Learning how to use a thermos for hot food is easy once you know two things: preheat the container and heat the food thoroughly before packing it. I’ll show you how to do both so you can pack soup, pasta, and leftovers for school lunches that stay hot longer.
A Thermos Makes Packing Lunch Easy
I’ve shared my kids’ school lunches online since 2012, and warm thermos lunches have been a staple from kindergarten through high school. Packing a thermos lets my kids skip the microwave line and enjoy a meal they already like at school. Here, I’m sharing what I’ve learned over the years about using a thermos to keep their food hot until lunchtime.
How to Use a Thermos for Hot Food
The secret to keeping food hot in a thermos is starting with a hot container and hot food. Here’s the routine I use when packing school lunches:
- Preheat the thermos.
This step takes 5 minutes and helps keep the food hot until lunch. - Heat the food thoroughly.
Use the stove or a microwave-safe dish to heat the food until it’s hot throughout. Reheated leftovers should reach 165°F. - Fill the thermos with food and close it immediately.
Pour out the water, fill it with food, and close the lid to keep the heat in.
TIP: Close the thermos lid until it’s snug, but don’t overtighten it. As the food cools, a vacuum can form inside, making the lid harder for kids to open at lunchtime.
How to Preheat a Thermos
Preheating is the step I never skip because pouring hot food into a cold container immediately starts cooling it down. You only need hot water and about 5 minutes.
- Heat water.
Use a kettle or small pot until very hot (or boiling). - Fill the thermos and close the lid.
- Wait 5 minutes.
While the thermos preheats, warm up the food. - Pour the water out.
And immediately fill the thermos with your hot food.
Here’s how I preheat a thermos before packing lunch:
How to Keep Food Hot in a Thermos
Besides preheating the container, the temperature of the food makes a big difference. Heat soup until it’s steaming hot, and make sure pasta, rice, and leftovers are heated all the way through. Stir food while reheating so there aren’t cold spots, especially when using a microwave.
Another tip is to fill the thermos with enough food to reduce empty space, without going past the manufacturer’s fill line. A properly filled thermos generally retains heat better than one that’s only partially filled.
Here’s how I heat food before transferring it to a thermos:
How Long Does Food Stay Hot in a Thermos?
Most insulated food jars are advertised to keep food hot for several hours, but the actual time depends on the thermos, how hot the food was when packed, and how much food is inside. Preheating helps, but it doesn’t guarantee the food will stay at a safe temperature until lunch.
For food safety, the USDA recommends keeping hot perishable foods at 140°F or above. If you’re unsure how well your thermos works, test the food temperature at home at the time your child normally eats lunch. You can also see how long a thermos will keep food hot in my comparison of popular containers.
Can You Microwave a Thermos?
Absolutely not! Never put a stainless steel thermos in the microwave. The metal can cause sparks and damage your microwave, and the thermos isn’t designed for reheating food.
Instead, heat the food in a microwave-safe bowl or on the stove, then transfer it to your preheated thermos. The same goes for preheating: heat the water separately, never inside the thermos.
Packing Hot and Cold Foods Together
You can pack a thermos alongside cold foods in the same lunch bag. Keep the thermos closed and place cold items like yogurt, fruit, and cheese next to an ice pack, away from the hot container. This helps keep the cold foods chilled without cooling down the hot lunch.
If you prefer everything in one container, a lunch box with a thermos is another option. These bento-style lunch boxes let you pack a hot main dish with cold sides in separate compartments.
Watch how I pack hot and cold foods together for school:
If you’re shopping for a food jar, I’ve shared my best thermos containers for kids, including options that are easier for little hands to open. And when you’re ready to fill it, these thermos lunch ideas for kids will give you plenty of options beyond soup.
Still wondering what to pack?
Download my Printable Lunchbox Builder to turn the ingredients you already have into lunch ideas in minutes, or browse hundreds more kid-approved lunch recipes here.
Jennifer says
Hi,
What about grilled cheese sandwiches and heated pizzas? How do you warm that up and keep it warm or hot for longer? Still use a thermos or is aluminum foil enough? Do you think we need to throw in a hot pack?
Thanks!
Laura Fuentes says
Grilled sandwiches and pizza will get soggy inside a thermos. There’s no great way to pack them and keep them hot. A lot of people will wrap them in foil but it’s not a great insulator. This is how I pack grilled sandwiches for school.