How long was the pizza left out? So is it still safe to eat? We sliced to the truth at a local science lab

University of North Florida biology professor helps test 3 pizza scenarios to show bacteria growth

Is pizza still safe to eat if it was left out for hours? Scott Johnson slices to the truth at a Jacksonville biology lab.

JACKSONVILLE, Fla. – Now that the big game is over you might be wondering what to do with any leftover pizza you might have. Some refrigerate it immediately, but others will keep it out for several hours before putting it in the refrigerator.

So how does that affect the safety of eating your slices?

We took our questions on this issue to the University of North Florida’s biology department, where professor Dr. Dale Casamatta helped us put some pies to the test.

We brought three pepperoni pizzas that were just baked. (WJXT)

We brought three pepperoni pizzas that were just baked. The first one we had immediately refrigerated. The second one we had handled with multiple people touching the pizza and then refrigerated after two hours. And the third one we also handled with hands and then refrigerated after four hours.

We then put the pizzas that were left out in a sterile solution so we could test them 48 hours later for any contamination.

While the test didn’t cover every situation, it did give us some guidance on whether pizzas left out can generate much bacteria.

We found high levels of contamination in the pizza that had been left out for four hours before refrigeration. (WJXT)

“The nice thing about pizza is pizza is a very xeric environment, so it’s very dry,” Casamatta said as we began our experiment.

What we found was the pizza left out for four hours had considerably more contamination than the one that had been immediately refrigerated.

The pizza that was left out for two hours had a different type of growth we didn’t expect.

Casamatta said that was likely because of how that specific pizza was handled.

“Whoever touched this one just has some bacteria on their fingers,” which shows that it’s a good practice to not touch any slices of pizza until they’re ready to be eaten, Casamatta said. “It is horrifying to know how many organisms we have on our skin.”

Dr. Dale Casamatta, a biology professor at UNF, helped us put several pizzas to the test. (WJXT)

Because pizza is so dry, Casamatta said, it can last several hours and still be generally safe to eat and really it comes down to a judgment call.

Although he did point out that “the bacterial counts were above the critical threshold” according to FDA standards on the four-hour pizza we tested, so technically that means that would not be safe to eat.

“That said, I have eaten pizza that lurked about for several days and survived,” Casamatta said with a smile.

But there are foods you should avoid leaving out and should refrigerate quickly. Foods with cream or eggs as ingredients and most soups. Those have higher liquid content and can spoil much more quickly.

Dr. Dale Casamatta, a biology professor at UNF, helped us put several pizzas to the test. (WJXT)

Casamatta also said with respect to pizza an unscientific visual or smell test can be useful.

“If you ever saw a growth or smelled something that’s kind of askew, always let your nose be your guide in these kinds of situations,” he said.

We also want to point out that our experiment only tested pepperoni pizza. If you have another type with higher moisture contents like fruit or vegetable toppings, it’s advisable to refrigerate those sooner.

And if you are worried about bacteria but still feel comfortable eating the pizza, you can always reheat the pizza in the oven or throw it in the microwave. It can help kill some bacteria.

About the Author:

Specializes in Clay County issues, general assignment reporting and stories off the beaten path and anchors weekend evening newscasts.