Warming up nicely

14 March 2005 (Last Updated March 14th, 2005 18:30)

Dr Aaron L Brody of Packaging/Brody Inc assesses the development of the technology used for the packaging of microwave and enhanced microwave heating appliances.

Since their inception during the mid-1940s, microwave ovens have undergone profound changes, in economics, popularity, functionality and now relationships with analogous devices.

Originally conceived as a replacement for the conventional conduction/convection/radiation ovens and ranges, the then-expensive microwave ovens were shown to be technically better suited to reheating aqueous foods, particularly hot water for rehydrating instant coffee.

When the price of the key magnetron component plunged and microwave ovens became widely available during the 1970s, two new industries flooded the marketplace with their offerings: microwaveable foods and microwaveable packaging.

Foods were engineered initially for cooking. ‘Clever' foods, such as ice cream sundaes, jam-filled pastries, brownie mixes and par-fried chipped potatoes were introduced with apparently little regard for whether or not they actually could be cooked in widely varying microwave environments. Surviving beyond the basic reheat categories were foods that could be cooked, such as popcorn, hand-held pastries and some variations of pizza, but few others.

Materials

Microwave packaging has had a chequered history: from being virtually non-existent to an entire market explosion. The initial thrusts were aimed at ‘dual ovenability' a compromise that acknowledged the singular properties of microwave and conventional heating technologies and attempted to accommodate the two in single structures.

In retrospect, the offerings appeared to be a case of suppliers promoting their own materials to try to penetrate the markets, with little consideration for the technical requirements. From this came materials, such as polyester and acrylic-coated paperboard and pulp trays, thermoset polyesters, crystallised polyester, nylon 11, polysulfone, polymethylpentene (TPX), polystyrene copolymers and microwaveable aluminum foil.

Susceptors

Almost simultaneously with the rush to dual ovenability came the stampede to susceptors, the semi-conductive materials that absorb microwave energy and increase their temperatures to levels that can generate surface drying, browning, crisping and crusting desirable for baked goods, deep-fried items and meats.

Among the array of susceptors offered were thin coatings of stainless steel, ceramics, carbon, alloys and aqueous salt solutions on polyester and paperboard. Additionally, patterned depositions were proposed to more uniformly disperse the energy, and thus the final heating.

However, the migration of compounds formed at the high temperatures of susceptors had not been previously measured for safety. Since that fateful period when regulatory authorities measured the effects of migratory compounds and these and other alleged hazards were found to be imaginary, foods heated in microwave environments have been generally regarded as safe to consume. The fact that only vacuum-deposited thin film aluminum on polyester film (laminated to virgin paperboard) exists today undelines the technical and economic challenges of marrying the basic susceptors concept to practical packaging.

Shift into today

Now that the initial hype for microwave packaging (and foods) has subsided, more rational views derived from an understanding of the fundamental engineering principles prevail.

Consumers, whether retail or food service, have classified foods according to their ability to be heated by microwave energy and have engineered foods and their packaging accordingly. Consequently, microwave food heating and packaging are comfortably evolving in their own niche, with the reheating of prepared foods, and new products arising from synergies with related technologies.

Electromagnetic interaction

To comprehend the role of microwave energy in food preparation requires an insight into the science. Microwaves are a portion of the electromagnetic spectrum closely related to radar, television, and visible and infrared waves.

As such, they are not capable of exciting nuclear reactions and so are not radioactive nor can they induce radio-activity to allay those misconceptions. The effect of the 2450MHz waves (the major frequency permitted by regulation) is solely absorption by polar molecules such as water and conversion of the incident energy into heat.

Contrary to popular belief, microwave energy does not heat from the inside out. Rather, about half of the incident microwave energy penetrates an absorbent material to a depth of nearly a half inch – losing about half of its energy in this distance. It then loses half of the residual in the next half inch or so, and so forth.

