Hot and Cold: Thermos 101

Foogo and LunchBots thermoses

So many folks have had so many great questions about using thermoses, I decided to dedicate an entire post to them.

Admittedly, I was pretty late to the thermos game in terms of packing Parker’s lunches. For the first couple years (including most of the first year of this blog) I didn’t use a thermos. At the time, Parker’s lunch box didn’t have room for one.

But this summer, I upgraded him to a much larger lunch box (you can read the whole story here, including the blow-by-blow of all the gear I have for his lunches). And that gave me way more flexibility. So I decided to embrace the hot-cold equation.

Let’s start with thermos basics. Regardless of their shape, most thermoses are made using a double wall construction — essentially two bottles, one inside the other. The air in the space between the two bottles is sucked out, creating a vacuum. That vacuum dramatically reduces temperature fluctuations inside the bottle. That means that hot items placed inside the thermos stay hot longer than in a regular container. Ditto for cold items.

These days, most thermoses are rated for how long they will keep items cold and hot (most models keep things cold a bit longer than they can keep them hot).

Which means the first step for using a thermos in your child’s lunch is to sort out what time lunch is eaten at school. Count back from then to the time you pack the lunch in the morning, then you know the minimum rating you need.

The next step is to sort out what size you need. I have three — two 10-ounce Foogo food jars (rated to keep hot items hot for five hours, and cold items cold for seven hours), as well as one 16-ounce LunchBots food jar (keeps things warm for up to five hours).

I’ve found it handy to have multiple sizes. The large one can even handle warm sandwiches (that I cut to fit using a large biscuit cutter).

The trick to getting the most thermal bang for your buck out of a thermos is to heat or chill it before adding the food.

To heat the thermos, boil a pan of water and pour that water into the thermos. Screw on the thermos’ lid and let it heat while you prepare the lunch. When you are ready to add the hot food, simply pour out the water and add the food.

When I need to chill the thermos to keep food especially cold, I simply open it and place it in the freezer while I prepare the lunch. When I’m ready to add the food, I just fill it and screw on the lid.

What can you pack? I use them for just about anything. Hot items include cut up or sliced meats, included roasted chicken or steak, pasta, grilled cheese sandwiches (if you cut it to fit), pot stickers and other dumplings, chili, etc. And, course, soup (though I have never tried this because Parker thinks soup is evil. Whatever…).

There aren’t a whole lot of cold items I put in the thermos. I usually add an ice pack to the lunch box, so any item not stored in a thermos stays cold anyway. But I do sometimes pack Parker’s favorite mango “ice cream” and I’ve found that it lasts just fine in his thermos.

The ice cream is made from a 12-ounce bag of frozen mango chunks. I place the mango, a splash of lemon or orange juice and a bit of water (and sometimes a banana, frozen or fresh) in the food processor, then process until it reaches the consistency of soft serve. Sometimes he has this for breakfast, too.

So that is what I know about using thermoses in my son’s lunch box. Anything I missed?

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10 Responses to “Hot and Cold: Thermos 101”

  1. Maria M. says:

    Mango ice cream?! Hel-lo!!!

    Thanks for the info on packing food into a thermos. The info and as well as the previous post on Lunchbots was quite a revelation in terms of what works. My crew hates anything that resembles a cold-sandwich-and-a-bag-of-chips kinda lunch. Now, I definitely feel more confident packing something other than “evil” food, aka soup or hot pasta dishes.

    (Scribbling note to self — must make mango ice cream, if only to treat myself. Ha! )

    • J.M Hirsch says:

      The best part of the mango ice cream is that when a certain 7-year-old wakes up grumpy (note that this EVER happens!), I can offer him “ice cream” for breakfast and not feel even a little bad about it.

  2. Katie G. says:

    I am enjoying cooking my way through your book “High Flavor, Low Labor” and that your mango-lemon sorbet recipe is a gem! My two year old loves it, and I really appreciate that it as an unsweetened treat! Thanks.

  3. Amanda says:

    Glad to hear the mango ice cream holds up on the thermos! We have that all the time, as well as pureed frozen banana.
    For a special treat I sometimes turn them into sundaes – drizzle a scoop of mango with agave nectar and top with sliced almonds & drizzle maple syrup on a scoop of the banana and top with walnuts.

    • J.M Hirsch says:

      Mmmm… Maple-banana ice cream sounds wonderful right now. Frozen bananas are a totally underappreciated food. They work wonders for smoothies (or “smoothlies” as Parker calls them), as well as ice cream.

  4. Christine says:

    You’re my thermos and mango soft serve hero.

  5. Jen says:

    This is so helpful!! I think we need a more powerful thermos.

    • J.M Hirsch says:

      Which sounds kind of funny. Makes me think of some big rig with a powerful engine. Which, come to think of it, would be a pretty cool thermos!