Fire and ice. Steam and water. Resistance and radiation. Exothermic and endothermic reactions. And now, micro/nano electronics.
Mankind has used heat to enhance and preserve foods since the beginnings of our meanderings on this planet. And cold has been used to preserve foods for many millennia. Invariably, until last century, the delta temperatures have come from independent external sources. Cookfires, fireplaces, and sawdusted ice blocks are legendary in folklore and history. From the onset of packaged foods during the eighteenth century, we have removed the food from the package to heat and immersed the beer or sarsaparilla bottle in ice to chill for serving. Acceptable even today, such actions may be perceived as “inconvenient” by consumers when offered an easier alternative.
The notion of self-heating packages has been with us for some time. Soldiers in the field, campers, truckers, and kids in a hurry have all expressed favor for a package of macaroni and cheese or chili that could be quickly heated to serving temperature for eating on the run, on the bike path, or in the classroom. And self-cooling packaging certainly has appeal when you think of a cool beer on a sunny afternoon at the ball game.
Here’s a look at key developments in the area of self-heating and self-cooling food and beverage packaging.
Exploring Self-heating Options
Exothermic chemical reactions are a perpetual demonstration by high school and college chemistry teachers. When these reactions are captured in proximity to food in packages, the contents on the other side of the package material may be rapidly heated and consumed at serving temperatures within minutes of activating the reaction. Among the better-known reactions applied are calcium oxide plus water and magnesium oxide plus water. The former was/is used commercially for plastic cans of liquid coffee, chocolate, and tea. Water in a separate compartment usually surrounding an inner content-barrier plastic can is mixed with calcium oxide to produce calcium hydroxide in a heat-generating reaction. Alternative anhydrous calcium oxide generates heat of solution. Other chemicals demonstrated to react exothermically include copper sulfate plus powdered zinc.
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The structure of the cans employed commercially is effectively a barrier plastic can within a plastic can surrounded by an expanded polystyrene thermal insulating label. The exterior shell can has two parts—one with water and the other compartment containing the calcium oxide. The consumer punctures the can base, causing the calcium salt to contact and react with the water to increase the temperature and, by conduction/convection, the liquid content temperature in three minutes or so for an 8-oz can.
To assert that the self-heating cans applying this system were either a technical or commercial success would be an overstatement. Among the more obvious issues were the very high cost of the multiple-barrier can within a can; the volume, which led many to believe that much more was contained; the complexity of activation; and other issues that became the subject of litigation.
The military “Flameless Ration Heater” is/was a water-activated exothermic heater in a flexible MRE (meal ready to eat) barrier pouch, which raises the temperature of the contents to 100ºF in less than 12 minutes—hardly rapid, but effective. The chemistry for this system is magnesium metal and water to produce magnesium hydroxide plus heat. Magnesium metal particles are mixed with sodium chloride and iron particles to effect the reaction, which has a much higher calorie value than calcium oxide.
HeatGenie (www.heatgenie.com) is a patent-pending, solid-fuel technology that integrates into food and beverage packaging. To activate, the consumer presses a button at the bottom of the package. The compact modular heat source at the base of the package is about the size of a small tea candle and weighs about 1.3 oz. The result is a cup of hot coffee or a bowl of soup, safely heated in less than two minutes. HeatGenie is small, occupying just 1 oz of the total container volume, leaving 11 oz of volume to be filled with food or beverage in a typical 12-oz can.
The heating element contains aluminum and silica, which, in an intimately mixed powdered state, can undergo a chemical reaction to generate heat. Aluminum can react with a source of oxygen such as silica to release thermal energy through oxidation.
HeatGenie’s technology precisely controls the oxidation reaction to generate heat energy. The button is a thermo-mechanical device that when activated catalytically generates a localized hot spot on the surface of the fuel that starts the oxidation reaction and creates heat. Once the fuel is spent, the heating process stops. The amount and rate of heat generated and released into the food or beverage can be precisely calibrated based on the mix of the fuel in the HeatGenie heater. This is important because the specific properties for a given food or beverage affects its thermodynamic characteristics.
