New Food Packaging Polymer and Processing Techniques

The IFT Packaging Division’s symposium on packaging polymers at IFT’s 2000 Annual Meeting was an outstanding event. The following are some of the highlights:
One growing area is the role of packaging in hot-fill-hold processing operations. Long-time academician Ian Britt of the University of Guelph set the stage by detailing the requirements for food and beverage packaging, especially the oncoming gas barrier plastic packaging. The systems approach, which integrates product with process and packaging to achieve food quality retention and safety, is not a new revelation. Amazing, however, is how few recognize the key role played by packaging (and its kin, distribution) in food preservation.

Heat plus its adjuncts, pH, water activity, etc., retard spoilage and pathogenic microbiological growth. Pasteurization is mild heat treatment, usually below 100°C, to destroy or reduce vegetative microorganisms and reduce enzymatic activity; it may be either a complete process or a hurdle in this important new technology. Pasteurization processes may be for aseptic or extended shelf life, using kettles, tunnels, or heat exchangers.

In hot-fill-hold, product is heated in batch or continuous units, and the packages are filled with hot product that commercially sterilizes the container and closure. The target process is 4D reduction of Listeria monocytogenes. Pasteurizing values may be calculated based on initial microbiological counts, pH, and temperatures—so far, fundamental food science.

Packaging considerations include filling and sealing, temperature, package material permeability, package integrity through distribution, end users, and environmental insult. Filling and sealing variables encompass container and closure types; seal temperatures, pressure, and dwell; fill temperature; and vacuum. Temperature performance is governed by processing conditions and the ability of the package to resist temperature (of filling and holding).

Heating and cooling of the product must integrate the effects of the product and its package and demands the engineering of package structures for enhanced heat transfer. Cooling curves for the product after packaging must account for the product thermal transfer characteristics as well as the ability of the package structure to maintain its integrity.

New opportunities in barrier films were discussed. Cryovac’s Nate Miranda provided a comprehensive picture of today’s barrier plastics situation, including some interesting insights. Early barrier films were developed for meat packaging, but as lower-fat foods have become available, flavor barrier materials have become important. To be commercial, regardless of the technology employed, barrier materials must deliver competitive performance for cost.

Typical barrier materials contain a substrate, usually on the exterior, often polyester, polypropylene, or nylon, often biaxially oriented, and sometimes printed. The gas barrier component is typically ethylene vinyl alcohol (EVOH) or polyvinylidene chloride (PVDC) coated on or coextruded with the substrate. Inside is a sealant, often polyethylene or ionomer. EVOHs have excellent oxygen barrier properties, but suffer from deterioration of those properties with increasing moisture. On the other hand, PVDCs, although not as good an oxygen barrier as EVOH at low relative humidities, do not change their gas barrier properties with changing humidity and so are better gas barriers at high moisture.

Of the two major types of nylon, nylon 6 is not as good a gas barrier as EVOH or PVDC, but nylon MXD6 is about as good as PVDC. Among the other gas barrier plastics, polyacrylonitrile (PAN) encountered regulatory problems and, although vindicated, suffers from ancient history.

On a cost-per-barrier basis, PVDC and EVOH are the least expensive barrier plastics offered commercially in significant/m² of oxygen barrier for 1,000 in² of film.

Emerging gas barrier technologies include:
• Nanocomposites. Treated clays dispersed within a polymer matrix create a tortuous path for the passage of gas, and about a 2–3 times gas barrier improvement is achieved. However, barrier properties are inconsistent as a result of variable dispersion and alignment. Cost is approaching that of PVDC and EVOH.

• Liquid crystal polymers. Polymers that form ordered phases in the melt state are extremely good barriers, even at high humidities, but have poor mechanical properties in the transverse direction. They can be very expensive.

• Coatings. Glass or silicon oxide applied as a very thin coating (20–40 nm) can be very good gas barriers that are not sensitive to moisture, but they can be prone to flex cracking and yellowing, and are high-cost solutions.

• Polysilicate. Alkali metal cations such as lithium or calcium oxides are more resistant than glass and form a better bas barrier than PVDC. Cost per barrier is unknown.

• Melamine can be vapor deposited to form a highly crystallized layer that provides a very good oxygen barrier, at an unknown cost.

• Blox®: Dow polyhydroxy ethers and similar products provide a wide range of barrier properties with excellent adhesion to substrates and excellent durability.

• Sealica®: Tetra Pak’s tradename for PET with Blox layer can provide bottles that can be made on existing blow-molding equipment. It should be orientable in film form.

• Oxygen scavengers. These are available in limited capacity, and must be combined with another barrier.

• Metallocene EVOHs should deliver precise properties but are not known to Be commercial at this time. 

