As children head back to school, the hot-lunch dilemma returns -- and "hot" is the key word.

All insulated food containers -- even the most expensive and well-known -- lose heat over time, we discovered in a test of five models. That's not a fact made apparent on labels or directions. Yet our tests showed that if food is not warmed to nearly boiling (212 degrees) before it goes into the container, it won't be hot at lunch.

If you're using a microwave for a morning warmup, the food may be hot enough to eat immediately. But that heat won't be maintained, even in an insulated container.

Another factor that can affect food temperature is the quality of the container. The five we tested lost between 35 and 100 degrees in 41/2 hours. To put the degrees into perspective, know that a standard cup of Starbucks coffee is 160 degrees. At that temperature, you're taking sips rather than big gulps. Once the coffee drops to 100 degrees, you definitely want a warmup.

We purchased the insulated containers from a Target store, but they are widely available. We tested them three times, each time heating them with boiling water for five minutes before adding the food.

The first time, we brought canned chicken noodle soup to boiling on the stove. We filled the containers and noted the temperature: about 200 degrees. The lids went on tight at 7:30 a.m. and came off at noon.

The winner was the mega-industrial Stanley "vacuum food jar," which yielded 165-degree soup. That was almost too hot for children who barely have 20 minutes to eat lunch. But if you want food to stay piping hot, Stanley will do it.

For our second test, we heated soup in the microwave. It took more than 8 minutes on high for 4 cups of cold soup to reach 200 degrees. It would have been just as easy to use the stove. We realized most people using the microwave don't heat their food enough.

A third test confirmed the theory. We heated soup in the microwave to a suitable temperature for immediate eating, about 150 degrees. In just three hours, the soup in all but the Stanley was barely lukewarm.

Our advice is to buy the best container you can afford and make sure it suits your needs. Open it at the store and look at the serving-cup lid. Does it come with a spoon? Inspect the opening and make sure it's wide enough to clean and load.

And, lastly, crank up the heat on the food.

Stanley

17-ounce capacity, stainless, $21.99. Hand-wash. Temperature of soup after 41/2 hours: 165 degrees. Recommended, especially for adults and teenagers with big appetites.

Pros: Kept food hottest of five products tested. Good size for adults and teenagers. Has a 12-ounce serving cup. Wide mouth makes it easy to clean and load. Lifetime warranty.

Cons: Not suitable for younger children because of size: At nearly 71/2 inches high, it won't fit in many lunchboxes. Price is higher than others.

Thermos

16-ounce capacity, stainless, $19.99. Dishwasher-safe. Temperature dropped to 150 degrees. Recommended for middle-school students and older.

Pros: Size (less than 6 inches tall) fits most soft-sided lunch boxes. Wide mouth makes it easy to clean and load. Comes with a small serving cup.

Cons: Foldable spoon is a bit difficult to handle and easily could be lost.

Embark

12-ounce capacity, stainless, $10.49. Dishwasher-safe. Temperature dropped to 100 degrees. Not recommended; difficult to open and container's exterior gets too hot.

Pros: Good size and weight for most lunchboxes. Wide mouth makes it easy to load, clean and eat from.

Cons: Food temperature dropped to an unappetizing level, and exterior gets too hot. Difficult to open.

Aladdin

12-ounce capacity, $9.99. Dishwasher- and microwave-safe. Temperature dropped to 112 degrees. Recommended only for adult and office use with microwave access.

Pros: A wide, mug-looking container that's easy to clean and load. No Bisphenol-A on the container's surface. Can be popped into the microwave.

Cons: Unwieldy size for many lunchboxes. Also, spilling is a concern. Did not hold heat well compared with other containers.

Thermos

10-ounce capacity, $13.99. FUNtainer stainless. Hand-washing recommended. Temperature of 200-degree soup after 41/2 hours was 125 degrees. Recommended for young children who don't have more than three hours between when food is packed and eaten.

Pros: Cute Hello Kitty container appeals to children (among other styles). Very wide mouth makes it easy to load, clean and eat from. Lightweight and small, just 41/2 inches tall.

Cons: No serving cup (but fewer parts to lose), and stainless rim is a bit sharp.