How to Select and Use a Thermos

It’s no secret that I like to send warm soup (or leftovers) to school with my girls in insulated Thermos food jars. I’ve actually made a habit of doing this on Wednesdays because I like to have one night per week when I don’t have to be “creative” with packing school lunches.

I make big batches of their favorite soups in advance, freeze them in individual jelly jars (leaving room at the top for the soup to expand), and then “voila!” lunch is pretty much done one day a week for weeks to come.

Tomato Soup for school lunches - in Thermos - by 100 Days of Real Food

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Not All Thermos Jars are Created Equal

So you can imagine my surprise when one Wednesday I had lunch with my daughters at school and asked to take a bite of each of their soups. Some readers have told me their insulated containers keep their kids’ food piping hot and others have said it’s so cold their kids won’t even eat it. So I decided to see for myself how warm the food actually was by lunchtime (about 5 hours after packing it in the morning).

Their lunch periods are only 20 minutes apart and the first daughter’s soup was fairly warm – even warm enough for a cold-natured person like me that prefers things pretty hot (soup, mochas, showers…you name it). Then I sat with my other daughter less than a half hour later and realized her soup was MUCH colder (she still ate it anyway – good girl LOL)!

But, from the outside their food jars looked identical…same brand, same Hello Kitty logo, etc. and after packing them at the same time using the same method/soup I just knew something was not right.

So I of course turned to my engineer husband who conducted a very scientific, controlled test to make sure the dramatic difference in temperature wasn’t just my imagination…and this is what he says about it:

Not all Thermos food jars are created equal! by 100 Days of Real Food

“First I inspected the food jars and even though they looked the same on the outside I found there was actually a difference on the inside. For one, the vacuum seal (beneath the bottom cover) on the colder jar looked inferior, and two, the shape of the bottom of the container looked like it would facilitate more heat transfer, which is not what you want when you’re trying to keep the heat in the container.

But to prove this theory I ran a simple test using one cup of boiling water (212 degrees) in each. Thermos brand food jars claim to keep food warm for five hours and after four hours the water in the ‘bad’ food jar (pictured on the left) was at 85 degrees, whereas the water in the ‘good’ jar (pictured on the right) was at 120 degrees…35 degrees is quite a difference!

I’m not sure if the vacuum seal was in fact broken or if this was just a knockoff/poor design, but regardless after calling the Thermos Company they sent me a new jar for free in exchange for me mailing them the ‘bad’ food jar at my own expense. Now we can be certain that both of our kids are consistently getting warm soup (or leftovers) at school!” – Jason Leake

Is “Pre-Heating” Food Jars with Boiling Water Worth It?

I’ve been told by dozens of readers that “heating up” Thermos jars with boiling water prior to adding the soup/food would keep it warmer longer. This theory does sound plausible so I started doing it myself hoping it would help!

But, being the skeptic my husband is, from day one he said he did not think the boiling water would make much of a difference for our application. He also didn’t like having to go through this extra step in the morning when he helped me get the girls off to school. :)

So once again my husband’s engineering background kicked in and he conducted another scientific test to determine if pre-heating the Thermoses made a difference or not:

“To test my hypothesis I got two identical food jars and labeled one as “pre-heated.” I filled that jar with boiling water, let it sit for 7 minutes, and then dumped out the hot water. I then immediately filled both jars (one pre-heated and one not) with exactly one cup of boiling water and closed the lids tightly.

After 4 hours I removed the tops and measured the water temperatures with our kitchen thermometer. The pre-heated one was only half a degree warmer…not even enough to notice!

So I’d say the pre-heating is not worth the extra time, at least for this scenario. That being said, insulated container manufacturers do recommend it for optimal performance, so perhaps it is worthwhile if you live in a very cold environment.” – Jason Leake

Our Advice

We think that good quality, insulated food jars and drink bottles can be incredibly useful when it comes to transporting and keeping your “real food” meals and beverages warm (or cold). In fact, at the pool one day I found myself drinking water out of my children’s Thermos bottles because their water was still icy cold while the water in my non-insulated steel bottle had turned lukewarm. I now have an “adult” Thermos bottle that I love and seriously take almost everywhere (in the car, on the plane, etc.).

So we highly recommend them, but when it comes to selecting a food jar, if possible, try to avoid the “bad” style shown on the left in the picture above. Also when it comes to adding food to the jars remember you can skip the pre-heating (i.e. boiling water) step. Simply warm up your food, add it to the food jar, put on the lid, and you should be good to go for about 5 hours.