Good gas barriers such as EVOH or PVDC are not necessarily barriers to all flavors, since flavors are complex mixtures. A single-component model of plastic barrier is not an accurate predictor of behavior toward flavors: real tests with sensory panels are required. None of the commercial gas barriers is a perfect flavor barrier, although EVOH appears to be better than the others. The position of the barrier relative to the source of the flavor is important: if the objective is to preclude flavor from the exterior from reaching the interior, the barrier is better placed toward the exterior. If the objective is to reduce scalping, the barrier should be toward the inside and in fact, in contact with the contents, is not always feasible. 

It should be evident that no universal plastic barrier exists, and that even combinations do not necessarily lead to an ideal barrier. As we shop through the array of polymers from suppliers, we must be cognizant that these folks are not omniscient. This enumeration by several of our leading packaging experts demonstrates that suppliers are developing materials faster than they can be realistically evaluated objectively. Furthermore, the technologies are sometimes offered commercially before they are truly ready for application. There are many such examples: the relative inability of many of the so-called beer plastics to withstand pasteurization; the concern about adverse odors arising from oxygen scavenger reactions; the limited capacities of oxygen scavengers; the durability issues with glass coatings; and so on. 

As food scientists and technologists know, food protection during distribution is very, but not completely, dependent on proper packaging. Food packaging experts are aware of the multiplicity of challenges they face when they are charged with developing new packaging. Polymer technologists and marketers don’t always fully appreciate the complexities of food needs. One striking example is the long history, still unresolved, of the interaction of flavor and plastics. Simple, one-component models are no longer accepted, and we now realize that flavors are mixtures and do not necessarily react in neat linear fashion. 

The results of this IFT Food Packaging Division symposium should be another very loud call for the close cooperation of plastics suppliers and food professionals in the early stages of development. Cooperation early in the game may minimize problems and perhaps optimize the plastic structures that might more closely approach our ever-elusive perfect barrier. 

We proudly note that IFT and our contentious Food Packaging Division have percolated to the top as the best source for information on the relationship between food and package. The presentations at this year’s Annual Meeting, and at next year’s as well, should be studied and used by every polymer and food packaging technologist interested in advances in plastics packaging technology. 

PATENTS
Selected patents are described below. Patents can be searched or downloaded, free of charge, from www.uspto.gov.

Food packaging for microwave cooking having a corrugated susceptor with fold lines. U.S. patent 6,137,099, filed 5/16/1997, issued 10/24/2000 to G.R. Hamblin, assigned to Pak Pacific Corp. Pty. Ltd. Describes packaging for causing the outside surface of a food product to crisp and/or brown when the food product is heated in a microwave oven. The packaging includes a corrugated sheet of a susceptor material (e.g., metallized polyester) adapted to be wrapped at least partially around the food product. The corrugated sheet may be adhered to a backing sheet of paper or cardboard.

Rigid polymeric beverage bottles with improved resistance to permeant elution. U.S. patent 6,136,354, filed 11/10/1998, issued 10/24/2000 to W.E. Wood et al., assigned to Cellresin Technologies, LLC. Describes biaxially oriented thermoformed beverage containers that enable carbonated beverages to have a substantially reduced concentration of water-soluble materials derived from the containers. Such containers can include a permeant barrier and an active trap for water-soluble materials that can be removed from the thermoplastic by extraction into the carbonated beverage. The improved container material includes a blow-molded thermoplastic polyester web with a compatible modified cyclodextrin material. The cyclodextrin has pendent moieties or substituents rendering the cyclodextrin material compatible with the container thermoplastic. The cyclodextrin material, after it is added to the polymer material, acts as a barrier and traps extractable materials as they permeate through the thermoplastic polyester. The cyclodextrin molecule has a large center cavity having properties that increase the likelihood that organic molecules will be absorbed and trapped in the center pore. The resulting polyester is substantially resistant to any extraction of soluble materials from the polyester material by the carbonated beverage.

Sterilant degrading polymeric material. U.S. patent 6,132,825 , filed 5/12/1996, issued 10/17/2000, to P. Frisk, assigned to Tetra Laval Holdings and Finance SA. Describes a polymeric material integrated with a metal catalyst for promoting the degradation of a sterilants such as hydrogen peroxide and ozone. Examples of possible polymeric materials are PET, COPET, and any mixture of them. The polymer materials are usually configured into containers for aseptic processing. The metal catalyst may be selected from the group consisting of iron, cobalt, nickel, ruthenium, palladium, osmium, iridium, platinum, copper, manganese, salts thereof, oxides thereof, and mixtures. The metal catalyst is less than 10% of the total weight of the modified polymeric material. The metal catalyst may be a plurality of nanosize particles or microsize particles. The metal catalyst may also be applied as a thin film onto a polymeric resin for absorption thereby. The sterilants usually become entrapped in amorphous zones of the polymeric material, which results in higher than acceptable levels of sterilants in the containers. The novel polymeric material allows for application of a sterilant in the vapor phase, and substantially reduces the residence time of the sterilant as a container undergoes aseptic processing, thereby expediting the aseptic process.

