La quimica de los alimentos

In the Classroom

The Chemistry of Self-Heating Food Products An Activity for Classroom Engagement
Maria T. Oliver-Hoyo* Department of Chemistry, North Carolina State University, Raleigh, NC 27695; *maria_oliver@ncsu.edu Gabriel Pinto E.T.S. de Ingenieros Industriales, Universidad Politécnica de Madrid, 28006 Madrid, Spain Juan Antonio Llorens-MolinaE.T.S. del Medio Rural y Enología, Universidad Politécnica de Valencia, 46010 Valencia, Spain

Self-heating or self-cooling containers for meals and beverages are excellent examples of chemistry in action for the everyday life of consumers. Such containers consist of dual chambers where the food is usually contained in the internal chamber while the chemical process that would heat or cool the food or beverage occurs in the other. A common coolingsystem consists of ammonium nitrate and water while a common heating system consists of the reaction of calcium oxide in water. The heating or cooling chamber requires the reagents to be separated until ready to use. These hydration methods are simple and do impart heat effects; however, they present some limitations such as heating times of 5–15 minutes (required to generate the necessarytemperature increases) and the need for large amounts of the reagents because of the low heat energy yield, which then must occupy a considerable space in the containers. Emerging technologies are making self-heating foods more accessible to the general public. One such technology is the self-propagating high-temperature synthesis (SHS) that involves the oxidation of a mixture of aluminum and other metalsby iron oxide, Fe2O3. The change in enthalpy of this reaction is greater than 3 kJ/g of reactants, making it more than 4 times higher than the heat energy evolved when limestone reacts with water (1). This technology ensures heating a beverage from 2–3 °C to the boiling point in less than 90 seconds and cutting the heating time for food to less than four minutes. In Spain and other Europeancountries self-heating beverages are known as “autocalentables” and are easily found at gas stations, airports, and highway rest stops. A variety of these products are available including different types of coffee (black, with milk, or cappuccino), chocolate, and tea. In the United States commercialization of these food products has been targeted toward outdoors enthusiasts and the military; however,companies such as Starbucks and Wolfgang Puck are advancing into this market. In the 1980s the U.S. Army took the lead to further develop the technology required to enhance the Meals, Ready to Eat or MREs that were used a decade earlier by the U.S. Space Program. One of these advancements included the Flameless Ration Heater (FRH) that allows military troops in combat to have a hot meal. These MREs,which include snacks, main entrees, and desserts, are now sold through a number of online sites and are available individually or in packages of “emergency supply” or “disaster preparedness” units (2).

These products provide an excellent means to promote interest in chemistry. The activity described in this article uses these commercial products to study the chemistry that produces theself-heating mechanism. Concepts such as stoichiometry, enthalpy of reaction, enthalpy of solution, heat transfer, and density of liquids are the core principles involved in these reactions. Creative ways to use these products may also be discussed. For example, the FRH of the MREs have been used in combat situations to warm intravenous fluids before administering them to patients as deployed medicalunits often do not have means to heat these fluids and by doing so they may prevent hypothermia in patients (3). Methodology We have used this activity with two different methodologies in the classroom: as the foundation for problem-based learning (PBL) and as the framework for inquiry-guided instruction (IGI). Even though this activity could be easily performed in the laboratory, it...