How long can you leave pizza out? We scientifically tested it. Here are the results

It’s the pizza test you’ve been craving!

When you have a pizza, how often do you refrigerate it within two hours? That’s the timeframe recommended by many official food safety guidelines when it comes to handling pizza.

However, according to a food safety expert at Michigan State, that might be overly conservative. So we decided to put it to the test using a simple experiment. The food safety expert is Dr. Theresa Bergholz, I was fortunate to find her thanks to the recently formed academic affiliation between Henry Ford Health, and Michigan State University.

I should point out from the start, while every effort was made to make our simple experiment as valid as possible, this was not meant to be the final word on this topic. People should use their own judgment and sensibilities when it comes to the safe food handling.

🍕 The pizza test

We started out by baking three identical pepperoni pizzas. To simulate real world conditions, after removing them from the oven and allowing a brief period of cooling, the pizzas were cut, and then entered our experiment.

Pizza A, our control pizza, was placed directly into the refrigerator after it was cut. Pizza B and C were served and handled normally by a group of college students. After those pizzas were handled, Pizza B went into the refrigerator (within two hours of baking), and pizza C was simply covered and left out on the counter.

From the standpoint of the experiment it was important that pizzas B and C were handled normally since this is when contamination was most likely to occur.

🍕 The cheesy results

Six hours from the time they came out of the oven all three pizzas were cultured. Culturing food is a more complicated process than I expected. First, a 25 gram quantity of the pizza was put into a sterile bag with a sterile buffer. Next, that bag goes into a device that’s actually called a stomacher. The stomacher essentially mushes the sample into a semi-liquid suspension that can be spread onto culture plates. The liquid suspension was plated out onto different culture plates, including a culture medium specifically used to detect staphylococcus aureus. This was done because staph aureus is a common cause of food borne illness related to handling. The culturing was repeated at 12 and 24 hours.

After incubating an appropriate time, all of the culture plates were evaluated and we returned to Dr. Bergholz’s lab two days later for the results.

Pizza A, the pizza that went directly into the refrigerator, had 30 microbial cells per gram after 6 hours. “It’s a very low level, you could consider it to be almost sterile.” Said Bergholz.

Pizza B, the pizza that was handled but refrigerated within 2 hours, had 140 microbial cells per gram after 6 hours. That’s more than the control pizza and really highlights how much handling adds to potential contamination. Still Dr. Bergholz said it was, “still a very low level.”

Finally, Pizza C, the pizza that was handled and left out of the refrigerator, had 2,600 microbial cells per gram after 6 hours. This is the pizza we were most interested in since it simulates the real world circumstance many of us replicate. Clearly the combination of handling and leaving it unrefrigerated caused more microbial growth. However, the actual number of 2,600 microbes per gram needs to be put into some perspective. According to Dr. Bergholz, while this was an order of magnitude higher than the refrigerated pizza, it was still, “a fairly low level of microbes.” She went on to explain, “things like fresh produce and fresh fruits can have anywhere from 1000 to 10,000 microbes per gram on them.” Concluding that ,”what we see here is that even if we had it sit out for 6 hours that still seems to be relatively low risk.” The microbes growing on the culture plates all appeared to be ordinary harmless contaminants. There was no potentially harmful staph aureus found.

The cultures done at 12 and 24 hours had a “significant number of molds kind of taking over,” according to Bergholz. “Finding yeast and mold on your food is really not harmful, we know that there are mold spores circulating in the air, that was probably the most likely source.” She said.

🍕 What does it all mean?

Big picture, nothing we do is risk free and everyone needs to keep that in mind. For many, the idea of eating pizza with 2,600 microbes per gram on it may seem gross. For me it’s perfectly reasonable, given the perspective that many things we eat have a similar or higher level of microbial contamination.

There are some important caveats to our experiment.

First, as I said at the top, if you have any doubt throw it out. Also, the result really only applies to pepperoni or cheese pizza. Other toppings, especially if they have a higher moisture content, might be more prone to bacterial contamination.

The experiment really highlights the effect of handling on contamination. Ideally, if you are like me and tend to leave pizza unrefrigerated for long stretches, you should avoid touching the uneaten slices. The more they are handled, the more likely you are to introduce bacteria to the surface where it will multiply at room temperature. While we didn’t detect any harmful bacteria on our pizzas after 6 hours, you should still do everything you can to minimize contamination.

Related: Jason Carr: These are the 3 best pizza spots in Metro Detroit

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.