How to Keep a Packed Lunch Cold Until Lunchtime

To keep a packed lunch safe until lunchtime, pack it in an insulated, soft-sided lunch bag with at least two cold sources, such as a frozen gel pack and a frozen water bottle, and put the perishable food between them. That's the USDA's advice, and it's simple once it's part of the routine. Perishable food that sits at room temperature for more than 2 hours, or 1 hour when it's above 90°F (32°C), should be thrown away, and a school morning can easily last longer than that.

Which foods need to stay cold

"Perishable" means anything that would normally live in your fridge: sandwiches with meat, eggs or cheese spreads, yogurt, milk, cut fruit, pasta or rice salads, cooked chicken and leftovers.

Some foods are fine without a cold source. USDA lists whole fruits and vegetables, hard cheese, unopened canned meat and fish, bread, crackers, chips, mustard and pickles among them. A lunch made only of those doesn't need an ice pack, though most kids' lunches include at least one perishable item.

Where to put the ice packs

Two cold sources work better than one because they cool the food from two sides. Put one flat gel pack along the bottom of the bag, the perishable food on top of it, and the second cold source on top or pressed against the food.

A frozen water bottle or juice box counts as a cold source and is usually thawed enough to drink by lunchtime. Freeze it overnight. Fill bottles only about three-quarters full, because water expands as it freezes. Wrap it in a napkin if your child doesn't like condensation on their sandwich.

Pack the night before

Making lunch the evening before saves the morning rush, and it helps keep food cold. Food that goes into the bag already chilled from the fridge stays cold longer than food packed at room temperature. USDA suggests preparing cooked food ahead of time so it has time to chill thoroughly in the refrigerator.

1. Make the sandwich or main and store it in a sealed container in the fridge.
2. Put the gel pack and water bottle in the freezer.
3. In the morning, assemble: ice pack, food, frozen bottle, close and go.

If your child's school has a refrigerator they can use, the lunch can go in there on arrival. It still needs the ice packs for the trip to school.

Insulated bags vs paper bags

An insulated soft-sided bag or box keeps food cold far better than a paper bag. If a paper bag is all you have, USDA suggests double-bagging to add a layer of insulation, and the two cold sources matter even more.

Wipe the inside of a reusable bag after each use, and let it dry with the zip open.

Keeping hot food hot

For soup, chili or pasta, use an insulated food jar (a thermos). USDA's method:

1. Fill the empty jar with boiling water and let it stand for a few minutes. An adult should handle the boiling water.
2. Pour the water out and add the food while it's piping hot.
3. Close the lid and keep it closed until lunch, so the food stays at 140°F (60°C) or above.

Leftover chili or the chicken from sheet-pan chicken fajitas work well this way, as long as they're reheated until steaming hot first.

Packing for children under 4

Preschool lunches need one more check: food that's round, firm or hard to chew is a choking risk for children under 4. Before it goes in the box:

• Cut grapes and cherry tomatoes into quarters lengthwise.
• Grate raw carrots or cook them until soft, instead of packing sticks or rounds.
• Cut hot dogs lengthwise and then into small pieces, never into rounds.
• Cut cheese into small, thin pieces.
• Leave out popcorn, whole nuts and hard candy.

After lunch

USDA advises throwing away perishable leftovers and used food packaging after lunch instead of bringing them home to eat later. If a yogurt comes home unopened after a long, warm day, it's safest not to put it back in the fridge for tomorrow.

For ideas to fill the bag, our nut-free lunchbox combinations note which items need a cold source.