According to the USDA (United States Department of Agriculture), perishable foods, including pizza, shouldn't be left at room temperature (between 40°F and 140°F) for more than two hours. This is because bacteria that can cause foodborne illness can grow rapidly in this temperature range.
Here are some additional factors to consider:
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Temperature: If the ambient temperature is above 90°F (32°C), the safe holding time is reduced to one hour.
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Spoilage: Even if the pizza is not yet unsafe to eat within the two-hour window, it might start to show signs of spoilage, such as off-odors, mold growth, or a slimy texture. If you notice any of these signs, it's best to discard the pizza.
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Refrigeration: After two hours, you should either refrigerate the leftover pizza or freeze it for longer storage. Properly stored, leftover pizza can be kept in the refrigerator for 3-4 days or frozen for up to 2 months.
Remember: When in doubt, throw it out! It's always better to err on the side of caution and avoid the risk of foodborne illness.