As a food safety expert with extensive experience in the field, I'd like to address your concern about the safety of consuming pizza that has been sitting out. The question of whether pizza is safe to eat after being left at room temperature is a common one and is crucial for understanding food safety guidelines.

Step 1: Understanding the Risks
The primary concern with any food that has been left at room temperature for an extended period is the potential growth of harmful bacteria. Bacteria are naturally present everywhere, including on food, and they can multiply rapidly under the right conditions. The "danger zone" for bacterial growth is generally considered to be between 40°F (4°C) and 140°F (60°C). Once food is in this temperature range for more than two hours, it can become unsafe to eat.

Step 2: The Two-Hour Rule
The two-hour guideline mentioned by the United States Department of Agriculture (USDA) is a conservative approach to ensure food safety. It's based on the assumption that harmful bacteria can double in number every 20 minutes under favorable conditions. After two hours, there could be enough bacteria present to cause illness.

**Step 3: Factors Affecting Bacterial Growth**
Several factors can influence the rate at which bacteria grow on food, including the type of food, the initial level of contamination, and the environmental conditions. For example, high-acid foods like tomatoes or vinegar-based dressings can slow down bacterial growth, while high-protein foods like meat and cheese can be more conducive to bacterial growth.

Step 4: The Smell Test
While the smell test is a traditional method of determining if food has gone bad, it's not always reliable. Some harmful bacteria produce toxins that may not alter the smell or taste of the food but can still cause foodborne illness.

Step 5: Safe Handling Practices
To ensure the safety of leftover pizza, it's best to refrigerate it within two hours of cooking or purchase. Refrigeration slows down bacterial growth significantly. When you're ready to eat the pizza, reheat it to a temperature of at least 165°F (74°C) to kill any bacteria that may have grown.

Step 6: Exceptions and Considerations
There are some exceptions to the two-hour rule. For instance, during the winter months or in cooler environments, the rate of bacterial growth may be slower, and food might be safe for a longer period. However, it's always better to err on the side of caution.

Conclusion
In conclusion, while it might be tempting to eat pizza that has been sitting out, it's important to follow food safety guidelines to prevent foodborne illness. If the pizza has been at room temperature for more than two hours, it's safer to discard it. Always practice good food hygiene, and when in doubt, throw it out.

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It still smells fine -- is it safe to eat?" Answer: Sorry, but you're out of luck. According to the United States Department of Agriculture, it's dangerous to eat any cooked food (including takeout foods like pizza, fried chicken or Chinese food) that has been allowed to sit at room temperature for two hours or longer.read more >>