How to Pack a Hot Lunch in an Insulated Food Jar So It Stays Hot

A hot lunch stays safe and warm until lunchtime if you do three things: heat the insulated food jar with boiling water first, fill it with food that is steaming hot, and keep the lid shut until your child opens it to eat. USDA guidance is to keep hot food at 140°F (60°C) or above, and a preheated, full, closed jar is how you do that without a fridge or microwave at school.

Morning time: about 10 minutes, mostly waiting for the water and the food to heat · Best for: soups, chili, pasta in sauce, rice dishes, meatballs · Kid-friendly: very, once they can open the lid on their own.

Why preheating matters

A cold jar soaks up heat from the food you put in it. If you spoon hot soup straight into a jar that has been sitting in a cupboard, the walls take a big share of that heat in the first few minutes. Fill it with boiling water first, put the lid on, and leave it for a few minutes while you heat the food. This is the method USDA recommends for hot packed lunches.

Food between 40°F and 140°F (4°C and 60°C) is in what USDA calls the danger zone, where bacteria multiply fastest. Perishable food should not sit in that range for more than 2 hours, or 1 hour when it is above 90°F (32°C). A hot lunch that has cooled to lukewarm by mid-morning has been in the danger zone for hours by the time it is eaten.

Step by step

1. Heat the jar first. Boil the kettle, fill the jar to the top, put the lid on and leave it for a few minutes. An adult should pour the boiling water.
2. Meanwhile, reheat leftovers until they are steaming hot all the way through, 165°F (74°C) in the middle, which is the USDA temperature for reheated leftovers. Stir halfway if you use a microwave, because microwaves leave cold spots.
3. Pour out the water, then spoon in the hot food right away. Fill it close to the top: a full jar has less air inside and holds heat better than a half-empty one.
4. Screw the lid on firmly and keep it closed. Every peek lets heat out.
5. Stand the jar upright in the lunch bag, away from the ice pack and the cold food.

How long a jar holds heat depends on the jar. Check what its maker says, and on the first try, open it at lunchtime yourself on a weekend to see how warm the food still is.

What to put in it

Good hot lunches are wet or saucy, because liquid holds heat well and stops the food drying out. Some ideas:

• Soup, especially thick ones like lentil, minestrone or chicken noodle
• Mild family chili, with a small container of grated cheese on the side
• Chicken and rice leftovers, with the chicken off the bone and a splash of broth stirred in
• Pasta in tomato sauce, or macaroni and cheese loosened with a little milk
• Turkey meatballs in sauce, cut in halves or quarters so they are easy to eat with a spoon

Skip foods that go soggy, like fries or anything breaded. Pack crunchy things such as crackers or tortilla chips separately so they stay crisp. A frozen mini banana muffin makes an easy side, since it thaws in the bag by lunch.

Keeping hot and cold apart

A hot jar and an ice pack can share a lunch bag, but not the same corner. Put the jar on one side and the cold food with its ice pack on the other, or use a bag with two compartments. Perishable cold food, like yogurt, cheese or cut fruit, still needs an ice pack. The details are in our guide to keeping a packed lunch cold.

For younger kids

• Practise the lid at home. Many jars have tight lids. Let your child open and close it a few times so they are not stuck at lunch.
• Food packed hot can still steam when opened. Teach kids to open the lid away from their face.
• For children under 4, cut round foods before packing. Slice hot dogs lengthwise and then into small pieces, quarter meatballs, and cook carrots and other hard vegetables until soft. The AAP lists these among common choking hazards for young children.
• On the first day, ask how it was. If your child says the food is too hot at lunch, let it cool for a few minutes after heating before filling the jar. It should still go in steaming.

School rules and allergens

Many schools are nut-free. Check that sauces and soups are free of nuts and nut oils, and label the jar with your child's name. If your child has a food allergy, the label is also a good place to note it. For cold lunch ideas that suit nut-free schools, see these nut-free lunchbox combinations.

Cleaning the jar

Wash the jar, lid and seal in hot soapy water after every use and let them dry with the lid off. Most lids have a rubber ring that pulls out; food can collect behind it, so take it out to wash now and then. If a jar starts smelling of yesterday's chili, a soak in hot water with a spoonful of baking soda usually clears it.