NATICK, Mass. — Soldiers thirsting for a cup of coffee on the front lines of World War II could quickly heat up the beverage inside their "steel pot" helmets that served as both head protection and a handy container for campfire cooking.

That isn't an option for today's soldiers, whose Kevlar-fiber helmets can defend against bullets but don't work so well for fixing food.

So researchers at the Defense Department's Combat Feeding Program in Natick cooked up another way for troops to make a hot cup of joe: A thick, resealable polyethelyene bag that can be used anywhere.

Soldiers mix instant coffee with water in the bag, then slide it into the flameless ration heater bags that troops use to warm their meals. A magnesium and iron oxide pad within the bag transfers heat to the water. Within minutes, the coffee is steaming hot.

Soldiers slip the bag into an envelope-like cardboard carton, which can be used like a cup to drink the coffee and protect their hands from the heat.

Barbara Daley, a food technologist at the Combat Feeding Program, said many soldiers on the front lines were going without coffee because there was no easy way to make it. Researchers at the program based at the U.S. Army Soldier Systems Center spent about two years developing the Hot Beverage Bag, or HBB, as it is known in military speak.

"There are coffee lovers out there and they wanted a way to make a hot cup of coffee. We found a simple, dependable, inexpensive way to do it," said Daley, who helped develop the bag.

Army Sgt. 1st Class Rick Haddad, who spent two months in Afghanistan last year, said the lightweight, easy-to-use bag was a big hit with the soldiers in his platoon, who previously often went without coffee.

"It's a morale thing," Haddad said. "Any time in a cold weather environment, if you can have something hot once a day, that definitely improves morale — especially if you are pulling guard duty for 12 hours a night with no sunlight."

Still, the instant coffee won't be mistaken for Starbucks.

The bags were first introduced to troops last year and are now being included in every meal pack, known as Meal, Ready-to-Eat, or MRE. They cost about 6 cents each to produce.

World War II veteran Tom Blakey said the hot beverage bag would have come in handy while he was serving in the 82nd Airborne Division. Although helmets were a popular way to make coffee or cook meals for a group of soldiers, that could be done only when they were sure the enemy wasn't close enough to see or smell campfire smoke.

"You had to have the proper circumstances ... because of the fire situation and the proximity of the Germans," he said. "When we found a fire they were building, we bombed it, just like they did to us."

David Stieghan, U.S. Army Infantry Branch historian at Fort Benning, Ga., said that in the Civil War, Union soldiers were issued unroasted and unground coffee beans, along with a 1-quart tin-plated steel cup. Soldiers would roast a few coffee beans at the bottom of their cups or in a small frying pan, then use their bayonet socket to crush the beans. They would then pour water into the cup and boil it on a campfire, Stieghan said.