May 5, 2010
Chemistry Improves Battlefield Food
When Neil Gussman joined the Army in 1972, meals for the battlefield were served in little green cans. Open those tins, recalls the Chemical Heritage Foundation's communications manager and Army sergeant, and you were likely to find culinary delights like "gelatinous, fat-coated Spam slices" and "big wads of grease."
Known as C rations, "the 12 main courses were ham and eggs, beans and franks, spaghetti, ham slices, and permutations of Spam," Gussman says.He reenlisted in 2007 and, to his pleasant surprise, found that the green cans had been replaced with sleek tan packages stamped "MRE," for Meal, Ready-To-Eat.
"When I got my first MRE, I was in gastronomic love," Gussman says. Inside were crunchy crackers, brand-name candy, and a heating bag that gave off no smoke or light signature. Tactical eating no longer meant meals of congealed fat, he says.
But moving from cans of "green eggs and ham" to pouches of moist lemon poppyseed cake and hot beef ravioli requires a lot of scientific innovation. "Everything in the MRE involves chemistry in some way," says Jeremy Whitsitt, the Department of Defense's combat feeding outreach coordinator. From the packaging designed to withstand downpours and airdrops to the chemical heater that warms meals and beverages, the Combat Feeding Directorate at the U.S. Army Natick Soldier Research, Development & Engineering Center (NSRDEC), in Natick, Mass., has spent years developing the modern MRE.
The unpredictable nature of military life means that battlefield meals must meet a set of strict criteria. MREs need to maintain their freshness for three years when stored below 80 °F, or six months when stored below 100 °F. "They must also be able to withstand rough handling conditions and airdrops from altitudes of 100 feet by helicopter, without a parachute, or 1,200 feet by plane, with a parachute," Whitsitt says.
An MRE's packaging presents the first line of defense in keeping it from getting beaten up during transport and in preventing oxygen, water vapor, and insects from infiltrating and spoiling the food. "It's a critical part of the overall MRE," says Danielle Froio, an NSRDEC materials engineer.
It's also the first thing you notice about an MRE as you pull apart the seal of its tough, tan meal bag made of low-density polyethylene. The food inside this bag is stored in two types of pouches, Froio explains. There's the retort pouch, which holds food that's been sterilized, and the nonretort pouch, which houses food that doesn't need sterilization.
Both pouches have a polyester outer layer that's easy to print on, so nutritional information is included with each of the MRE's components. Beneath the polyester is a layer of foil, which, Froio says, is the ultimate barrier to oxygen, water vapor, and light. A polyolefin layer also makes it possible to seal the package. And retort pouches have a fourth layer of nylon to make them durable enough to withstand the rigors of the sterilization process.
Researchers at NSRDEC are currently working to find a replacement for the foil layer in both types of pouch. "In low-temperature situations it can develop pinhole cracks that reduce the shelf life of the package," Froio explains. Although the group has examined many different polymers as possible replacements, all are permeable to oxygen or water vapor, she notes.
The researchers are now looking toward nanocomposite materials, Froio says, because their nanostructure "creates a tortuous path for the water and oxygen" to travel. They've had some success with montmorillonite- and kaolin-based nanocomposites, and a low-density polyethylene nanocomposite has been used as a meal bag in field tests.
Of course, the guidelines that govern MREs aren't limited to their durability. MREs are, after all, meals, and they have to provide enough nutrition to sustain a soldier engaged in intense physical activity. By regulation, Whitsitt says, each MRE must provide approximately 1,300 calories.
And then there's taste. Responding to the complaints about the old C rations"”the ones Gussman describes as variations on Spam"”NSRDEC has made an effort to create meals soldiers actually enjoy.
So how do you make cooked pasta and cake that can sit on the shelf for three years? "We build hurdles into these different food matrices to make it hard for 'bugs' to grow in them," Whitsitt says. Using acidic tomato-based sauces keeps the pH low, for example, thereby preventing bacterial growth. As for the baked goods, by tinkering with dough conditioners and adding iron-based oxygen-scavenging packets, the researchers are able to control pH and water levels to keep the breads and cakes tasting fresh.
