How to Keep Lunch Warm

Co-authored by wikiHow Staff

Last Updated: September 22, 2021 References

Keeping food warm that you have heated up in the morning and packed in a lunch can pose some challenges. Using a thermos is necessary for keeping a packed lunch warm, but it helps to know the proper way to use one. There are also some tricks you can use to keep warm items and cold items in the same lunch bag. If you continue to have problems or you just want to see how long your thermos will keep food warm, then you can also run a simple test on your thermos.

Method 1

Method 1 of 3:

Using a Thermos

1
Fill the thermos with boiling water to heat it up. Let the thermos sit for about 10 minutes with the boiling water inside of it to help heat the container.^{[1]
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FoodSafety.gov
Online portal combining food safety information from the U.S. Food and Drug Administration, the Food Safety and Inspection Service, and the Centers for Disease Control and Prevention
Go to source}^{[2]
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Research source} This preheating treatment will help to keep the food that you put into the thermos warm until lunch time.
- Pour out the water after 10 minutes.
2
Place hot food into the thermos.^{[3]
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Trustworthy Source
FoodSafety.gov
Online portal combining food safety information from the U.S. Food and Drug Administration, the Food Safety and Inspection Service, and the Centers for Disease Control and Prevention
Go to source} Heat up the food in the microwave or on the stovetop so that it reaches a boiling point of 212 degrees. This will help to ensure that it is warm enough to eat at lunch time.^{[4]
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- Remember that the food will be consumed hours later, not right now. It is okay to make it too hot to eat right away.
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3
Seal the thermos well. Close the thermos immediately after you put the food into it to lock in the heat. Make sure that the thermos has a tight seal to prevent heat from escaping from it.^{[5]
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Online portal combining food safety information from the U.S. Food and Drug Administration, the Food Safety and Inspection Service, and the Centers for Disease Control and Prevention
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- If the food in the thermos is for your child, then instruct him or her to ask for help opening the thermos at lunch time. Do not try to seal it loosely so that your child can open it him or herself.
4
Do not open the thermos until lunch time. Keep the thermos closed tightly until you are ready to eat it. If you open it before lunch to check on the temperature, then some of the heat will escape and it may be cool by lunch time.^{[7]
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Online portal combining food safety information from the U.S. Food and Drug Administration, the Food Safety and Inspection Service, and the Centers for Disease Control and Prevention
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- If you are sending lunch for your child, then make sure that you tell him or her to keep the thermos sealed tightly until lunch time.

Method 2

Method 2 of 3:

Packing Warm and Cold Foods Together

1
Use an insulated lunch bag. To ensure that the warm food stays warm and the cold food stays cold, make sure that you pack the entire lunch in an insulated lunch bag.^{[8]
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FoodSafety.gov
Online portal combining food safety information from the U.S. Food and Drug Administration, the Food Safety and Inspection Service, and the Centers for Disease Control and Prevention
Go to source}^{[9]
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Research source}
- Do not use brown paper bags. A brown paper bag will not provide any insulation for the food you pack in it. This means that the food you pack may no longer be warm or cold by lunch time.
2
Wrap the thermos in a paper towel or napkin. Wrapping the thermos in a paper towel or napkin will help to ensure that it stays warm. It will provide an extra layer of protection against the cold from the cold food items in the lunch bag. Wrap a paper towel or napkin all the way around the thermos.^{[10]
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- You may want to use a rubber band to keep the paper towel or napkin in place.
3
Place cold items on one side and warm items on the other. Separate the warm and cold food items as best you can. Place the thermos on one side and any cold items on the other side of the bag. You may also separate the items with neutral temperature items, such as napkins or utensils.^{[11]
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- If the lunch is for your child, then instruct him or her to leave the items packed as they are or the warm food might cool down.
4
Use a divider to separate the warm and cold items. If you have a divider for your insulated lunch bag, then you can use the divider to separate the warm and cold items. Place the divider between the warm and cold items and then seal the bag.^{[12]
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- If you don't have a divider, then you could use a piece of cardboard cut to fit in the bag.

Method 3

Method 3 of 3:

Testing Your Thermos for Heat Retention

1
Heat your food to 212 °F (100 °C). Remember that you should heat food up to the boiling point of 212 °F (100 °C). You can also just heat up some water to do this test.^{[13]
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- Heat the food or water in the microwave or in a pan on the stovetop until the liquid is boiling.
- If you have a food thermometer, then you can also measure the temperature to ensure that it is at the boiling point.
2
Place the food in your preheated thermos. Next, transfer the food or water to your preheated thermos. After you have placed your hot food in the thermos, seal the thermos tightly.^{[14]
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- Remember that you should always preheat your thermos by filling it with boiling water and letting the water sit for about 10 minutes. Discard the water after 10 minutes and immediately put in your hot food or water.
3
Note the time and determine how long to wait. Note the time that you put your food or water into the thermos for your test. Consider how long after packing the lunch you would usually be eating or when your child would usually be eating lunch.^{[15]
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- For example, if you normally pack lunch at 7:30am and then eat at 12:30pm, then you should wait five hours to determine if your thermos is effective.
4
Check the food when time is up. After you have waited for the amount of time that your food would normally be in the thermos, open it up and check the temperature.^{[16]
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- If the food is still warm, then you have a good thermos that will keep your food warm for lunch. If it is cold, then you may want to find a different thermos or try the test again and heat the food or liquid to a higher temperature.
- If you have a food thermometer, then you might even check the temperature to ensure it is near the safe range of 140 °F (60 °C). If it is a degree or two off, then this is nothing to worry about. However, if the temperature is much lower, such as 100 °F (37.8 °C), then it might not be safe to eat.

Community Q&A

Question

How can I keep it hot without a thermos?

Le'Aisya Fernandez

Community Answer

Take a sock and fill it with white rice and heat in the microwave for 15 minutes. Put it in the container. It will help keep the food warm for 4 hours.
Question

Does the thermos only come in one size?

Community Answer

No, you can find them in many sizes at a hardware, camping goods, or discount store, as well as online.