History

The history of the retort pouch covers initial curiosity-based exploratory experiments with polymeric films, recognition of the potential advantages as incentives to further delve into the concept, and performance of experiments to establish a database for the eventual design and production requirements of the pouch. The earliest formally recorded activity for applying flexible packaging to thermoprocessed foods is that of Hu and coworkers (2,3), which expanded on earlier graduate studies by Hu. Their results revealed that polymeric pouches of high- and low-acid foods could survive water processes using superimposed air to prevent bursting and also pointed out a need for stronger, heat-sealable materials with low oxygen- and water-permeability rates. Wallenberg and Jarnhall (4), showing European interest, confirmed the concept's feasibility. Limited production was reported in Italy as early as 1960 (5). In Japan, the growth of the retort pouch benefited from lack of competition from other preservation methods and from post-World War II development of pasteurized (up to 100°C) high-protein entrées (6).

During this period, up to approximately 1960, many fundamental questions were answered: materials were screened and their performance specifications established; durability of the basic package was confirmed, product formulations were optimized, processing techniques were further refined, and heat-sealing methods were studied and improved. Key contributions were made by film converters who developed and supplied the heat-resistant three-ply laminates. Equipment manufacturers proffered rotary and in-line equipment, ranging from use of preformed pouches to form, fill, and seal operations starting with roll stock film. Thermal processing centered on water cooks with superimposed air pressure.

Near the end of this early period of development, field tests performed by the military revealed that defects at the point of use occurred at a low frequency and were related to manufacturing deficiencies—mostly contaminated or leaking seals, but also body cuts from poor in-plant handling. In other words, if made and processed well, the pouch, as part of a ration meal, performed well. Similar manufacturing problems were reported in northern and southern Europe (7,8) and Japan (9).

The next sphere of activity, from 1961 through 1975, centered on transition of the retort pouch from a pilot plant operation and test production runs to commercial-level production systems. The approaches to success in this time frame universally reflected an acknowledgment that the production-oriented technology—as borrowed from the canning and frozen food industries—existed, and that standard equipment could be used, but that equipment and unit operation performance had to be at a high level. References 6 and 9-11 are reports on ensuing representative manufacturing system developments. If, for example, the pouch forming and filling operations started with roll stock and were performed on a standard intermittent motion machine, at least 14 additional functions need to be performed. A summary of production systems up to 1977 is presented in reference 12.

TECHNOLOGY BASE Materials

During the earlier development phases, the film structure consisted of three plies, built to take advantage of and support the oxygen and water vapor barrier properties of aluminum foil. At that time—the mid-1950s to late 1980s— shelf life of over a year, and for the military, 3 to 5 years, was the given requirement. The three-ply materials consisted of:

1. The outer ply, offering some barrier and strength properties but mainly relied on for abrasion resistance, is usually polyethylene terephthalate (the polyester Mylar) in thicknesses of 9 to 12 An adhesive system has been used to combine the outer ply with the aluminum foil.

2. The center material is dead soft aluminum foil in thicknesses of 9 to 12 //m. This ply is the barrier to oxygen and water vapor passage. Although some pinholes (foil breaks) are inevitable because of the foil's prelamination fragility, calculations of the percentage open area and storage tests both indicate that these breaks have no measurable adverse effect on performance. Neither do these breaks per se permit bacterial penetration; fractures must exist through all plies for bacteria to penetrate into the product.

3. The inner ply is the major performance contributor. It must be suitable for extrusion bonding to foil, where the bond must withstand thermoprocess temperatures as high as 270°F, transportation abuse, and exposure to a wide variety of troublesome food ingredients (lipids, salts, flavor oils) for long periods. It must be heat sealable on basically standard equipment; free of toxic or objectionable components, odor, or flavor; machinable; and capable of retaining its functional integrity at storage temperatures ranging from — 25°F to over 110°F. The first inner food-contacting material used in any significant testing was polyvinyl chloride, adhered by an adhesive system to the foil layer.

Problems with delamination and off-odors with polyvinyl chloride led to two alternatives that prevailed until the late 1970s: a modified medium- to high-density polyolefin called C-79 by its producer (Continental Can Company) and a blend of ethylene-propylene copolymers. These two materials were also combined with the foil by an adhesive (a polyester-isocyanate system) (10). These materials, or minor modifications thereof, were used successfully worldwide.

In late 1974 and into 1975, there was a hiatus in retort pouch development while the U.S. Food and Drug Administration reassessed extractives attributable to the polyester-isocyanate adhesive system. Because of concern over the adhesive approach and the precautions necessary to produce an acceptable material, an extrusion lamination approach was perfected; the current universal inner film is a cast polypropylene that is then co-extruded with the foil.

The three-ply foil laminate was satisfactory where preformed pouches were used or where pouches were made vertically on rotary or in-line form, fill, and seal machinery. Because the successful initial performance characteristics of the pouch were based on products made via such production procedures (both sides of the pouch equally manipulated and/or stressed), the only other film structures used were those without foil and, occasionally, also with a polyamide in lieu of the outer polyester. These other films were for products where a shorter shelf life (8-18 months) was acceptable.

In the late 1980s and into the 1990s, as experience and confidence accumulated with production procedures, distribution, and user performance of the retort pouch, some materials changes were made to accommodate horizontal form, fill and seal equipment, and the greater range of materials requirements engendered by consumer needs. Relative to use of horizontally oriented equipment, the alternate use of drawn semirigid polymer trays and the desirability of microwavability were also pertinent factors. Currently, for shelf life of a minimum of 3 years, such as required for the U.S. military's meals ready to eat combat ration, the cast polypropylene/foil/polyester structure is used for any pouches still vertically formed and filled and as the top or cover layer when horizontal machinery is used. The bottom or drawn laminate in the horizontal mode consists of cast polypropylene as the sealant ply, then foil, and oriented polypropylene as the outer ply. In Japan and impending in the United States is a four-layer material—a polyamide ply will be inserted between the foil and the polyester for preformed pouches or between the cast polypropylene and the foil for horizontal form, fill, and seal operations.

Where a shorter shelf life (8-18 months) is adequate, that is, for practically all applications except the military, the central aluminum foil ply is replaced by a high-barrier polymer, ethylene vinyl alcohol (EVOH). The resulting polypropylene/EVOH/polyester film—on horizontal systems—presents an adequate 18-month shelf life and a microwave reheat feature. EVOH meets U.S. regulatory requirements for direct food contact (13).

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