Efficiency of energy conversion into heat is high – nearly 100 per cent. Thus, heat is generated below the surface of the food greatly increasing the rate at which the food temperature increases. Thermal energy may then travel internally by conduction and convection, supplementing the effect of the incident microwave energy.

Electromagnetic Radiation

Variations in the food alter the absorption: salt or ionic concentration, heterogeneity, lipids, ice (which absorbs at a lower rate than liquid water), corners that tend to concentrate the energy, and surfaces where water heats without evaporation and thus precludes high temperature browning and crisping reactions.

Therein lies some of the major challenges of microwave heating: each food component has its own singular absorption characteristic so that each area within the normally heterogeneous food heats differently than adjacent areas – one reason why homogenous liquids are so well heated in microwave fields.

Cooking with microwaves

As cooking involves sequences of chemical and physicochemical changes initiated and continued by heat, careful thermal inputs are often required to ensure the time and sequence to effect the actual changes. (In reheating, only rapid temperature increase is necessary.) Special thermal reactions require heat inputs at times and retardation of heating at other times.

Thus the timely inputs of below boiling temperatures is required in the mass (via microwave energy, often for periods longer than for reheating), high temperature in the surface (through susceptors), and of retarding overheating the regions that receive excess energy – for example, corners by radiation reflectors for part of the heating sequence.

Of the array of foods intended to be consumed hot or after heating, some benefit from microwave cooking: vegetables – where the heat is intended to soften tissues without overcooking (which damages flavour and colour; seafood (without surface crisping); stew meats; and sauces. By reducing the total thermal input with microwave energy, higher quality can be achieved than with conventional heating. In such situations, the incident microwave energy converts to heat in the aqueous environment and even produces steam, which adds to the rapid heating when the product is cooked.

Microwave Packages

The same principles of heating the water component of food and generating steam is operative in the reheating of many foods such as: water for coffee or rehydrating soups; chilled ready meals; and prepared dishes. In the old-fashioned microwave heating methods, the food may be removed from its package and rethermalised in a microwave-safe receptacle.

In today's convenience environment, the package as well as the heating container and the eating dish are the protective medium. In each instance, to achieve the low-standard one to three minute time for heating, package materials must be microwave transparent, resistant to steam temperature, fat resistant, especially at elevated temperatures, and offer no adverse flavour when heated. The materials must be engineered into structures that have high surface to volume ratios to maximise microwave energy input.

Although many microwave transparent package materials, such as glass and coated paperboard, might be applied, the commercially available materials meeting these criteria are polypropylene, multilayer barrier polypropylene/ethylene, vinyl alcohol, crystallised polyester and polyester-coated virgin paperboard. Selection of materials appears to be governed by economics and marketing since the technical differences are comparatively insignificant.

Microwave package structure

Structures are important, with flat trays, especially those with dished-in bases, being desirable. When the food is for cup consumption, the shape should include as much added surface as possible, as tapering and adding undulations to make clear the relatively difficult-to-heat pure cylindrical shape of cans, jars or cups.

Food-packaging technologists and food scientists must coordinate their efforts since the food formulation may have to be altered to accommodate the rapid heating time requirements of marketing and the package – for example, in the dashboard-dining soups where particulates have been downsized for ease of heating and sipping from the cup-shaped multilayer barrier can. Heating when the contents include two or more different components, such as macaroni and cheese in one compartment of a retort tray and apple sauce in the other, is yet to be achieved.

Microwave complements

Microwave heating must be complemented by concomitant conduction/convection/ internal radiation. Furthermore, the generation of steam markedly increases the rate at which the temperature rise occurs, but, of course, differential steam pressures within the food often lead to ballooning of package materials, bursting and physical movement of the package while the food is exposed to the microwave energy.

This potential for explosive force leads to the need to release of steam pressure. In contemporary microwave packaging, incorporation of mechanical or melt-down pressure release during or after heating, and self-venting, enhances the heating effect while reducing the bursting hazard.

