Every other Friday, the Outside/In team answers a listener question about science and the natural world. This week's question comes from Anthony.
“I have a long running debate with my wife. We have 18-year-old twin boys and sometimes we leave pizza out on the counter. She's convinced it's bad for your health. I don't think so! Is pizza left overnight on the counter bad for you?
Host Nate Hegyi looked into it.
Transcript
This has been lightly edited for clarity.
Nate Hegyi: One night when I was about 12 years old, we went to my Auntie Jen’s house for dinner. She ordered pizza … and the next morning, some of the leftovers were still out, on the counter.
Being a kid, I went to grab a breakfast slice, and my dad told me… very sternly… No! If I ate it, I’d be sick. Ever since, I never eat pizza left out overnight. Turns out, my dad was probably wrong.
Don Schaffner: Generally pizza that has been left out overnight on the counter is fine to eat the next day.
Nate Hegyi: This is Don Schaffner. He’s a food microbiologist at Rutgers University, where he ran a study analyzing two decades worth of data from pizzas left out at the university’s dining hall.
He looked at bacterial growth, and found that a pizza could sit at room temperature without posing any problems for eight hours. And that’s being conservative! To understand why, we have to break down a pizza to its individual components.First, there’s the crust.
Don Schaffner: So the crust is essentially bread. And of course we leave bread out at room temperature right.
Nate Hegyi: Then there is the tomato sauce, which is somewhat acidic. That’s important here because acid slows the growth of bacteria.
Don Schaffner: That acidic tomato sauce helps to give it some preservative power.
Nate Hegyi: Next step? Cheese. It may not have as low a pH as tomato sauce, but because of the fermentation process cheese is also slightly acidic. And, especially when it comes to popular pizza cheeses like mozzarella, it’s a food with what Don calls “a relatively low water activity.”
Don Schaffner: “Water activity” is a measure of the water in the food that's available to support the growth of microorganisms
Nate Hegyi: Basically, the lower the water activity, the less growth of bacteria.
And, like any chef knows, all pizzas get thrown in an oven! This dries out the cheese even more, but it also kills any dangerous bacteria that may still have been hanging on.
Finally there are the toppings.
Don Schaffner: Meats that are on pizza are going to be already cooked. In the case of pepperoni, it's cured. And, most pepperonis are actually already shelf stable.
Nate Hegyi: But things might get a little dicey if you shake it up, say swap out pepperoni for hamburger. Some spore forming bacteria can survive the cooking process on beef, and multiply if a pizza’s left at room temperature for very long.
Don’s biggest concern was actually any veggie toppings. That’s because veggies have a higher moisture content and many aren’t very acidic. But again, the cooking process would kill any bacteria. And, unlike beef…
Don Schaffner: There's probably not a lot of spore forming pathogens on fresh vegetables.
Nate Hegyi: Now technically the USDA recommends throwing out any perishable food that’s been left at room temperature for more than two hours. But Don says that’s a broad recommendation that doesn’t take into account the individual characteristics of foods like pizza.
Don Schaffner: Am I saying that if you had a perfect storm scenario, you could not give someone food poisoning? No. But I would say, generally speaking, based on the characteristics of pizza and the way pizza is prepared, I'm going to say it’s not risky to leave it on the counter overnight.
Nate Hegyi: So, Dad, I could have eaten auntie Jen’s pizza. Take that.
If you’d like to submit a question to the Outside/In team, you can record it as a voice memo on your smartphone and send it to outsidein@nhpr.org. You can also leave a message on our hotline, 1-844-GO-OTTER.
