How to Keep School Lunches Warm

Packing a hot dish in your children's school lunches provides a home-cooked meal, but it's only safe if the hot food stays hot. The food danger zone that can lead to illness falls between 40 and 140 degrees Fahrenheit, according to the USDA Food Safety and Inspection Service. Warm doesn't work in terms of safety -- the food remains safe to eat only if the temperature stays above 140 F.

Generally, a hot packed lunch must stay hot for 3 or 4 hours. Wash your hands, any preparation utensils and the lunch containers thoroughly with hot, soapy water so you don't introduce any bacteria during preparation. Take the time to preheat the food completely before you pack it. The microwave can result in uneven heating, so the stove top works best. And warn your children to test the food for heat before they take a bite.

Liquid lunches, including soups, stews and thick dishes like chili, retain heat well if you prepare and pack them properly. Heat liquid food to boiling, generally at or near 212 degrees Fahrenheit. An insulated soup container helps retain the heat, but to keep food at its hottest, preheat the container first. Fill it with hot water and let it sit for about 5 minutes as you heat the lunch, and then empty out the water and promptly add the boiling food. Place the lid on the container as soon as you add the food so the heat isn't lost.

Good main course meals choices for a hot packed lunch include dense dishes such as casseroles, rice dishes or pasta. A hot sandwich or plain cooked meat is less likely to retain enough heat to stay safe until lunchtime, so these types of food are better served cold. Reheat cooked dishes to their original temperature, usually between 350 and 400 F. Store them in a preheated insulated container made for solid foods or soups, such as a lidded insulated bowl. The hotter the food when you pack it, the more likely that it will stay hot until lunchtime.

Although the main food container is insulated, each layer of insulation you add after packing retains even more heat. Use an insulated lunch bag or wrap the container in a towel to hold in the heat. If you are packing a combination of hot and cold foods, pack them separately. This is especially important if you use ice packs around cold food, because the ice can also speed up the cooling of your hot items. If you are in doubt about the insulation quality of your containers or lunchbox, perform a test run. Pack your lunch as usual and wait 3 or 4 hours, and then check the internal temperature of the food with a food thermometer to see if it stayed at a safe temperature.