All items packaged in a retort pouch are also sterilized by boiling. But certain foods simply don't hold up under the extremes of temperature and time"”120 °C for 30 minutes"”the military uses for this process. For example, says C. Patrick Dunne, the senior adviser in advanced processing and nutritional biochemistry for the Combat Feeding Directorate, "We have yet to get a really good mac and cheese out of the retort pouch."
To expand meal options and improve food quality, NSRDEC has been working on advanced processing techniques for sterilization that use microwave radiation and high pressure. "The challenge we have is to conquer the chemistry that happens during and after the sterilization process that will lead to degradation of quality," Dunne says. "You're not going to get fresh salads all the time, but we would like to give our guys something that goes beyond basic vitamins."
The novel microwave sterilization process that NSRDEC has developed in collaboration with researchers at Washington State University uses a microwave that operates at 915 MHz. This is a lower frequency than the average home microwave uses and penetrates the food to a greater extent. To keep the pouches from exploding, Dunne says, they are placed under pressure in a water bath. All told, the sterilization takes less than 10 minutes.
The one drawback to using this microwave sterilization process is that it doesn't work with the standard retort pouch. Its foil layer can't be placed in a microwave oven. Dunne says NSRDEC is looking into alternative packaging.
The military is currently testing MREs with chicken and dumplings sterilized via the microwave method. There are "benefits in taste, color, and texture" that come from microwave sterilization, Dunne points out. "You can make a salmon filet that tastes like the poached salmon you'd get in a restaurant. It does not taste like cat food."
NSRDEC is also working on high-pressure sterilization. The process places a food pouch under 100,000 psi for about three minutes. Heating is also used if the food being sterilized hasn't been pasteurized.
The result, Dunne explains, is greater variety of food that doesn't have the "tinny" flavor that comes from the current processing method. Mashed potatoes sterilized with this process have already passed the military's shelf-life and field tests.
Perhaps the most tangible chemical contribution to the MREs is the small chemical heater that makes it possible for soldiers in the field to enjoy the comfort of a hot meal and a hot cup of coffee. The heating technology, which gives off no light or smoke, makes use of the exothermic reaction of magnesium metal and water. The heater is composed of a postcard-sized polymeric "tea bag" filled with 9 g of Mg, which sits in a plastic sleeve. A solider adds just 1 oz of water to the sleeve, slips in the MRE entr©e, and waits about 10 minutes.
The temperature gets up to about 60 °C, according to NSRDEC chemical engineer Peter Lavigne. "The reaction product, magnesium hydroxide, is essentially milk of magnesia, so it's disposed of without environmental concerns," he says.
The Mg powder pack also contains a little bit of salt and iron. These penetrate the magnesium oxide coating that tends to build up on the metal and prevents it from reacting. Chloride ions react with the Mg(OH)2 product to form MgOHCl, which dissolves the MgO coating. The role of iron is less clear, but it's thought to cause bimetallic corrosion that promotes the reaction between water and Mg.
Although the heater works extremely well, Lavigne says that there is concern about the hydrogen gas generated in the reaction. "We've always had an interest in eliminating hydrogen from a user-safety point of view," he says. NSRDEC is currently exploring heaters based on calcium oxide and phosphorus pentoxide exothermic hydration reactions, as well as Mg oxidation coupled with manganese dioxide to quench hydrogen generation.
"The challenge is to get at the heating profile that's safe to handle yet capable of heating a food product in a short time and is suitable for use with food," Lavigne notes.
As far as soldiers like Sgt. Gussman are concerned, any further improvements to the MRE are a bonus. "The soldiers who only know MREs sometimes bitch about them," he says. "Old soldiers who remember the canned rations know better."
On the Net: