The Return of the Retort Pouch

The retort pouch is alive and well and growing in the United States! And it goes far beyond the realm of the Meals, Ready to Eat (MRE) for the military and the Afghani population, and even beyond the pet food business.

Certainly not yet capturing the entire metal (and barrier plastic) canned food market, retort pouches and trays are gradually penetrating the world of seafood and poultry and baby food and boiled peanuts and self-heating entrees—and probably cannibalizing both cans and glass jars. And intruding into this silently expanding market is another resurgent food package form targeted at nearly the same audience: aseptic packaging of low-acid foods. So, to expand the universe of food preservation to achieve higher-quality food products for consumers, 21st-century food technologists are reaching into our archives, because each of these holistic technologies has been in our quiver, and even in very limited application, for nigh onto half a century.

Thermal Processing
First and foremost, all of this technology is aimed at producing ambient-temperature, shelf-stable, high–water-activity (>0.85), low-acid (pH >4.5) foods with higher initial quality than can be achieved by conventional canning—although some remarkable advances have been made in Appert’s ancient and honorable technology.

Ambient-temperature shelf stability is really microbiological shelf stability, since biochemical changes occur during distribution, disturbing the comfortable myth of “shelf stability” and quality retention. The foods are subject to specific Food and Drug Administration and other regulatory oversight to ensure their microbiological safety, with such issues as temperature–time protocols, hermetic seals, records, etc., under explicit scrutiny. These rigid strictures have accounted for much of the delay in universal commercial acceptance of the retort pouch—as well as its cousins, the retort trays, cups, bowls, bottles, and jars—and aseptic packaging of low-acid foods such as soups, puddings, meats, fish, and vegetables, and ready-to-eat meals and meal entrees.

The notion of food acid level is critical and must be at least stated here, since so many have misunderstood this fundamental: food products with pH >4.5 are vulnerable to growth and toxin production from heat-resistant spores of pathogenic microorganisms. Thus, sterility attained by thermal means requires high temperatures in excess of the temperature of boiling water, thus mandating overpressure, e.g., retorting. Foods with pH <4.6, such as fruits and most beverages derived from fruit (including tomato) may be microbiologically stabilized by time–temperature processes far lower than those required by regulation for low-acid foods, hot fill and processes at <212°F followed by cooling being typical.

In traditional canning, the low-acid food is filled into metal cans or glass jars, hermetically sealed mechanically, and subjected to temperatures in the vicinity of 240°F under pressure to ensure that the slowest heating point within the food reaches a pre-established time–temperature integral. This thermal protocol—required to achieve statistical sterility—usually overcooks the product (unless, as is being increasingly done today, the entering food is virtually raw before filling and the thermal procedure is calculated to integrate sterilization plus cooking). Packaged product is cooled, with the cooling cycle calculated to be included in the total thermal sterilization process.

Definitions
Before continuing, a few definitions are in order:
• Retort Pouch. Basically, a retort pouch is a flexible pouch (or tray, dish, bowl, or cup) used in place of the cylindrical metal can or glass jar. The principles—and the regulations—are the same, but the implementation is much more challenging. The geometry of such packages is flat or oval or truncated conical or almost anything that might emerge from the creative mind of a designer. The material incorporates plastic—and almost every plastic is restrictive in terms of the maximum temperature it is capable of withstanding, with 250°F usually the upper limit and with thermally induced distortion possible at much lower temperatures. The hermetic seal is generally by heat and melt bonding and to date is not demonstrated to be as reliable as mechanical double seaming. And the oxygen barrier of plastic—even in combination with aluminum foil—is not that of metal or glass.

The advantages proposed so many decades ago with the invention of the technology remain today: much reduced time–temperature input due to much higher surface-to-volume ratio to foster much more rapid heat transfer from the pressure vessel into the package interior, thus suggesting higher initial quality. Lighter-weight packaging is an obvious benefit for military rations, where the concept received its greatest stimulus. Easier opening was among the major pluses touted for many years, although the introduction of the full panel easy-open aluminum end on barrier plastic bucket-type cans and the application of easy-open steel ends to metal cans may counter this perception. Retort pouches are modern and fit today’s convenience lifestyle, and marketers relish the larger surface area on which to print graphics to generate shelf appeal.

• Aseptic Packaging. Aseptic has been mainstream for more than 30 years in the U.S. Aseptic describes independent sterilization of product and package and assembly in a sterile environment, to achieve ambient-temperature shelf stability from a microbiological perspective. Aseptic processing is the thermal treatment of the product in heaters that can be engineered to heat and cool the product as it moves to accelerate the heating and cooling; thin films are not uncommon to treat milk, pudding, coffee, ice cream mixes, and other generally liquid products. Aseptic packaging is sterilization of the packaging environment to accept the sterile product, sterilizing the package structure, filling into the package, and sealing. Package sterilization may be by steam, as in the venerable aseptic canning process, or chemical, e.g., hydrogen peroxide, as in the Tetra Pak system.

