Developments in Microwaveable Food Packaging

Last year DuPont released a list of the top packaging innovations of the 20th century.
The microwave revolution qualified as innovation number two. Drastically reducing the time required to cook a frozen meal means increased convenience and greater variety, not to mention energy savings. Campbell’s aluminium-replacing CPET trays took home a packaging innovation award in 1986—the first in its category.

Microwaveable packaging has gone a long way. For the month of June I had in mind to write two articles about microwaving in packaging. Not only because recently various very revolutionary microwaveable packaging designs have been developed, but also because of the Symposium of the International Microwave Power Institute to be held in New Orleans this month.
The first article I have in mind will go about the latest development of microwaving packaged complicated  food products. Complicated in terms of its compilation of several ingredients each with its own characteristics to heating.
In my second article about microwaving I want to speculate about some ideas for incorporating microwaving into a packaging as a better alternative to the chemical heating process common in the self-heating packages currently in the marketplace. I will concentrate on solid food as that still is an area, which can’t be covered by the chemical heating process of self-heating packages. The actual self-heating packages only can handle liquids and in my opinion the future for self-heating is in the solid food packaging.

But first the recent developments in microwaving of complicated food products. As said microwaveable packaging has come a long way. From the simple popping of popcorn and the thawing of frozen food we have seen the first packaging designs with a susceptor to equalise  the heating of an unpackaged product.

The latest technology in this area is the MicroRite technology with trays laminated with a polyester film of thin patterned aluminium. The patterned aluminium is used to harness and control the microwave energy, as the MicroRite technology redistributes the microwave energy via a selective aluminium patterning, channelling it deeper into the frozen food for faster more even heating.

But it looks like that this technology already is surpassed by more advanced ones.

Compartment Microwaving
In my article “What’s going on in packaging for ready meals (part 02)”, which I wrote last year, I hinted at the possibilities of compartment microwaving of Shieltronics microwave-shielding technology, which makes it possible to control the intensity of microwaves in a microwave oven so that foods requiring less microwave energy than others can all be prepared in one convenient cooking cycle lasting four to six minutes.

To achieve this, the packaging has a special structure. It’s a two-compartment injection-moulded PP-tray that is filled with vegetables in one compartment and raw or partially cooked fish, chicken, or meat in the other.

The LDPE lidding film is laser perforated in three areas. Two sets of perforations are placed over the vegetable compartment, while the third set is perforated over the protein compartment. The protein compartment is evacuated and back-flushed with a gas mix that prolongs refrigerated shelf life to eight or nine days and the perforated lidstock is heat sealed on the rim of the tray and the wall that separates the two compartments. The end result is an MAP compartment for protein and a separate compartment for vegetables.

Two LDPE pressure-sensitive labels about 10 mm in diameter are applied over one perforation of the vegetable compartment and one over the perforation of the protein compartment. These labels function as valves and open slightly to release some of the internal pressure that builds up when the consumer cooks the meal in the microwave oven. The valve on the protein compartment also ensures that the modified atmosphere environment is maintained.

One of the perforations on the vegetable compartment remains uncovered, to allow the fresh vegetables to respire: oxygen in, carbon dioxide out. Without this controlled exchange of gases, the vegetables would spoil fast.

This technology lets dissimilar foods which require different cooking times conveniently be cooked in a single packaging in the microwave without requiring the consumer to cook them separately.

However the Shieltronics microwave-shielding technology is very interesting and useful as long as the ingredients of the food product (ready meal) can be stored in different compartments. But what to do with a food product that can’t be divided but still is composed of various ingredients. Take a look at a richly-decorated sandwich. Let’s take a Jimmy Dean breakfast sandwich, with its bread, an egg, a sausage and cheese, as example. When you’re dealing with four different components, each of them heats differently and reacts differently in the microwave. It make the problem more complicated because the components are stacked one upon the other and not stored in different compartments. Hillshire Brands claims to have the ideal solution and it looks very interesting indeed.

Heat Fresh Pouch
Hillshire Brands baptised the new development Heat Fresh Pouch, a thermoform-fill-seal package that uses a nonwoven film laminate, made from polypropylene and cellulose, on the bottom of the packaging. The formable top film is a proprietary multilayer structure. Both forming and nonforming webs films are supplied by Curwood Inc.

There are three main things the packaging does. It’s a moisture-equilibrating, self-venting, absorbent packaging for one-step microwave heating, so that the consumer gets the same texture on the bottom as on the top. The nonwoven film is the absorbent component of the packaging.

Each of the components has different water, fat and salt contents and what Hillshire is trying to do is using the steam and driving that to each of the components, so that everything heats up. The consumer doesn’t want one thing overheated and another component still cold.
By venting excess steam and absorbing condensation, the packaging lets the product heat evenly, with each component maintaining appropriate mouthfeel. This is a feat, as the sandwiches include a croissant or biscuit, cheese, egg and sausage.

Hillshire Brands and Curwood combined two proprietary technologies to optimally control moisture, heat, and pressure in the package during microwave heating. The nonforming web, which becomes the base of the packaging, is a two-layer extrusion lamination of oriented polypropylene and a non-woven material. This laminated structure, which has a cloth-like look and feel, has moisture-absorbing properties.

The forming structure is a proprietary coextruded 6-mil ICE R film consisting of a nylon exterior along with a PE sealant. (ICE is an acronym for Internal Coolant Exchange, a proprietary rapid quench process that’s exclusive to Curwood). The two webs, forming and nonforming, essentially create a moisture equilibrating “steam dome” that allows for efficient use of steam while excess steam is absorbed.

During heating, moisture created by the heat cycle rises to the top of the packaging, rolls down the sides, and is absorbed by the non-forming film. This eliminates the problems of soggy or cardboard-like bread. At the same time, pressure is created within the packaging to drive heat into the sandwich core for controlled, even heating of meats, cheeses, and other sandwich components. The packaging is also designed to self-vent so that it doesn’t burst in the microwave.

Steam is getting a very popular way of heating food products and packages with an automatic venting valve are common.  The steam-in-the-pouch packages for frozen vegetables require as much moisture as possible in the packaging. But with a product that contains a bread component, careful control of the moisture so the bread doesn’t get soggy or hard is crucial. While Hillshire Brands leverages the steam dome for cooking, it added the equivalent of a paper towel on the bottom to absorb moisture. What it comes down to is an extraordinarily effective use of steam that makes this sandwich packaging special.

Compared with sandwiches in the old packaging it’s quite an evolution. In the old times,  preparation consisted of removing the sandwich from the pouch, wrapping it in a paper towel, microwaving it on 50% power for 1.5 minutes, flipping the sandwich over and heating it for one minute at full power and finally letting it stand for a minute before unwrapping and eating.

In the new packaging, the consumer puts the pouch into the microwave, paper side down, for anywhere from 80 to 90 seconds. There’s still a stand time, but the consumer doesn’t have to remove the product from the packaging, he doesn’t have to wrap it, or flip it, he doen’t have to worry about 50% power or full power.

Keep the Shieltronics and the Hillshire/Curwood technologies in mind when we discuss in my next article the possibilities of incorporating microwave technology in packaging. The developments in packaging material and the packaging structures are of crucial importance for an eventual packaging with a built-in microwave.