There’s one other tip in regard to the lid though…I’ve found that I have to screw on the lid all the way and then slightly unscrew it (by maybe a quarter of a turn) to make it easier for my children to open it by themselves at school. After you loosen the lid, simply hold the jar sideways and make sure no food leaks out in order to test the seal.

Please share with us in the comments what your experience has been with using insulated food jars!

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174 thoughts on “How to Select and Use a Thermos”

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  1. I am getting my daughter ready for summer school next week and it will be my first time packing a lunch. I bought the freezer packs recommended in a different post and my question is – if I pack milk in a thermos, do I need to add ice packs around it to keep it cold until lunchtime? Thanks!

    1. Assistant to 100 Days (Amy)

      Hi Lauren. It won’t hurt to add ice packs but the thermos should keep the milk cold on its own. ~Amy

  2. Jazmine Holguin

    I I may I would like to reccommend you check out the Hydro flask. It keep beverages cold for almost 24 hours and beverages hot for around 9 hours. I once left it full of Ice in my hot car one summer day and went on a hike thinking I could just drink the melt when I got back… to my surprise hours in my hot car didn’t even give me a drop to drink. Anyways its worth checking out, I am on my second one I love it.

  3. Question… after the soup defrosts in the frig overnight how do you heat it up in the morning to pack in the lunch? Thx!

  4. Do you thaw the soups in the fridge or out on the counter? I need to find good thermoses, I know my kids would love to have warm pasta and soup for lunch!

  5. My son loves to take soups and pastas to school. The thermos is great. Now I have the one on the left and he says sometimes its so hot he has to wait to eat it. I am hoping his continues to keep his foods nice and warm/hot. I love all you ideas for packing kids lunches. Waiting for my silicone pop makers to arrive!!!!

  6. What age is a good time to start using the thermos. I’d like to do one for my 3 year old but I’m worried it would be too much work for the teacher. He loves soup and drinks it rather than eating with a spoon. I was also thinking of introducing the idea of the thermos with his snack time yogurt or oatmeal. Any ideas or observations?

    1. Assistant to 100 Days (Amy)

      Hi Serena. I think introducing a thermos now with items that are not too hot would be fine. All a teacher might need to do is open it. You might try it at home a few times before packing it for school. ~Amy

  7. i have purchased an actual thermos brand thermos- walmart approx 16.00 – camping section. it holds 16 oz. i always bring the soup to a boil before putting in container- i fill at about 3-4am. one day husband didnt eat his food. by 7pm soup was still hot!!! he complained that i shouldnt heat so hot- but- THIS IS IMPORTANT!! if food stays at warm temp and isnt boiling hot it is unsafe! IT WILL BLOW THE LID OFF !!!if its not boiling hot when it goes in it spoils!

  8. Warming up the container before putting food in has nothing to do with keeping the food warm longer and has everything to do with not dropping in temperature when you put food in.
    When putting food into a cold metal container that metal wants to be at the same temperature of the food so it sucks the heat from the food so everything is at the same temp and from the very start it loses some of it’s warmth. I do the same thing with my ceramic and stainless steel coffee mug every morning, warm it up before the coffee goes in as sucks away the heat.

  9. I see that you use Thermos brand for your water bottles and thermos. I have been hesitant to use this brand because I do not see anything on it saying they are BPA free. While other brands to promote the BPA free aspect on their product. I assume this is something that you would have looked into when choosing your food containers.

    1. Assistant to 100 Days (Amy)

      Hi Lainy. We’ve been on the lookout but have not found anything other than a traditional type thermos. ~Amy

  10. I totally agree with the pre-heating being a sham. My problem is that my daughter can’t open my thermos at school. I make sure it is about as loose as possible. Is there any truth that preheating makes the thermos open easier later? Maybe she just needs some strength training. Great blog!!!

    1. Assistant to 100 Days (Amy)

      Hi Heather. I am still a pre-heater (gasp!) and my boys do not have trouble opening their thermos. :) ~Amy

  11. When my children were younger I found that if I put a large/wide rubber band (like from broccoli) around the lid, their small hands had an easier time removing the lid!

