Has the Era of Retort Packaging Arrived?

To understand a column on the previously much misunderstood retort pouch and tray and their recent analogs, it is necessary to define and describe as many relevant pieces as can fit. This is especially true because such data is in short supply among many within the industry.

Low-Acid Foods
Retort describes a mechanical means to elevate food product temperature to above 100ºC in order to facilitate the in-package thermal destruction of microbiological spores and thus effectively sterilize the contained food product. Also known as autoclaves and pressure cookers, retorts have been used widely for more than a century in the food industry for post-fill thermal processing of canned foods.

Retorts, or steam or water pressure vessels, are applied for treatments of low-acid foods in which low acid is defined as having water activity of 0.85 or above and a pH of 4.6 or above. Such foods may contain pathogenic microbiological spores capable of surviving temperatures above 100ºC so sufficient heat must be applied to ensure the statistical absence of such spores or their vegetative offspring during ambient temperature distribution of the heated and cooled product.

Cylindrical metal cans are the most common package for thermally sterilized low-acid foods, but during the past fifty years, other package structures such as pouches, trays, bowls, and cups have been proposed, developed and commercialized, and virtually all have been prefaced by the word retort to describe their applications.

High-Acid Foods
Another class of wet foods that is canned includes those that are high acid, with a pH below 4.6, mostly fruit and tomato products, usually devoid of pathogenic heat-resistant microorganisms. These are often microbiologically stabilized by heat but at temperatures below 100ºC. A common technology for foods is hot fill, meaning that the product is heated to temperatures sufficient for killing heat labile “high-acid” microorganisms and subsequently filling while still hot into metal cans or other packages such as glass or plastic bottles (carefully selected for temperature resistance), pouches, bowls, cups, etc.

--- PAGE BREAK ---

The entering food functions as a thermal sterilization medium, destroying airborne and surface microorganisms. Thin-wall, two-piece aluminum cans are often employed today to contain hot-filled products. Such products are filled with the simultaneous application of liquid nitrogen, which serves primarily to expand as it converts to gaseous form and to provide internal pressurization and render the can walls rigid—a physical stabilization engineering action. Being effectively inert, the nitrogen gas displaces air inside the can and thus biochemically acts as a counter-oxidation mechanism. Analogous counter-pressure technologies are applied for hot-filled plastic bottles that would otherwise collapse under internal vacuum. Some high-acid foods receive additional heating post-fill but not under atmospheric conditions, and so the word retort is not usually applied to hot-fill or low pH food packages even though they are the same as retort packages.

Aseptic Processing and Packaging
In very recent years, aseptic processing and packaging technologies have been applied for both high- and low-acid liquid, fluid, and particulate-containing foods, with one interesting result being that some of the packages are incorrectly described. Some of you who read the Packaging column in the February issue of Food Technology may have been somewhat confounded by the references to microwave sterilization, some of which were for post-fill and some of which were specific to aseptic. Obviously, boundaries between retort, hot fill, and aseptic—especially particulate—have been blurred so that the words retort, canned, and aseptic must be more carefully applied lest the scientist, technologist, and engineer err and cause a major problem such as some of those that have occurred but have not been publicized to date.

Retort Pouch/Tray Packaging
The term retort pouch/tray appears to describe post-fill sterilization of low-acid particulate foods in a package other than a cylindrical steel can. The traditional package for ambient temperature shelf-stable foods with high water activity has been a steel can or a glass container since just after the time of pioneering food scientist Nicholas Appert two centuries ago. Rigid steel and glass met the food requirements as described by the pioneers—heat resistant, more or less chemically inert to the food contents, relatively (for the period) lightweight, capable of pressure/vacuum containment, resistant to physical abuse, and, with some difficulty, accessible. About 50 years ago, aluminum foil lamination pouches were introduced as substitutes, and about 30 years ago, the notion of barrier plastic was proposed for many reasons. For example, because they are lighter weight, flexible pouches should be much less expensive than rigid containers as well as easy to open. Also, because of the much larger surface-to-volume ratio, much less heat is required to achieve sterility. Marketing professionals were enamored with the large surface areas for communicating product information on the store shelf.