Because the thermal process is much lower than the terminal process of canning, the resulting product is of much higher quality. And the packaging may be almost any material structure or shape: witness the honorable Tetra Pak Brik rectangular solid, the widely used bulk pouch and bag; the cup of pudding, and—the surprise of 2002—pudding in a tube.

A somewhat more conventional implementation of the low-acid aseptic concept is upon us in 2003: coffee and dairy beverages from the new Morningstar and Dairy Farmers of America installations. Both were accepted in mid- to late 2002 by regulatory officials for low-acid foods and are operating, producing coffee beverages such as Frappucino and chocolate milk in plastic bottles of much better quality than the terminally sterilized, glass-packed products that have become so popular in very recent years. Managed by Stork, Gainesville, Ga., the Dairy Farmers’ operation employs an entire Stork system that includes a spiral tubular heat exchanger to sterilize the product. Bottles are sterilized with hydrogen peroxide and aseptically filled on a four-lane machine capable of outputs of up to 300 bottles/min. So, finally, America has sterile low-acid beverages in plastic bottles, even if, at the moment, some are distributed under chilled conditions to better retain biochemical quality.

• Nontraditional Variations. There are also packaged products that do initially appear to be not retort or aseptic but some nontraditional variation. Plastic cans were developed about 20 years ago, with the notion that they would replace metal. Using oxygen-barrier ethylene vinyl alcohol coextruded with polypropylene, these are double-seam cans hermetically sealed with metal ends and subjected to controlled thermal sterilization. Control must be tighter than for metal, since the plastic is thermally sensitive. The process is virtually traditional canning with some variations to account for the shape engineered for microwave reheating. A 2002 version is the “vertical” Campbell Soup at Hand™ can apparently designed for sipping from your automobile beverage carrier.

The polyester jar for tomato soup represents another “think quality” products: rather than traditional terminal sterilization, the single-strength product’s pH is reduced to below 4.6 and thus hot filled into easy-open, reclosable polyester jars for the convenience of consumers who want microwave reheating.

I would be remiss not to remind readers of Tetra Pak’s award-winning Recart paperboard carton engineered for retorting being used for pet foods in Europe (see the Packaging column, “Thinking Outside the Box: Tetra Pak’s Past and Future,” on p.66 of the November 2002 issue)

• Aseptic Packaging of Particulate Foods. Foods that contain large particles, such as New England clam “chowda,” could not be sterilized by aseptic processing technologies because no one knew the time–temperature history within the particles and so the product could not be guaranteed sterile. A coalition of university, government, and industry professionals under the umbrella of the Center for Aseptic Processing and Packaging at North Carolina State University invested more than a bit of effort to develop a protocol resulting in FDA approval of a protocol for aseptic processing (and by extrapolation, packaging) potato soup. Unfortunately for those in the commercial world, no one in North America has seized upon this opportunity to lead us into the technology, although several products have been in the European market for years.

• Aseptic Canning. Aseptic metal cans have been in the market for more than a half century, containing more or less homogeneous soups, puddings, and, much more recently, milk in aluminum cans, so this variant is hardly new. If any aseptic packaging in plastic cans has occurred, it has escaped my attention—but it is a logical extension.

• Ohmic Heating. Processes such as electrical resistance (ohmic) heating of the fluid contents require aseptic filling. This has been used in the United Kingdom but not commercialized in the U.S.

• Microwave Sterilization. Microwave sterilization is the latest improvement on this ever-promising technology. Because of its ability to penetrate food deeper than conventional conduction/convection heating, microwave heating of foods can be many times faster. But microwave sterilization is still solely a thermal process and so must be regarded by technologists and regulators as another terminal thermal sterilization technique.

By performing the microwave heating within a pressure chamber, however, the requisite temperature of the slowest heating point within the necessarily plastic package is achieved within a few minutes, and so the quality of the product emerging from the process is superb. Whatever happened to the brilliantly conceived and executed Swedish Multitherm process—microwave plus pressurized hot water—that sterilized products such as fish or potatoes in less than 7 min? Classica, Lakewood, N.J., burst on the scene in 2002 with great promise for those who will boldly dare to reach out for the quality-conscious consumer.

Retort Pouch and Tray Technology
The retort pouch is a misnomer, since the name today denotes a flexible pouch for low-acid foods terminally sterilized in a pressure vessel often called a retort. Until someone redesignates the technology, it encompasses all manner of tray, cup, bowl, carton, bag—and any other variation on the cylindrical metal can, although the traditional can is not necessarily a cylinder nor metal any longer.

• Retort Pouch Materials. Developed by and for military rations and derogatorily called MRE by too many, the packaged products are manufactured in the U.S. by a very limited number of “authorized” processors employing the preformed pouches or trays plus lidding from a very limited number of package material converters.

The three major “authorized” retort pouch processors on the North American continent are Wornick, Cincinnati, Ohio, Sopakco, Mullin, S.C., and Ameriqual, Evansville, Ind.; other processors include Masterfoods (formerly Mars) Kal Kan and probably Nestlé, both for pet foods.

