We all know that leaving leftover pizza on the kitchen table overnight and eating it the next morning is probably not the safest thing to do, so it might be true that the opposite is true – putting hot leftover pizza in the fridge straight away… right?
While this sounds good in theory, tossing a steaming, freshly cooked meal straight into the fridge raises new questions: Does it impair the food's ability to cool properly? And won't any leftovers already in the fridge end up warm and inedible? SELF reached out to a few food safety experts to find the answers.
There are some solid reasons to keep leftovers cold.
Before we explain the safest way to refrigerate meals, let's take a step back and talk about why it's important. Properly chilling leftovers is “a very important element of food safety,” says food safety researcher Nicole Richard, MSc. The University of Rhode Island and the chair of the RI Food Safety Commission told SELF.
Doing so limits the time your dishes spend in the “danger zone” where pathogens (bugs that can cause disease) thrive — that unstable temperature range between 40 and 140 degrees F. When it comes to refrigerating food, the main threat comes from two specific strains of bacteria: Clostridium perfringens and Bacillus cereus, Donald Schaffner, PhD, chair of food science at Rutgers University, tells SELF.
These bacteria are different from notorious food-poisoning bacteria such as Salmonella and E. coli, which are killed by sufficient cooking. Clostridium perfringens and Bacillus cereus, on the other hand, form heat-resistant spores and can survive at high temperatures, so they can also be present in properly prepared meals, Dr. Schaffner said. According to Dr. Schaffner, these bacteria can multiply and cause illness if food is left in the danger zone for too long. As a result, severe food poisoning symptoms such as vomiting and diarrhea occur.
Some foods are more risky than others, Dr. Schaffner said. These include meat and poultry (associated with Clostridium perfringens) and grain products such as rice and pasta (associated with Clostridium cereus). So if your leftovers include steak, spaghetti, paella, etc., it's even more important to chill them properly.
Does that mean it's best to put leftovers in the fridge as soon as possible?
Not necessarily. While proper cooling is essential to prevent foodborne illness, you can also put leftovers in the fridge for a little longer, but not too long: Food should be fine if left out for up to two hours, says Dr. Schaffner.
In fact, he says it's probably best practice not to freeze leftovers right away. First, it doesn't actually speed up the process in any meaningful way. According to Dr. Schaffner, hot food actually cools on the counter almost as quickly as it cools in the refrigerator. This is because the difference between the temperature of the food and the temperature of the room is “not that big.”
