Slice science part 1: How long is it safe to keep pizza out?

Experts say to refrigerate cooked pizza within 2 hours -- but who does that?

Is there other life in the universe? What happens to us when we die? How was Stonehenge constructed?

Unfortunately, I can’t answer any of these questions ... So I’m going to take a stab at something more tangible: How long can pizza be left out and still safely be eaten?

This may seem like a trivial question, but I assure you it is not. It is a question that affects millions of people, including yours truly.

Even though I am a doctor and should be eating healthy, I have to admit, I love pizza -- and I have always played fast and loose with the rules governing its storage.

I pride myself on giving practical advice to my patients and viewers. Advice that I would follow.

When you look for an official answer to the question of how long pizza can be left out, the actual number is two hours. This number in several official publications, as well as articles written by respected journalists. To me that sounds crazy and unrealistic.

Maybe I am an outlier, but in my world, a cooked pepperoni pizza can be left out of the refrigerator for almost a solid day and still be eaten without thinking twice. As you know, I am a physician, and I don’t want to give any advice that would put anyone’s health at risk, so here is my caveat: Don’t take this as medical advice. I am simply telling you what I do.

The question is, is that safe?

I can honestly say that in more than 30 years of practicing emergency medicine, I have never seen a patient say that they became sick from eating old pizza. Lots of other foods have been blamed for intestinal complaints, but pizza has not been on the offender list in my experience.

Clearly, that alone does not make it safe, as my experience is not all encompassing. I am sure that some of you reading this may have had a very different experience, and if you have, I hope you recovered quickly. That said, I am a realist and I do not know anybody personally who consistently refrigerates their pizza within two hours of receiving it or having cooked it.

“These food safety recommendations from the USDA are based on validated studies, and also what we know about how different food borne pathogens grow at certain temperatures,” said food safety expert Dr. Theresa Bergholz.

Dr. Bergholz studies food safety at Michigan State University. I was connected to her through the recent academic affiliation between Henry Ford Health and MSU.

She agreed that there are reasons to believe the two-hour recommendation might be overly conservative when it comes to pizza, especially pepperoni pizza.

“If you’re following proper baking instructions, we know that should kill any microbes that are present on the pizza as you’re baking it,” Bergholz said.

Baking the pizza is what is known as a kill step, essentially sterilizing it in the oven. Bergholz also points out that toppings with a high salt content and low moisture content will either slow or eliminate microbial growth.

After discussing the matter, she agreed that whether pepperoni pizza could safely be left out for more than two hours was a legitimate question. Better still, she agreed that it was worth studying with a simple experiment.

“Honestly, I think it’s something that people do and it’s probably reasonable,” Bergholz said. “I’ve done it myself.”

So, together, we devised an experiment to test microbial growth on cooked pizza and determine just how long we can leave it out and safely eat it. We’ll explain how the experiment worked, and just how close our predictions were, when the next article releases after 5 p.m. on Thursday, Nov. 10.

Stay tuned!

About the Author:

Dr. McGeorge can be seen on Local 4 News helping Metro Detroiters with health concerns when he isn't helping save lives in the emergency room at Henry Ford Hospital.