How long is it safe to keep food hot in a thermos?

Are you wondering how long it’s safe to keep food hot in a thermos? Don’t worry, you’re not alone.

how to keep food hot in a thermos safely for school lunches or office lunches via MOMables.com

I mean, you heat it up in the morning, and the food is cold by lunch! How does that happen!?

Each week, I receive dozens of e-mails from MOMables subscribers wondering the same thing.

Since the invention of a thermos, people want to know how long will the food last hot, how long should the food be heated, and of course, if it’s safe to eat.

First, watch this video so you see the do’s and don’ts to make sure your food stays hot in the thermos.

Is your child’s lunch going to make him or her sick? Probably not, so relax a little. Most common food poisonings result from eating foods that contain large numbers of harmful bacteria.

The likelihood of the lunch you pack for your child in the morning growing serious amounts of bacteria in 4 to 5 hours, when packed properly in a lunch box, inside an insulated lunch bag, and with an ice-pack is slim (but sure, it could happen).

Today, we are talking hot foods. Hot foods you want to pack for lunch in a thermos.

After a lot of testing in the kitchen and purchasing different thermos containers, I believe I have a pretty good guide for you to follow.

How long is it safe to eat? It depends on the container and how you heat up the food.

  • Bacteria grows quicker between 40 and 140F. Perishable foods should not stay at that temperature range longer than 3 hours.
  • Food packed inside any lunch container should be cooked thoroughly at a safe temperature (no raw eggs, avoid raw onions, undercooked meats or fish).
  • Soups, pasta, and any other hot foods should be heated on a stove to a boiling point (212F, 100C) and microwaved piping hot prior to packing.
  • Thermoses must be preheated prior to filling.

The problem is that most parents heat up foods as hot as they would normally serve it for dinner. Remember, you are not heating up the food to eat now, you are heating it so that it can be safely eaten 3 to 5 hours later. When you heat food like this, the lunch will be cold, regardless of the container you use. 

I tested the thermos containers three times by preheating the thermoses before adding the food each time. The food was filled at 7:30 am, and the lid came off at noon.

Here are the five results:

  1. Thermos Brand 10-ounce capacity Funtainer stainless. The temperature of the soup began at 210F, and after 4 to 5 hours, it was 138F. Would I recommend this container? Yes. It’s great for younger kids, easy to open, and fits a good portion of food.
  2. Thermos Brand 10-ounce capacity Foogo stainless. This one is similar to the Funtainer. The temperature began at 208F, and after 4 to 5 hours, was 134F. Would I recommend this container? Yes. I own one. Like the one above, it’s great for younger kids.
  3. Aladdin 12-ounce BPA–free plastic. The temperature began at 210F, and after 4 to 5 hours, it was 110F. Would I recommend it? Yes and no. Yes, if you are an adult and have access to a microwave and can reheat the food. It’s difficult for younger kids to open because the lid is wide (my 7-year-old could not open it). Overall, this is the worst performing thermos and we don’t recommend it.
  4. Lunchbots 16-ounce capacity stainless container. The temperature began at 210F, and after 4 to 5 hours, it was 130F. Would I recommend? Yes. I own one. It has a wider mouth, so it can be difficult for the younger ones to open. It has a larger capacity, so it’s perfect for bigger appetites.
  5. Stanley 17-ounce stainless. The temperature began at 210F, and after 4 to 5 hours, it was 165F! It was nearly too hot to eat. Would I recommend? Yes. I bought it for this test and will be using it for my husband’s lunches because he typically leaves at 7 am and doesn’t eat until 1 or 2 pm sometimes. It’s a great option for older kids with bigger appetites. It wasn’t difficult to open because the mouth isn’t as wide as #3 and #4. The only con is that it is bigger and bulkier.

To prove the test, I heated up a bowl of soup to a temperature my kids would eat, 150F (and even this was a bit hot for my son). I filled the thermos with the hot soup, and after 4 to 5 hours in containers 1 through 4, it was cold (barely warm at best), and in #5, the Stanley, it was warm and edible.

My advice is for you to make sure you preheat your thermos, heat up the food to a boiling point or piping hot, and purchase a thermos that suits your needs. 

Ask yourself: Will the thermos fit into my child’s lunch box? How much food will I be filling it with? Will my kindergartener be able to open it? Remember, with any new container you purchase for your child, you must teach him/her how to use it first at home.  

If you are pretty new at lunch packing, need school lunch ideas, or need a little direction to make sure your child eats a varied diet, you are not alone. You can always subscribe to our school lunch plan.

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About MOMables

Laura Fuentes is the founder of MOMables and a mom to 3 kids. She's the Author of 3 cookbooks, has competed (and won!) on Food Network, is a TODAY Food contributor, has a column on the Huffington Post and creates recipes for numerous print magazines. She also produces recipe videos for the MOMables YouTube Channel.