FAQs on Reheating Food: Pizza, Chicken, and Everything Else

Let’s begin with some general advice from food safety experts about reheating perishable foods:

-Be sure to reheat leftovers to 165°F.

-Reheat them only once.

-Don’t reheat cooked food that’s been left at room temperature for more than 2 hours.  Just feed it to the garbage can. 

Of course, the three rules listed above are all designed to protect you from the growth of harmful pathogens and the risk of food-borne illness.  However, chances are (if you’re like most people) you’ve broken all of these rules more than once. The following FAQs, answered by 4 scientists on our site’s Advisory Board, will help you understand the science behind these rules and offer tips on handling perishable leftovers safely.

Q. Seriously now, how seriously should I take that rule about reheating foods only once?

A. Below are answers from four of our Board members.

Food scientist Dr. Karin Allen provides this explanation:

1.  While it is nice to think that we’re killing all of the bacteria when we heat something to 165°F, we’re most likely killing only a percentage. Although it’s a high percentage, it is not 100%.  For example, if 1% of a specific pathogen survive the heating process, they could multiply whenever that food is passing through the 40°F - 140°F “danger zone”  (either cooling or reheating).  The next time it’s heated, some of those survive and multiply, and this cycle will continue as long as you chill and then reheat the food.  The problem is that, if the ones that are left initially have a little more resistance to heat, their offspring will, too.  So next time it’s heated, you might end up with 10% survival, then 25%, then 40%, and so on.

2.  The other issue is Staphyloccocus aureus. This bactium is carried by about 2/3 of the population on skin and in nasal passages and is very easily introduced to food after it is cooked.  As S. aureus grows and multiplies, it produces a toxin that causes nausea, vomiting, fever, aches – symptoms we often associate with “stomach flu.”  When you reheat your leftovers, you destroy the bacteria (mostly), but the toxin may be left behind, not destroyed by normal heating.  When you eat the food, you ingest the toxin, and the more of the toxin you ingest, the sicker you get.  The more times you reheat the leftovers, the more the toxin could be building up, and the sicker you will get when you finally eat that food.

Food scientist Dr. Joe Regenstein tells us this:

The challenge is what type of degradation occurs while a food is in the danger zone.  From a pathogen safety point of view, heating kills most pathogens if done properly (but it does not destroy the S. aureus toxin). However, reheating is often done in a microwave, and that doesn’t assure proper uniform heating. Therefore, I would want to reheat foods such as soups and vegetables in a regular pot with stirring and put turkey or a casserole in the oven with sufficient bake time to heat the center.  (A food thermometer can tell you when it reaches 165°F.)

Food scientist Dr. Catherine Cutter tells us more about spores and toxins:

Spore-forming bacteria  (ex. Clostridium perfringens) can grow in foods with a lot of protein in it—for example a casserole with milk and cheese or food in gravy—especially if that food has been temperature-abused.  To avoid having spores germinate into actively growing or vegetative cells that could produce toxins or cause foodborne illness, avoid a lot of up and down temperatures and get the temperature of hot leftovers down fast. This task can be accomplished by putting a large amount of leftovers into 2 or 3 smaller pans and refrigerating as soon as possible or by putting a large pot of chili or whatever into an ice bath and stirring frequently to help reduce the temperature quickly. [Ed. note: For more information on this method, click here.]

Besides the risk of contamination, another reason to avoid repeated reheating of food is that it reduces the quality.

Food process engineer Dr. Timothy Bowser makes this confession:

I’m guilty of heating one particular leftover food again and again –lasagna. I think it tastes better after the third time or even later. There are probably other things that develop their taste after reheating, but I can’t think of any at this moment.  For most leftovers I take only the part that I expect to eat and re-heat that.

What can you conclude from these answers? There are increased health risks in repeated reheating, especially if it’s not done properly and the cooling is done slowly or the product is left out at room temperature. Reheating in the microwave may not always be adequate to give you food that’s safe to eat, so check the internal temperature with a food thermometer.  It should be 165°F.

Q. If  I left part of a cooked pizza on the kitchen counter overnight, and the next day I heated it to 165°F, wouldn’t that kill all the pathogens and make it safe to eat? 

Dr. Allen’s explanation in the above FAQ provides a good answer to this one as well, but here are some additional comments from two other scientists. Note that, in the responses below, Dr. Bowser also expresses concern about toxins and Dr. Regenstein expresses concern about reaching the right temperature to kill most other pathogens. 

Dr. Bowser:  If the pizza is left unrefrigerated overnight, the toxins that may be produced by bacteria will not be inactivated by heating to 165°F. The bacteria will die, but their toxins will remain.

Dr. Regenstein: You need to be sure that it is really heated to 165°F – not easy with pizza.  And the product shouldn’t be reheated in a microwave. [Ed. note: The microwave heats unevenly, so the pizza may not all reach the desired temperature.] But after the pizza has been left out overnight, it’s best to err on the side of caution and throw it out.

Q. My life is hectic, so I use a lot of ready-made food such as soups, mashed potatoes, macaroni and cheese, and vegetables in sauce. They’re sold cold (but not frozen) in plastic containers.  If I don’t finish them at one sitting, can I reheat the leftovers?

A.  Most of these products don’t contain big portions, but there may be too much for one person to consume in a sitting. Dr. Cutter suggests that, if you’re dining alone on these items, it’s a good idea to put the amount you expect to eat into another dish and just heat that.  Save the rest for another meal. Note: You should not heat a food twice in the plastic container it came in.  (This rule applies to frozen dinners also.)

Q. I sometimes buy a ready-made rotisserie chicken. It’s sold hot or warm.  I often purchase it several hours before my dinnertime. I put it in the fridge to cool it down and then reheat it. Will it cool too slowly to get below 40°F within the 2-hour limit?

A.  Maybe not. Dr. Bowser has some “cool” solutions to this problem:

-Cut the chicken up before refrigerating so it will cool faster, and individually wrap and spread out the pieces in the refrigerator/freezer.

-Stuff the bird with ice while cooling it in the refrigerator. A thin plastic bag of ice cubes should help to cool the carcass down quickly without getting it too wet. Make certain that there is good contact between the ice and the surfaces of the cavity.

-Put the bird in the freezer—but not long enough to freeze it if you want to eat it the same night. Pull out the bag of ice before it re-freezes into a lump that might be difficult to remove.

Dr. Regenstein suggests taking the chicken out of its plastic container before refrigerating it.  The box acts as an insulator, keeping the heat in.

One final tip about perishable leftovers: the usual advice is to save them in the fridge not more than 3 days and then discard them. For more information on proper handling of leftovers, type “leftovers” into the search box on this site’s home page.

Source(s):

Karin E. Allen, Ph.D., Utah State University, Dept. of Nutrition, Dietetics, and Food Sciences

Timothy J. Bowser, Ph.D. Oklahoma State University, Dept. of Biosystems and Agricultural Engineering

Catherine N. Cutter, Ph.D., Pennsylvania State University, Dept. of Food Science

Joe Regenstein, Ph.D., Cornell University, Dept. of Food Science