Product geometry

The need to increase the food surface area for microwave energy to penetrate the food during the brief exposure time is critical. The ideal shape for microwave heating is spherical since it offers the greatest surface-to-volume ratio, with the toroidal shape being a good alternative. Thus, many microwaveable food packages are engineered with concavities in the base, or instructions that include spreading fluid food components into specific patterns on the solids prior to microwave heating. Examples of these geometries are found in bucket-type plastic cans and trays for refrigerated meals.

Microwave reflectors

At the opposite end of the electromagnetic spectrum, electrical conductors, such as metals, reflect microwave energy, hence the need to avoid metal packaging in microwave cavities. When the mass of metal is small relative to the mass of food, the metal's contribution is trivial and can be effectively discounted.

Food packaging engineers have occasionally taken advantage of the reflective properties of metals to use these materials in structures to block the entry of microwave radiation – as in corners or appendages (edges of meat, fish or poultry) to reduce the heating and thus preserve the desired moist heating effects.

Distribution within cavities

Further complicating microwave food heating is the distribution of microwave radiation within the oven. Here, microwave energy patterns are irregular, resulting in modes of high and low energy partially obviated by multiple reflections from the metal walls. A rotating stirrer (fan) is intended to disrupt the incident waves and increase their uniformity in three dimensions, but the patterns have not yet been engineered for total uniformity, so regions within each cavity will still contain high and low energy. Another way to aid distribution of energy is the deliberate movement of the food on turntables.

Too many of these technical issues were either little known or ignored during the early years of microwave oven popularity, leading to a general malaise of microwave foods and packaging during the 1990s. Gradually, most of the challenges are being addressed from interesting perspectives.

Intelligent Heating

To compensate for the paucity of energy uniformity in microwave cavities, several electronic engineers have developed intelligence in which data from the package is read into the microwave oven so that the energy level and time programme is tuned to the combination of food and its temperature and geometry, oven geometry, configuration and age.

Thus, frozen food may receive a thawing programme – pulsed half energy – followed by heating to elevate the temperature after the mass of ice has been eliminated. Chilled ready meals might receive pulses of full energy depending on where the package is placed within the cavity. And popcorn would receive full energy until all the kernels have popped and signal that information to the sensor.

New heating technologies

Engineers have enhanced ovens to offer microwave and other forms of heating that deliver even faster temperature increases than pristine microwave energy. Some of the newer food-heating technologies suggest that they could replace microwave heating in the race to be fastest.

Combination forced-air convection with microwave heating greatly increases the speed of heating and has the ability to brown and crisp. Combinations of infrared radiation and microwave heating produce rapid heating with surface effects. Impingement – high-pressure jets of hot air on the food surface – coupled with microwave heating, is another effective variation on the theme of conventional plus microwave energy input to accelerate the temperature increase.

Most of the newer multi-heating technologies override some of the problems and are more effective than single energy sources to achieve rapid heating – and even cooking. Not yet mainstream for domestic ovens, such innovations are common in food-service applications.

It is evident that the food packaging has not yet been engineered for heating directly in the package in such combination ovens. Enclosing or sealing the microwave cavity and permitting steam pressure build-up increases the rate of heating, but with no surface effects.

Food of the future

More intelligent approaches are being applied to microwave food heating and packaging. Rather than one cycle and one package material and structure being imposed on all foods, recognition that each food is unique has led to a multiplicity of configurations to reach the goal of convenience and speed. Intruding into the mix is the new array of food-heating devices – many of which are being engineered to function in synergy with microwave energy and thus accomplish what one alone could not.

That the packaging for the new heating generation has not yet been developed represents an opportunity to meet a new consumer need. Assisting in the potential for growth is the application of electronic sensing and computer control not only to obviate previous problems but to anticipate future consumer issues, such as initial food temperature, eating schedule, food weight and moisture content, nutritional value after heating, and perhaps even feeding back information to the package developer about the package's structure.

This is the shape of microwave packaging to come.