Pizza contains numerous ingredients that can harbor dangerous bacteria, including meat, cheese and vegetables such as tomatoes. While cooking kills many bacteria, those that survive can multiply if you leave pizza at room temperature. Contamination also can occur after the cooking process from the hands of the person who packs or delivers your pizza. To keep delivery pizza from spoiling, follow the Department of Agriculture guidelines for food storage.
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Keep It Warm
If you're going to be eating pizza within a short time, keep it warm in the oven rather than leaving it out on the counter. Bacteria multiply most rapidly at temperatures between 40 and 140 degrees Fahrenheit, according to the National Institute of Diabetes and Digestive and Kidney Diseases. Pizza left out on the counter will cool to less than 140 degrees fairly quickly. Since most people don't live in houses cooled to 40 degrees, the pizza will remain within the perfect temperature for bacterial growth.
Cool It Down
Once you've eaten all you're going to eat, put the rest of the pizza in the refrigerator or freezer within two hours or sooner. Delivery pizza may have started cooling on the way to your house unless the company uses insulated bags, or it may have been sitting out in the store before delivery. Put pizza away as soon as you're finished eating. Set your refrigerator to 40 degrees or lower and your freezer to zero degrees to keep food safe. At zero, bacteria will not grow. When you thaw the pizza, however, bacteria will begin to grow again.
If you reheat pizza, reheat to at least 165 degrees. The best way to check the internal temperature of a food is to use a cooking thermometer. Reheating the pizza kills any bacteria it may have picked up before or after transport to your house. Warming your pizza in the microwave may not heat all parts of the pizza to 165 degrees, warns Lynn Paul, food and nutrition specialist at Montana State University Extension office, so make sure to check the temperature with a thermometer.
When to Throw It Out
Don't keep leftover pizza in the refrigerator longer than three to four days. When you put the leftover slices in the fridge, don't pile all the pieces up on top of one another, since the ones in the middle may not cool properly. Use a shallow dish to lay slices out rather than stacking five or six pieces on top of one another. If you keep pizza in the freezer, it's safe for one to two months and probably longer, according to Real Simple, but it may start to taste funny after that time. If you lose power, food in a separate full freezer remains safe for about two days, while a half-full freezer keeps food safe for around one day as long as you don't open the freezer door, according to the USDA. Food in the freezer section of the refrigerator won't last as long.