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.
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:
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 (they are made in China after all!). 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)
Debunking the Myth: “Pre-Heating” Food Jars with Boiling Water Doesn’t Help!
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 a difference. 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 myth is busted, and it’s not worth the extra time. And as it turns out I was right – it happens, occasionally. (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 luke warm. 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!
http://amzn.com/B0017IFSIS
This thermos keeps food steaming hot for hours. When tested without preheating, water that was boiling still measured 185F, 4 hours later.
For packing hot and cold together:
if you have a 2-compartment lunch bag, use that, with the ice pack placed so that it is not directly on top of the thermos in the other compartment.
otherwise, wrap the thermos in a small towel to insulate it. This can be done even if you’re not packing an ice pack, to prevent heat loss.
I used to send my daughter with a character themed food jar but she complained that it wasn’t staying hot. This school year I opted for the adult version by Thermos and it stays HOT even after I pick her up from school. I paid about $18 at Target so really about the same price as the one that didnt work. So I use the old one for fruit
I use a thermos for my kids many times a week. For me it’s so much easier than preparing an inviting cold lunch. I print out their school’s hot lunch menu for the month, then try to plan to send similar lunches for my kids. In this way, my kids are eating the “same” things as their friends, but I know how it’s made. Plus, they tell me they like it better than the school’s food. I do pre-heat…and I have found that some of the larger bowl shaped containers vacuum seal and the kids can’t open them. So I use the same Thermos’ you are showing.
Karen, that is BRILLIANT. It solves two problems at once: lunch planning (which I am not good at but now that you point it out, the school board has experts doing it for me) and the difference of home-packed lunches. I am going to use your tip! Thank-you!
Sarahbee, ah you make me feel so warm and fuzzy! I asked my kids if the food is still warm when they eat it, and they both told me yes it’s still piping hot and we use thermos brand. I also put the lunch in a padded (cooler?) lunch bag, and if I also have cold items with a cold-pack frozen thing(I often use a frozen sponge for fear of a chem leak from those blue things)I separate the hot thermos from the cold things with a folded paper towel. Works like a charm
Thanks a lot for the article and the tests. I have never heard of thermos containers to keep food warm before. I did not read the other comments, so excuse me if others have mentioned it before. But my gut feeling about keeping food warm for a long period of time (morning till lunch) is, that it is the perfect way to destroy vitamins as most of them are heat sensitive and the ideal breeding ground for micro-organisms.
I just had to comment on this post though because I saw so many flaws in it! I live in the Yukon, Canada, and it is a VERY cold place most of the year. I also work outside most of the year, and depend on my thermos for a warm-up!
The first major difference in good vs bad thermoses is the brand: it has to be either Thermos or Stanley, although more expensive, they are the only ones that work. It is also critical to pre-boil thermoses,when you are in cold temperatures this makes the difference between piping hot and barely lukewarm tea.
Moreover, trying your experiment once in only one brand of thermoses doesnt prove anything except that your husbands engineering background lacked statistics.
Likes it hot! – Thanks for your comments. This was not a review of different brands of insulated containers, but rather information on our experiences with the Thermos brand containers we use. The mass of the stainless steel liner in the food jars is quite small relative to the mass of the hot food placed inside (water, in the case of the test), and therefore it’s thermal capacity is low. In other words, pre-heating the container is kind of like holding a match under a pot of water. It doesn’t make much difference. But you are correct that I did not test a statistically significant number of containers. Perhaps I’ll do some more tests. I did take statistics years ago by the way
No! You did not “just have to comment”. Your opinion is as valid as the rest but please remember that it is just that, your opinion.
I think your experiment also failed because after pre-boiling it you only filled it with a cup of water (assuming of course that the thermos holds over a cup of water… but I have never seen a thermos that only hold a cup). Thermoses do not work as well if they are not completely full, after drinking your first cup the remaining liquid begins to cool down quickly, because the thermal energy moves from the hot sources to the cold source. The more space that is left in the thermos, the faster it cools down, no amount of pre-boiling will help if it is not filled to the brim. If the tea/soup/whatever is boiling when you put it into your thermos it also makes a big difference. I am sorry if you took offense to my opinion, but keep in mind that you are advising people about things that are also just your opinion…
Likes it hot! – I did not take offense to your opinion, and never gave any indication of such. The Thermos food jars we use have a maximum capacity of only 1.25 cups. I preheated them with 1 cup of water, and I tested them with 1 cup of water.
Likes it hot! – You clearly didn’t read the entire post, nor did you clearly read Jason’s response. They were testing their kids’ Thermoses. If you haven’t ever purchased kids’ Thermoses, then you’re probably correct in saying that you’ve “never seen a thermos that only hold a cup.”
My husband and I have recently discovered a brand that we have fallen in love with. Klean Kanteens have some vacuum insulated bottles that keep ice for 24 hours! and warm foods for 6 hours. (the ice I have witnessed, the warm foods I need to test out for the full 6 hours, I drink my tea in less than 6 hours) Anyway a ridiculous amount of time. They are stainless steel. A very environmentally conscious company. The Kanteens come in various sizes and in the spring they are introducing shorter food containers with wide mouths. And the best part is they have some holiday specials right now. Neither one of us work for the company, we just love their products. I mean really ice water after shopping in Texas heat, its amazing.
It looks like some people have already mentioned Klean Kanteen! When reading the post my computer skipped to the end of the comments.
Glad to see others like them too!
We love Klean Kanteen! It was our toddler’s first “real” water bottle (non-sippy cup).
I love this post! I’ve been sending heated leftovers in thermoses with my four kids to school for about 7 years now. I can’t tell you how many people have left horrified comments about food borne illnesses since I first started posting about this. BUT! if you put the food in while it’s boiling hot, and send it in an insulated lunch bag, it really is hot enough at lunch for them to enjoy.
