How To Use A Thermos

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Why a Thermos?

Well, here’s my quick story. Maybe it’ll sound familiar. My boys, who are now 12 and 14 years old, pack a lunch for school the majority of the time, rather than buy lunch. This is mainly because they eat so darned much! If you have a teenaged son, you know what I’m talking about. Even pre-adolescents can have mind-blowing appetites.

We got to the point a couple of years ago when my boys were actually buying multiple lunches at school many days because they were so hungry, and with 5 kids, that quickly became an expensive routine. Not to mention, school lunches, nutritionally speaking, are not usually the healthiest.

My growing kids needed a big lunch, but having a sandwich every day, or even two sandwiches, got old for them quickly. I kept thinking, “What would be really great is if I could send some real food with them. Like leftover chili or a couple sloppy joes.” Those are the kinds of things they had started eating for their afterschool snack instead of the bowl of Goldfish crackers that had been the norm when they were younger.

I think I was probably Googling for ideas, and that is when I discovered the potential of a thermos! In my mind, I’d always thought a thermos was for soup. It turns out, it can be used for SO MUCH MORE. You can use a thermos for basically any hot food that you want to keep hot.

What is a thermos, anyway?

A thermos, also called an insulated food jar, is a double-walled container that keeps food hot for a prolonged period. The space between the inner and outer walls is a vacuum. (The air is sucked out during the manufacturing process.) This makes it very difficult for heat to escape the inner cavity. That’s a good thing, because that would cause your food to get cold.

The Best Way To Use A Thermos

You should fill the thermos in the morning before school. I often have the kids pack the rest of their lunch the night before so the thermos is the only thing left to be added in the morning.

Heating your food and then loading it into a room temperature thermos just doesn’t work very well. If you do this, some of the heat from your food will be lost to the inner wall of the thermos and your food will cool down some before lunchtime.

If you want your food to stay nice and hot, you need to get the inner walls of the thermos hot first. That way, when you put your hot food in, there is no place for the heat to go, and your child’s lunch will stay piping hot, even until lunchtime!

Also, just a note: You should not microwave an insulated food jar. This could mess up the vacuum-sealed walls as well as cause major issues if your thermos is made of metal.

Step-by-step Instructions For Use

1. First, you need to preheat the thermos. Heat some water in a separate container to near boiling (in the microwave is fine). Then, pour the hot water into the clean, empty thermos. You can lightly place the lid back on the thermos to keep the heat from escaping. Leave the hot water in the thermos for at least five minutes.
2. While the thermos is preheating, heat up whatever food you will be loading into the thermos. This can really be practically anything that needs to stay hot. Pizza, burrito, barbecue chicken, chili, stew, spaghetti, chicken fettuccini…you name it. Just make sure you heat the food completely so it is very hot.
3. Once your food is heated and ready to be loaded, dump out the hot water from the thermos. Very carefully blot out the extra water with a towel. The metal inside of your thermos will be very hot, so don’t burn yourself!
4. When the excess water is dried out, load the food into the thermos. You may need to be a bit creative on how you do this, depending on what food you are packing. Pizza is one that doesn’t seem to lend itself to being packed in a thermos, but if you cut it into strips and either insert them vertically into the thermos or roll them into spirals and stack them, it works great. Being creative is key!

How to Choose the Best Thermos

Thermoses come in lots of colors, sizes, and varieties. The two main things to consider are capacity and construction.

For my hungry boys, I need a large thermos (at least 12 oz) to be able to fit enough food inside for them. If your child has not hit the hungry teen phase yet and you are simply looking for a way to pack more of a variety of foods in their lunchbox, then you can probably get away with a smaller thermos.

Make sure that you buy a thermos that has a stainless steel inner wall. There are some thermoses that are lined with plastic, and I really question the safety of that, even if it is BPA-free. Since thermoses are meant for hot food, and it’s never a good idea to heat plastic since that releases chemicals into your food, stainless steel is just a safer choice.  Plus, my guess is that plastic will not retain heat as well as stainless steel, making the thermos less effective at keeping the food hot. But that’s just speculation on my part.

Another aspect of construction for thermos food jars is the method of insulation. Make sure yours is vacuum insulated. All of my recommendations listed below in the next section use vacuum insulation technology.

Where to Buy the Best Thermos

Good food jars can be found in most stores, including Walmart, Target, and outdoor supply stores like Dick’s, Summit Hut, and REI. You’ll find the best selection in August and September, when it’s back-to-school time. Amazon has a great selection year-round, so if you need one right away or don’t have time to shop locally, that’s probably your best bet.

How to Clean Your Thermos

Thermoses need to be washed by hand in the sink. I usually use the same bottle brush that I use for our reusable water bottles. 

Thermoses are NOT dishwasher safe. If you put the main jar portion in the dishwasher, the vacuum insulation would likely be destroyed, making your thermos ineffective at keeping food hot. Even the lid portion needs to be hand-washed, or the silicone gasket that creates the seal will be ruined and not fit your lid anymore. I speak from experience.

Favorite Foods to Pack

The possibilities are truly endless as far as what food you can pack in a thermos. Here are some of our favorites, though.

Keep Food Cold, Too

Your insulated food jar can also be used for cold foods like yogurt, pasta salad, and cottage cheese. If you decide to use it this way, you can put the open thermos in the refrigerator overnight to get the inner walls nice and cold. When you’re ready to pack your lunch, just take out the chilled thermos, fill it with your cold food, and screw the lid on.

Length of Time Food Stays Hot (or Cold)

How long food stays hot or cold will depend on numerous factors including the capacity of the food jar, how full the jar is, and how hot/cold the food was when it was packed. Manufacturers give performance data based on testing they have done on their own products. In general, the thermoses recommended in this post can keep hot food hot ranging from 5 to 14 hours. They will keep cold food cold for 10-24 hours. Check the individual company for thermos-specific performance if you have particular requirements, but all of the products in this post are sufficient for school lunch use.

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And there you have it! The trick to being able to send a hearty and satisfying school lunch for your growing teen. If you already have a thermos and just haven’t been using it, I highly recommend you pull it out of the back of your cabinet and add it to the rotation of lunches for your guy or gal.

Your teen will love you for it.

Resource: How Thermoses (Vacuum Flasks) Work