether the food contains preservatives, which slow down spoilage.

Depends on the environment. If you freeze it, it should be able to last years (although it might not be that good to eat)...

That depends on how you store it. If you freeze it, it will probably last for years. You might want to store it in the refrigerator and eat it within a few days. It won't last as long at room temperature, but you can usually keep eating it for hours when it's at room temperature.

The answer largely depends on the conditions in which the pizza is being stored. Perhaps you left the pizza on the kitchen counter over night. At room temperature, bacteria that cause food-borne illness can contaminate the pizza. These bacteria grow best between 40-140 degrees Fahrenheit, known as the danger zone. According to the US Department of Agriculture, any perishable food (including pizza) is not safe to eat after sitting for more than a few hours at room temperature.

However, maybe you put the pizza in the refrigerator before going to bed. Refrigerators keep food below 40 degrees Fahrenheit, and thus can preserve food for longer by slowing down bacteria growth. Generally, pizza can be kept in the fridge for ~4 days and be ok to eat.

You could even store your pizza in the freezer. In an airtight container in the freezer, below 32 degrees Fahrenheit, a pizza could last 1-2 months. The pizza would probably be safe to eat after even a longer time, but the freshness and quality will decrease with time in the freezer. In principle, if you kept your pizza cold enough, for example cryogenic freezing with liquid nitrogen, the pizza could last indefinitely long.