Although I believe the question appeared fairly straightforward, and was answered appropriately as asked, I think the underlying question is really, “If I leave a pizza sitting out for 4 hours and then eat it will I get sick?”
Although microwaving pizza or other food will kill most bacteria that’s contaminating it, one of the major causes of food poisoning is not that bacteria is present, but that their waste products or other secretions are present, i.e., toxins. So even if you kill the bacteria you can still have problems.
Toxins aren’t always denatured by heating, so it’s often best to discard food that’s been sitting out for a while.
On t’other hand, I think that’s not a question that could be reliably answered. It depends on WHICH bacteria (or other toxins) are floating around in your kitchen, n’est-ce pas?
Meanwhile, welcome to the Straight Dope Message Boards, Threbus, we’re glad to have you with us!
Notwithstanding the caveats mentioned above, excellent experiment! I smell a course in microbiology here… the method looked like a more readable version of one of my undergraduate pracs.
Four points:
The amount of time the food is allowed to sit between about 20 and 40 degrees C is important - pathogenic enteric bacteria love this range. (Pizza may be safer than average on this count because of its inherently high surface area and thus fast cooling.)
Water content is important. This is why peanut butter can last a year, but algae soup on a warm counter, not so much. Pizza is probably lucky here as well.
Death curves follow an inverse power law, so you will always chase that last cell. There’s always a compromise to be found (perhaps most pressingly in the milk business.)
Microwaves heat water directly, including the water inside bacterial cells, so microwaves are probably quite effective at killing bacteria even without needing to heat the surrounding food.
The only caveat I have on this experiment is that the petri dishes should have been incubated at body temperature.
Fifty five years ago, I was working in a lab in which we worked regularly with E. coli (harmless strains). Out o curiosity we left a peti dish uncovered in a lab for two hours and then covered it and incubated it at 37deg C for a couple days. There was precisely one colony on it, presumably, though not necessarily E. coli. Although air contamination obviously happens it is not that common.
One more caveat, however is that not every bacterium that is harmful in a human body will necessarily grow on an agar medium. It might require a human body environment.
I think user Threbus has it right. But I think it is a combination of the actual bacteria along with whatever they do. Consider the term food POISONING. Poison. Right? Where do you get something that will poison food? Bacteria. You can kill the bacteria but either their dead bodies or their poop can and do contain toxins. The danger lies in the fact that at room temperature they multiply rapidly.
There may also be bacteria that make you sick with a parasitic reaction, but generally I think it is a toxin sort of deal.
Who microwaves pizza? Yuck! Will a non-minimal number of microbial life forms survive heating a slice of pizza in an oven for 15 minutes at 350°F? What about a toaster oven, where the heating is presumably less even and the temperature presumably less exact?
Everyone knows you don’t eat pizza if it has been sitting out uncovered. But I would be interested to see the amount of bacteria found on a pizza that was left in the closed delivery box, unrefrigerated, overnight. I must admit to enjoying this Saturday morning snack on more than one occasion without any negative effects. I’m sure the same can be said for many readers.
In my stupid youth, I decided one day to clean my toothbrush and then had a great idea - put it in the microwave to get rid of any bacteria!
So, I put my toothbrush in the microwave and set it to two minutes.
Voila! My brush did a perfect 360 and I had the cutest little toilet brush you ever saw, although I guess it was at least bacteria free.
Summary:
Bacteria gone, but probably a bad idea. Don’t do it.
Tune in next week to learn why it is a bad idea to throw all of your neckties in a washing machine - another youthful plan gone awry. And no, I am not a science major, why do you ask?
According to the United States Pharmacopia, General Chapter <61>, "MICROBIOLOGICAL EXAMINATION OF NONSTERILE PRODUCTS: MICROBIAL ENUMERATION TESTS ", the temperature you are supposed to incubate at depends on the organism you’re looking for and the agar you’re using. The most common temperature is 30 to 35 C, but for yeasts and molds 20 to 25 C is specified. None are as high as 37 C.
I was impressed with the thoroughness and rigor of the experiment. Without the use of positive controls, I would hesitate to say the reported results can be taken as quantitative. A recovery study would like need to be done. However, I think they more than adaquately support the conclusion on the question of whether microwave ovens will kill microorganisms recovered from pizza.
The idea behind how plate counting is supposed to work is that each separate, distinct colony found on the plate grew up from a single microorganism (the so-called colony forming unit, CFU). So, you want to incubate your plates under the conditions most favorable to the organisms of interest, to make those buggers grow fast and big so you can clearly see them to count them. But not so fast that they start running into each other making it difficult to count how many distinct cfu’s there are. Sometimes there are so many colonies that that is unavoidable, which is why they did the dilutions. When your 1:1 dilution plate has too many colonies to count, you move to your 1:10 and count. If that is too numerous to count, you move to the 1:100 plate and so on. When you get to one where you can count them individually, you take that number, multiplied by the dilution factor of the plate and that is how many cfu per mL (if you swabbed with 1 mL) you have.
One control I would like to point out for future reference is the delivery medium. Having worked in pizza delivery, not all delivery bags are created equal, and which bag being used could have affected the control. Also, the time from when the pizza was cooked (~600 degrees F) to the time it arrives at your door is also suspect. Bags are usually cleaned on a regular basis (roughly once per year or so,) but they aren’t sterilized.
Imho, to fully control these factors, I would recommend a frozen pizza cooked at the lab, then tested, with samples taken from the pizza while still frozen as well.
Firstly, there is no doubt that microwaves cannot kill bacteria directly. They can’t even roast ants, as the wavelength of microwaves is greater than the dimensions of the ant, never mind bacteria.
Secondly, forget the science, who would heat a pizza in a microwave anyway?. It has a much better texture and taste when heated in a conventional oven. Because it takes longer to heat up this way I suspect it is marginally more effective as a bacteriocide.
Still, I’m certain the results for pizza are equally applicable to other foods with the same moisture content, but not to much drier foods which would take much longer to heat up.
Anthony Kaye Bristol, UK.