  12. HI!! I really like your site. We are starting to pack for the whole family and trying to find the best combo of containers. Which Thermos containers do you use? I couldn’t find the info on the specific one and the link took me to a list of containers. :) I’m also looking at the larger 16 oz thermos King food jars for my hubby & 14 yr old son…thoughts? Thanks!! I love the tips about soup! :)

  13. I love the thermos for keeping soups hot, HOWEVER, my problem is leftovers. My son takes leftovets for lunch a lot as he is on GAPS and i cant find anything that would keep for example, leftover meatloaf warm or salmon patties and veggies. The shape of thermos isnt good for that kind of food. Thpughts?

    1. Assistant to 100 Days (Amy)

      Hi there Claudia. Sorry. Came up dry on this one. I’ve used more than one small thermos before but I totally get the dilemma. Seems no one has yet marketed a “thermos plate”. Someone needs to. Hmmmm. :) ~Amy

  14. Does anyone know if these stainless steel thermos’ are nickel free? That is super important to me, as I have an extreme allergy to nickel and have to avoid it. A lot of stainless steel things are off limit for me. I couldn’t find the info in the links above to amazon. Thanks! (18/0 is no nickel and 18/8 and 18/10 contain nickel. Just FYI!) :)

    1. Assistant to 100 Days (Amy)

      Hi there. You may have to call the company on that one. Sorry to not have an answer for you. ~Amy

  15. I put stew in my food thermos and was surprised it stayed very hot after a few hours. But when I used the same thermos for soup it cooled down to a lukewarm within the same time period. Maybe you could do an experiment with the thickness of the food in the thermos. I used a Stanley thermos.

  16. Once again, you are a lifesaver!!!! I have been putting boiling water in the thermos first. So glad we can skip this. Maybe my husband will be more likely to continue the hot lunches now that he will do morning duty.

    One questions – if I send Macaroni and cheese (which is less liquid than, say, soup…will it get cold faster??? What would Jason say?

    1. Assistant to 100 Days (Amy)

      Hi Kim. The “what would Jason say” made me giggle. I’ve observed that the items which are more dense seem to stay warmer longer. If I send a dense cream soup or, say, a thick black bean and rice as opposed to something more “brothy”, it is always hotter at lunch time. No scientific testing behind this, however…no Jason either. :) ~Amy

  17. I DO preheat my thermo and it is effective. But I use it to pack stir fried rice and vegetables. My kids are not crazy about raw broccoli but they love it when I stir fried them with rice ( just like you get them from Chinese take outs). When you are placing non liquid food such as stir fries, preheating is very effective. I place boiling hot water and close lid and leave it for about 5 min, then dump all the water. I wipe the inside the thermo before I put food in it. 4 hours later, rice is still very hot!!

  18. Is it really safe to eat food that is kept at below the “safe” zone temp? It seems to me that the temp of the foods isn’t hot enough to be considered safe if it’s not maintained at above 135F. (Between 40-135F is considered the “unsafe” zone.) I ask because I have wanted to try at thermos forever but am afraid of having unsafe food by the time I get to eat it.

  19. I boil water in a 2 cup measuring glass in the microwave and after filling up the thermos, I turn it upside down in the remaining water so that the lid also heats up. Can your husband do an experiment trying it that way and tell us if it makes a difference? I don’t even own a thermometer to test it :)

  20. I would also like to know the answer to that Donna. I was wondering the same thing when I go to pack soup for my daughter next week! Does it need to be packed separately than items to be kept cold?

  21. Hi Lisa,

    How do you heat the jelly jars- do you microwave them from a frozen state the morning of or defrost them in the fridge the night before? And how long do you heat it? I’m going to try this!

    1. Assistant to 100 Days (Amy)

      Hi Robin. You defrost them overnight. I usually transfer the contents of the jars and heat it on the stove. If you use the microwave for the jars, make sure you don’t forget to remove that metal lid. :) ~Amy

  22. With regards to your “myth busting” – Maybe you should try it again, but not by refilling them with boiling water. How about refiling them with hot soup? I’m just wondering if the boiling water is so hot that even the “cold” thermos has plenty of joules. You might see more of a difference in something that is, say, 110 degrees.

  23. A question for you. Do you pack the thermos of hot soup/leftovers into your kids lunch box if there is an ice pack in it to keep other things cold? Or do you keep the thermos separate in their back pack? I would think that putting it in a lunch box with ice pack would cool the thermos down faster

    1. Assistant to 100 Days (Amy)

      Hi Donna. I put both in one lunch box. My boys say their food is still plenty warm at lunch time. One of them is pretty picky about “hot” food being “hot”. ~Amy

  24. I have the same thermos as you have on the right. It does not keep food warm at all. I’ve tried both preheating it with water and not. The food is always barely warm at best.