Why then didn’t the retort pouch capture any significant share of the canned food market throughout the 1960s, 1970s, and 1980s? Among the challenges were (and, to a great extent, still are) the enormous multi-billion dollar investments can-making companies had in can-making equipment; the enormous investments food processors/packagers had in can filling and closing and thermal sterilization equipment; major regulatory issues; and the fact that the cost of steel cans (and glass jars) was below those of retort pouches, especially adding in the cost of protective secondary packaging that was mandated at the time. In addition, shelf life, however measured, was less than that of metal cans and glass jars and retort pouch filling equipment was much, much slower and less reliable than canning machinery.

Because of advantages of weight and profile, opening, and scrap packaging, retort pouches seemed better suited for specialized markets such as camping, military, and humanitarian aid, not to mention space travel. And so was born the linkage of retort pouch and its descriptor MRE (meals, ready-to-eat) for military field rations, not a booster for mainstream consumers.

--- PAGE BREAK ---

Barrier Plastic Structures
Meanwhile, the concept of barrier plastics (plastic materials that could slow the passage of oxygen as did steel and glass) emerged with polyvinylidene chloride (PVdC or saran) and ethylene vinyl alcohol (EVOH) oxygen barrier plastics. To compound the confusion, crystallized polyester, silicon oxide, cobalt-catalyzed oxygen scavengers, and liquid crystal polymer oxygen barrier structures also emerged in the marketplace. Steel food cans would be usurped by the 21st century because of the multiplicity of alternatives that could function nearly as well as the steel can and glass jar, or so stated the soothsayers of 25 years ago. By combining the barrier plastic material with structural polyolefins in can or can-like shapes, plastic could provide all the food protection properties required, went the litanies. Into the mix during the 1980s popped bucket/bowl-shaped cans—co-extrusions, co-injections, and melt-to-mold multilayer structures of moisture barrier polypropylene and moisture-sensitive EVOH, ultimately engineered for microwave rethermalization and similarly constructed steel can clones.

Multi-Material Structures
In the beginning of this millennium the notion exploded that materials and structures need not be single materials processed through traditional equipment. Combinations of plastic with other materials could be employed. Such structures had no imperative for thermalization in static, rotary, or hydrostatic retorts.

Alternative heating technologies could be applied with satisfactory results. For example, microwave energy sources using either 2,450 OR 915 mHz frequencies and agitating and reciprocating pressure cookers were the new mechanical heat applicators. And the traditional cylindrical shape was not sacrosanct: trays, cups, stand-up pouches, bowls with handles, bricks, and the like could be sterilized for ambient-temperature distribution. And edging the entire post-fill/hot-fill were the newer versions of aseptic processing and packaging: baby foods, particulates, and thick fluids such as puddings. But the major thrust for the retort pouch/tray technologies came from pet foods with unit-portion wet cat and dog food being packaged in pouches and small barrier plastic trays—by far the largest-volume applications and eminently successful commercially.

From offshore—where the retort pouch concept has been commercially implemented—came millions of aluminum foil lamination pouches of seafood in both flat and stand-up formats to complement the mainstream flat steel can and to spark sales in this category. Also from offshore came silicon oxide coated stand-up flexible pouches of cooked rice in all of its many variations, using the glass coating to provide visibility for the contained product. And during the past few years, from a handful of contract processor/packagers came domestic sources for retort pouches and trays.

Where Retort Packaging Is Headed
Steel cans have not yet been disrupted by retort pouches and trays, which appear to be yet limited to a select few products, but the innovations are growing in stature and unit volume as technologists, marketers, retailers, and consumers have begun to accept them. It should be disturbing to food and food packaging scientists and technologists that the driving forces for these categories are from offshore sources—that U.S. universities and food industries are not scaling up or updating their laboratories, pilot plants, and production facilities to accommodate for retort pouches, trays, and their ilk. It might even be upsetting to our profession that so much of the change is being derived from aseptic processing and packaging technologies—and that from overseas. Despite the undeniable limitations of retort plastic packaging, the technologies are generally sound and economically viable and could be implemented not just as alternatives to steel cans and glass jars but as mechanisms to delight retailers and consumers with new food products in safe, high quality, and convenient formats.

Aaron L. Brody, Ph.D., Contributing Editor, President and CEO, Packaging/Brody Inc., Duluth, Ga., and Adjunct Professor, University of Georgia ([email protected])