In addition to soup, I send reheated casseroles (like spinach enchiladas and chicken divan), bean dips, and homemade nacho cheese in our thermoses. And I haven’t had a single incident of food poisoning in 7 years, so I trust this method.
I admit I’m still so skeptical of your test with heating the thermoses first. It looks like good science, but I don’t know if I can make myself put hot food into a cold thermos. I think I’ll at least run them under hot tap water before filling them, perhaps more for my peace of mind than for any scientific reason. Thanks for doing the research, Jason! So interesting.
As a chef, I’ve done quite a few tests on how various food storage containers keep food hot and cold. It’s been 6 years since I packed lunches, but I still have my notes from making sure my kiddo wasn’t getting dangerous food since she had the late lunch and work schedules meant her lunch was getting made quite early in the morning. I took a weekend and did some tests to figure out the best course of action on planning and packing lunches.
Putting in water that is boiling at 212 degrees, my thermos has it hovering around 145 at the 5 hour mark–so, not in the danger zone yet. Brothy soups can be added at about 200 degrees and have similar results (just entering the danger zone at 5 hours) but thicker soups can’t be heated that high without scorching to begin with, so they go in at about 160 degrees and enter the danger zone at the 4 hour mark–which means if being eaten by the 6 hour mark there is very little danger to even the most immuno-compromised persons like cancer patients, pregnant women and children.
Now that being said, I put BOILING water in and let sit for about 10 minutes to heat the thermos up to get those results. Failing to do so meant that we lost a good HOUR and a HALF (sometimes two!) of time outside the danger zone–my boiling water was at 135 degrees at the 3.5 hour mark. Tomato soup at 200 degrees hit 140 degrees at about 3 hours and 45 minutes. Chili at 160 was 140 in a little over 2 hours. Boiling water in the thermos for 5 minutes gave us no noticeable change in time from not putting water in at all. Boiling water for 15 minutes gave us about an extra hour of time across the board, but we didn’t really need it so 10 minutes seemed to be the sweet spot.
So certainly, specific containers may have different results than others, and the heat/density of the food when it goes in is going to have significant changes in the results too. As is how often you open the container to get those results. I took tests every hour until I got to the danger zone, then tested again using the same method but without opening until I hit that time–I found I lost about 10 degrees in 5 hours by opening each hour. But the results as listed here are for the second test, assuming it sat in the kid’s locker and wasn’t opened until lunchtime.
I love this response. I felt like the test with hot water was missing something, but I couldn’t figure out why it would be different with soup vs. hot water. This makes total sense. Also, one of the websites for the food thermos’ specifically states to pre-heat the thermos with boiling water, so at least they think it is important too. Very interesting either way.
Great post! I was surprised as well when I had lunch with my daughter the other day that her soup wasn’t very hot. As a Farmers Market Manager I am versed in food safety and was wondering about “keep ing hot foods hot” and her soup not being very hot by her late (12:30) lunchtime. I’ll take a second look at the thermos design now.
Seems like some people on here just like to disagree with anything that is said.
I love this page and it gives me a lot of helpful ideas. I dont always agree with everything and I even sometimes will say i dont agree but its in a way that is a conversation..not as a way of what seems like calling people out.
If you have ACTUAL INFORMATION than i think that is great but to just br critical of the page seems pointless and well, annoying.
Thanks for the information. We have two of these Thermos’ at home that I bought with the intention of sending hot food, but my husband has never trusted that they would stay hot. I think I will have to run the experiment at home so that he can see the results for himself. Sure would be great to be able to add soup to the lunch list! I freeze my soups in the jelly jars too (of course based on your recommendation), so this would certainly be convenient for a once a week option!
I bought the thermos you had a link to but realized it is a drink thermos (we can always use that!). I was actually looking for one for soup for the kids’ lunches. An 8 oz. seems so small- is that big enough for a main portion of soup for an 11 year old boy? Just wondering what size you use for soup.
I got a great “adult” size thermos at Target for my 10 year old son. It came with a lid that held a fold-up spoon. He loved it mostly for the manly-geekiness of the folding spoon (it IS a plain black thermos) but it holds 1 1/2 times more than the small kiddie thermos containers for food.
I found that I have to place boiling water in the thermos I have for at least 30 minutes to make any difference in keeping the food warmer longer anything under 30 doesn’t make any difference to temperature. That helps with keeping the food warm until my kindergartener lunch of 11 am and I put the food in about 7:15am. I use a thermos brand like the one tested in the post on the right only a different pattern of course! I sometimes will put boiling water in the thermos before I got to bed and it is still warm in the morning. Would love to see a test redo leaving boiling water for a longer period of time to see if you notice a difference.
Hi Allie – Sorry for my late reply! Pre-heating will result in a slight improvement in performance, but my point is that for our purposes (kids’ snacks and lunches) the improvement is so small that it is not worth the additional 5-10 minutes it takes to pre-heat the thermos. I certainly don’t want to spend 30 minutes doing so…I am not a morning person!
At some point I would like to test ALL of our insulated containers (we have some different styles and brands) with and without pre-heating to add some statistical significance. Not sure when that will happen though since we use them constantly.
Okay, I just noticed you have both the food jars and a regular thermos. I bought the thermos thinking I could put soup in it, is that not true? Would I need to buy a food jar as well?
Daniele – Either would keep soup warm, but the food jars are much easier to eat out of with a spoon.
Where do you find a good food jar/thermos that is not made in China?
Hi Carey. Sorry, did a bit of research but came up empty on this one. Goodluck. ~Amy
Aaah! Of course we have the one on the left – that explains why her hot lunch is always cold!!
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.
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.
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, http://www.swellbottle.com. 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.