  25. These are the BEST water bottles to keep things super cold! I live in Austin, Tx and if you put a few ice cubes in your water/water bottle…your water will still be cold even if you leave it in your 100 plus degree car for hours. Also they don’t sweat so they won’t ruin kids books in their back packs. Apparently they will also keep hot drinks hot but I don’t like drinking hot out of stainless…but it is OUTSTANDING for cold drinks.

  26. Just ordered two of those thermos containers last night!! We sent lunches every day last year and am looking for a way to change it up a bit more this year. I see sending lots of your mac and cheese in these!!!

  27. Hi, I was hoping to find the inside of the new thermos bottle I bought would be the flat one, but of course my luck is it wasn’t. I wanted to get you opinion between two thermos. Initially I bought a 5 hour hot thermos ( flat bottom inside), but I found it wasn’t very hot when I used it, so I bought a 7 hour hot one, (havent tried it yet). BUT when I just opened it, the bottom is like the one you pictured that wasn’t as hot (not flat bottomed). Do you think the insulation difference between the 5 hour hot vs. the 7 hour hot, make it a better thermos? or even if its 7 hour hot the bottom would let heat out just as quickly? I just don’t want to use it and not be able to return it, their so pricey, and school starts on Monday:/ Really looking to invest the money in the best option that works.

    1. Jackie – I would not hesitate to return if the product states 7 hours and it doesn’t work for 7 hours, even if I have used it. That’s really the only way you will know for sure. Read the instructions to see the recommendations for your model on pre-warming the bottle and the food, follow them, and see what happens!

  28. As a mother of two young ones, I appreciate this information! My daughter’s Thermos has not been satisfactory (now I know why), and my son is entering Kindergarten. Lunch is important for these little ones! I have read reviews, shopped, and tried a couple other containers (they didn’t work). Again, thanks for your efforts and information!

  29. I am looking for the flat bottom thermos containers to use in a science lab. What style was the flat bottom one?

    1. Assistant to 100 Days (Amy)

      Hi there. Click on the “thermos food jars” at the beginning of the post and it will take you to each style. ~Amy

  30. How do you get the soup from freezing in a mason jar to steaming in a thermos? Do you defrost in the fridge over night in the jar and then cook it in a pot in the morning? Embarrassingly, some of us are working through the simplest logistics. Thanks!

  31. Hi,

    We also use the kids Thermos food jars to send leftovers with my son for school. After he told us that his food was not very warm, I discovered the directions to put boiling water in prior to use. I am not sure why it made a difference for us, and not for the boiling water test, but he reported that his food was staying quite hot after the boiling water.

    On a side note I just discovered a new small double-walled insulated lunch bottle. It is from the company S’well, I love the small 9oz size. It fits in my son’s lunch bag. It claims to keep cold drinks cold for 24 hours, which I have not tested, and hot drinks hot for 12 hours. I can definitely say that ice is still in there at the end of the school day. I like that the size is packable and that I can send my own milk, water, or healthy homemade juices to school and they are definitely refrigerator cold at the end of the day. The downside is that they are pricey. Also follow the wesite’s directions for care (handwash only, and don’t submerge the cap, and don’t put in freezer). I don’t work for the company — just happy that I found this bottle.

  32. We have two Thermos 10-Ounce Stainless Steel Food Jars. We can not get the lid off of one of them. Do you have any suggestions? They are not the Thermos character jars. One is plum and the other silver.

  33. I didn’t read ALL of the other comments, so I apologize in advance if someone else already said this….but the point of “priming” a thermos isn’t necessarily to help keep the food hotter for longer, but just to keep the cold metal of the inner part of the thermos from snatching a few degrees out of the food when you pout it into the thermos for the first time.

    Make sense? Same reason I pour hot water into my coffee cup in the morning before I fill it with coffee and restaurants warm up the dinner plates. It’s just a little help. If your thermos isn’t particularly cold when you start out (because your house is a comfortable temp or it’s summer, for example), then it probably won’t make any difference. But if it’s winter or early in the morning and still cold in your house, it might save you a few degrees!

    Hope that explanation helped :-) I remember helping my dad prime his thermos before he went to work when I was a little girl.