HeatGenie has partnered with can maker Crown (www.crowncork.com) to jointly develop the integrated package solution to be commercialized with leading consumer packaged goods manufacturers.
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There is no need to separate the HeatGenie heating device from the beverage or food can after use. The heating unit components are claimed to be environmentally safe and recyclable before and after activation.
HeatGenie’s approach to self-heating is unlike other self-heating systems described here. Previous self-heating technologies involved mixing calcium oxide or other chemicals with water. With the efficient, solid-fuel system used in HeatGenie, consumers should enjoy a larger serving size of product in a lighter, recyclable package. Heating times are four to six times faster, and the heater itself is eight times more compact than earlier self-heating systems. As an example, a 1-oz HeatGenie heater can heat 8 oz of coffee from room temperature to serving temperature in less than two minutes. HeatGenie will initially target metal packaging, i.e., cans, due to heat conductivity.
The desire for beer, carbonated beverages, tea, and other fluid foods to be consumed at chilled temperatures is nowhere better reflected than in the application of thermochromic ink labels that change color with reduced temperature of beer bottles. Over the years, cooling of beverages from ambient to “optimum serving temperature” has been a target of numerous patents and ventures—each introduced with much fanfare.
Evaporation of water has been engineered into beer kegs to chill and maintain the low temperature. Crown applied Tempra heat pump technology in metal cans: water is bound in a gel. When water is released, it evaporates and chills the contents, dropping the temperature by about 30ºF in about three minutes in a 12-oz can. In the I.C. Can™, an activated carbon desiccant contained within a vacuum draws heat from the beverage through an evaporator into an insulated heat sink container.
The Joseph Company Chill Can (westcoastchill.com) appears to function employing the same or very similar technology. The inner unit is a heat exchanger, in their parlance called an HEU. The coconut shell activated carbon adsorbs carbon dioxide at 10 bar, which is released when activated to remove energy from the contained beverage by sublimation. The technology has been incorporated into a commercial product: Blizzard Energy Drink.
Incorporation of fluorocarbon refrigerants to vaporize removing heat from beverages content was highly effective during the 1990s technically but was stymied environmentally by the notion of emitting Freon—a greenhouse gas—into the atmosphere.
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From Hot to Cold
Despite Tempra’s early claims of reversible heat and cooling mechanisms applying the activated carbon/carbon dioxide adsorption/release system, no known (reversible) implementation resulted. But long before this hydrothermodynamic hypothesis, there was the Peltier or thermoelectric effect: the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, it creates a temperature difference. The applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side.
This effect can be used to measure temperature as in thermocouple sensors in instruments, generate electricity, or change the temperature of objects. Because the direction of heating and cooling is determined by the polarity of the applied voltage, thermoelectric devices are efficient temperature controllers with no moving parts.
Researchers at Intel, Arizona State University, Nextreme, and RTI International have reportedly integrated a thermoelectric cooler into a tiny computer chip. The semi-conductor-based device uses electric current to transfer heat; a static nanostructured thin film with thermal properties better than bulk thermoelectric materials uses electrons to pump heat. Such thin film devices are not ready for large cooling or heating capacity required by packaged food masses yet, but they are useful for small-volume chilling and heating and are far more compact (capable of fitting into food package structures without being visible to the eye).
Thermoelectric heater-coolers will require power—from nano generators or from micro-solar generators—all of which could also be applied to power interactive communications and quality signalers, which will certainly be integral to food packaging in our future. As always, these future developments will be an extension of our past—as witness thermoelectric thermodynamics.
Aaron L. Brody, Ph.D.,
President and CEO,
Packaging/Brody Inc., Duluth, Ga.,
and Adjunct Professor, University of Georgia