Guy Crosby, “The Cooking Science Guy” has some tips to keep you enjoying your Thanksgiving food beyond the holiday. Guy Crosby, an adjunct professor in the Department of Nutrition at Harvard T H Chan School of Public Health, teaches chemistry and food science.

Crosby put his years 30 years in food-related research and development, and his decades in food science academia to give us at Food Safety News insight into how to store Thanksgiving leftovers properly and prepare them a second time.

When should leftovers go into the fridge?
Crosby recommends waiting until your food is cooled before putting it into the refrigerator. If something large is placed into the fridge, like a chunk of turkey or a big bowl of vegetables, it can contain a large amount of heat. That heat will increase the temperature of the refrigerator.

Refrigerator temperatures should stay below 40 degrees F. If the refrigerator gets warmer than the 40 degrees bacteria starts to grow rapidly. The U.S. Department of Agriculture Food Safety and Inspection Service (FSIS) calls temperatures between 40 and 140 degrees F the “Danger Zone” for food safety.

Although it takes time to cool down food outside the refrigerator, it’s advised by the Food and Drug Administration and USDA so that you’re not affecting the temperature of the refrigerator. Warm and hot food can be left out for up to two hours without safety risks, as long as ambient temperatures are not above 90 degrees F.

Crosby warned that the food shouldn’t be left out too long.

“You really shouldn’t exceed letting it stay out for more than two hours and even one is preferable. Just give it a chance to cool before putting it in the refrigerator,” he said.

How should leftovers be stored?
Leftovers should be sealed in an airtight container. Most plastic containers with snap on tops keep air out. This is important because an excess of air leads to increased oxidation and aerobic bacteria growth. Crosby said that your best bet for keeping your leftovers safe is to place them in a good container, “as opposed to just putting them in an open container that’s not sealed or protected.”

How long do leftovers stay good?
The FDA recommends a three to four day limit on leftovers. However, Crosby explained that public health officials are being conservative when they give this estimation. They are doing this because there are certain foods that permit the growth of bacteria more readily than others. He emphasized that each food is different.

The science behind it has to do with foods’ varying pH levels and the amount of moisture in a food item. The pH scale range goes from 0 to 14. A lower pH number means the food is more acidic — 7 being the neutral number. When the pH is below 4.6 bacteria are less likely to grow and the food will remain safe to eat longer. Tomato-based products are an example of a food with a low pH, which is why you can generally leave tomato-based products in the refrigerator for a lot longer than three days and they will still be safe to eat.

The other factor to consider is the amount of water in the food item. Water is required for the growth of bacteria. The drier the food, the longer it will stay safe to eat.

“Foods that are somewhat acidic, below Ph 4.6, and foods that don’t have a lot of water in them, will actually last longer in your refrigerator than three days,” Crosby said. “But three days is the safe limit just to make sure everything is covered.”

Where in the fridge should I put my leftovers?
It is important to remember that even though refrigerators are supposed to be 36 to 40 degrees F, their inside temperatures vary. Just like your oven is often hotter in the top and back than the front, refrigerators have different temperatures in different locations. It depends on the design of the refrigerator, but as a general rule, they tend to be colder in the back and at the bottom.

If you have any concerns about the temperature of your fridge you can always move your leftovers to the colder locations. Crosby recommends keeping a thermometer in your refrigerator and moving it around from time to time to make sure it is functioning and keeping your food at safe temperatures.

Should I worry about the fridge door opening and closing too much?
With family, guests, and kids, there can be a lot of people opening and closing the refrigerator door. It’s a good idea to keep an eye on the amount of time the door is open. Leaving the door open can change the temperature quickly. The more the door opens the more the temperature drops and risks not only the growth of harmful bacteria but also oxidation and the loss of flavor.

What about the freezer?
If you want to store food longer than the recommended time in the refrigerator, it should be frozen. This will ensure that no bacteria will grow. The only risk the freezer poses to the food is hurting the texture and potentially causing an off flavor. The possibility of freezer burn exists for certain foods like meat. Ice crystals forming in the meat can ruin the texture, but this does not impact the safety of the food. This is the same for the oxidation of fats — it can cause an off flavor.

“It’s a matter mostly of texture and flavor in terms of how long you can let something stay in the freezer, rather than safety,” Crosby explained. “Because you could certainly leave something frozen for six months and not have a safety issue, but the quality may suffer.”

For how long specific foods stay good in the refrigerator and freezer, the FDA has a reference chart.

How should I reheat my leftovers?
It is safe to reheat frozen leftovers without thawing, either in a saucepan, microwave or in the oven, Crosby said. When reheating leftovers, make sure they reach 165 degrees F as measured with a food thermometer. Getting the food to this temperature will kill off any bacteria that has grown during the time that it was stored in the refrigerator. Make sure that when microwaving leftovers, the food is covered and rotates for even heating.

The USDA recommends bringing all sauces and gravies to a rolling boil when reheating.