The major suppliers of retort pouch materials include Pechiney (formerly American Can), Chicago, Ill., Pyramid, LaHabra, Calif. (imported), Kapak, Minneapolis, Minn, Curwood, Oshkosh, Wis., and Reynolds, Richmond, Va. Sonoco, Hartsville, S.C., announced its developmental entry into the market in late 2002, and Israel’s CLP Packaging, Fairfield, N.J., in 2003.

More significant, the 2001–03 commercial—and apparently successful—entries of retort pouch–packed tuna and salmon are all made from structures fabricated in Asia and supplied by Pyramid, with processing in Thailand or Ecuador, if I read the labels properly. All are subject to FDA oversight of both package material and process—even if made offshore—since they fall under the definition of low-acid foods. And in 2003, Sara lee introduced boneless chicken breast in a standup flexible retort pouch.

Materials are generally structures of polyester/aluminum foil/polypropylene prefabricated by the converter and filled and heat sealed largely on Japanese equipment—probably Toyo Jidoki or Mitsubishi, with others such as Korea’s Hyewon offering new types for solid packs.

• Retort Process. Pouches are carefully laid on racks to maximize the heat transfer and minimize potential damage to seals or foil structures within the retort; this is usually, but not always, accomplished by water immersion. Retorting heat-sealed pouches is a delicate balance between effecting heat penetration and avoiding fusion heat-seal bursting. Retort pressure is controlled very carefully to counter steam pressure developed within the pouch from the heating and the residual internal steam pressure during the cooling portion of the cycle. Among the retort engineering firms which have ventured into the pouch sterilization field are FMC Corp. and Stock America, Grafton, Wis., as well as Japan’s Hasaka.

Despite glorious pronouncements, output speeds from the preformed pouch/fill/ seal machines is slow—one deterrent to rapid cannibalization of traditional cans. And the indispensable seal inspection has not yet been obviated by any of the many “automatic” devices, meaning that visual inspection is required to minimize the probability of seal defect and content contamination.

• Retort Trays. During early development of the retort pouch, many of us thought that the true path should be the tray because filling could avoid seal contamination—a challenge for the pouch, since the filler nozzle moves past the seal area—the tray seal area was always flat, thus increasing the probability of a reliable seal; and consumers tend to eat from dishes not unlike trays and not from pouches. So several organizations developed and introduced the retort tray. Remember General Foods’ extensive flirtation with Impromptu, whose “only” flaw was that the contents tasted like canned food? And the Campbell Plastigon barrier soup bowl that was so beset with problems that it is now a classic Harvard Business School case study cast upon MBA students?

Hormel took a different route employing custom-engineered automatic equipment from FMC Corp. and a marvelous sealing mechanism marrying amorphous and crystalline polypropylene that would bond throughout processing and distribution and fracture upon stress during opening. The result was the Top Shelf brand that is still in Hormel’s portfolio in the form of its Dinty Moore brand of beef stew and other meat products.

If you stroll down the baby food aisle of America’s supermarkets, you might note a paperboard folding carton bearing the name Gerber Lil’ Entrees™. Inside is a compartmented tray containing a low-acid meat/poultry entree in one compartment and a fruit dessert in the other. The concept for toddlers is an ambient-temperature, shelf-stable, easy-to-microwave-reheat, high-quality food for mom to serve and junior to eat. The tray is thermoformed multilayer barrier plastic with an all-plastic-barrier, flexible, peelable heat-seal lidding. Packaged on custom-engineered, preformed tray deposit/fill/seal equipment in the Wornick plant, the product is retorted and inspected, then quarantined for a period to further reduce the probability of a contamination.

Despite a paucity of manufacturing sites for retort trays, the time might be right for them. Several pet foods have been successfully marketed in barrier plastic retort trays for more than ten years and have demonstrated reliability, and a range of consumers are accepting the retort tray concept, just as they are accepting retort pouches, which seem to increase in number by the month.

• Shelf Life. But visible success does not signify absence of challenges. These include seal reliability, output speed, cost, constant regulatory oversight, some suppliers’ reckoning that the numbers mean easy entry, and the secret that biochemistry alters the products during distribution. Consumer sensory analyses published by Soldier Systems Center, Natick, Mass., show color and flavor deterioration on many of their dozens of retort pouch products within months—and this in a context in which three years of “shelf life” is the target. Some argue that no such change occurs, but all of us who have witnessed the phenomenon know that the military is sponsoring research on active packaging, with one objective being to obviate the adverse oxidative effects and better delight the troops with food.

Looking to the Future
Low-acid foods may be rendered microbiologically safe through a range of thermal processes in and out of the package. Reduction of the cumulative thermal input has been a direction that delivers higher-quality foods. Aseptic packaging and retorting in new configurations have afforded technologists the opportunity, but none of the newer technologies is free of issues that should alert the managers that dealing with low-acid food preservation is not a casual enterprise.

Retort pouches have been an available technology for many years. But they have not yet been proven to be as reliable as the old-fashioned ways, nor to finally deliver to consumers all they are capable of doing. With more research on the visible problems, they will achieve even more than their original expectations.

by AARON L. BRODY
Contributing Editor
President and CEO, Packaging/Brody, Inc.
Duluth, Ga.