Modified polymers having controlled transmission. U.S. patent 6,124,006, filed 5/29/98, issued 9/26/2000. Issued to I.M. Hekal, assigned toCapitol Specialty Plastics, Inc. 

The present invention includes processes and resulting structures for producing a modified polymer having channels. The channels act as controlled transmission passages through the polymer. A polymer is caused to assume a molten state, typically by applying heat and melting the polymer. A channeling agent is blended then reacted into the polymer so that it is distributed within the polymer. In one embodiment, an absorbtion additive is blended into the product so that the additive is distributed within the product. The product is solidified so that the channeling agent forms passages in the product through which a desired compound is communicable to the additive that is entrained within the product. The solidified product may be used to form a desired shaped article such as plug type inserts and liners for closed containers, or it may be formed into a film, sheet, bead or pellet.

Disposable food contact compatible microwaveable containers having at least one micronodular surface and process for their manufacture. U.S. patent 6,120,863 filed 10/8/97, issued 9/19/2000. Issued to C.M. Neculescu , et al., assigned to Fort James Corp. 

Disposable food contact compatible microwaveable containers having one or more micronodular surface are disclosed. These containers, including plates, bowls, cups, trays, buckets, souffle dishes, and lids are prepared from a polyolefin selected from the group consisting of polypropylene, polypropylene polyethylene copolymer or blends, and mixtures of these, mica, and pigment and are thermoformed into the shape of a the aforementioned containers exhibiting (a) a micronodular surface on at least one side of the surface; (b) a melting point of not less than about 250.degree. F.; said containers being dimensionally stable and resistant to grease, sugar, and water at temperatures up to at least 250.degree. F. and being of sufficient toughness to resist cutting by serrated polystyrene flatware.

Resealable closure and method of making same. U.S. patent 6,076,969, filed 12/1/1998, issued 6/20/2000 to R.F. Jaisle et al., assigned to Sonoco Development, Inc. Describes a flexible package that includes a resealable closure for resealing one portion of the package to an opposing portion of the package. The resealable closure is formed by applying a layer of pressure-sensitive adhesive on the inner surface of at least one of the opposing portions of the package, and applying a layer of cohesive to the inner surface of both of the opposing portions so that the pressure-sensitive adhesive layer is covered by a layer of cohesive. The pressure-sensitive adhesive has a greater affinity for adhering to the cohesive than to the inner surface of the package. Accordingly, pressure-sensitive adhesive is detached from the inner surface of the package when the opposing portions are pulled apart, and the portions can be resealed by pressing the portions back together to cause the pressure-sensitive adhesive to adhere to the portion from which was detached.

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Products & Literature
PICKLES being test marketed by Vlasic Foods International are said to be the first food product pasteurized in polyethylene terephthalate containers. The package is a wide-mouth PET jar that withstands temperatures up to 215ºF. The degree of crystallinity in the material is controlled to create a container with a higher degree of thermal stability in the sidewalls. The crystallized finish provides thermal and mechanical stability to ensure seal integrity throughout extreme processing conditions. A stabilized, champagne-style base accommodates the pressure created during pasteurization. For information about the jar, made by the True Heat Set™ process, contact Schmalbach-Lubeca Plastic Containers USA, 10521 S. Hwy. M-52, Manchester, MI 48158 (phone 734-428-4515, fax 734-428-4622)—or circle 327.

TRAY PACKAGING for meat and poultry now includes a superabsorbent soaker pad using a polymer manufactured by Stockhausen. It is currently the only superabsorbent polymer accepted by the Food and Drug Administration for poultry and meat packaging. The pad is manufactured by Paper-Pak Products, and trademarked UltraZap Supersorb. For more information, contact Paper-Pak Products, Inc., 1941 White Ave., La Verne, CA 91750 (phone 909-392-1200, fax 909-392-1732)—or circle 328.

METALLIZED ORIENTED POLYPROPYLENE FILM developed to Nabisco’s specifications for its SnackWell® cookies offers a high-speed sealing window, improved hot tack, and consistent peelable films. The film, manufactured by Huntsman Corp., can be used by a number of convertors of packaging for Nabisco. The food contact surface is based on DuPont’s Elvax® ethylene-vinyl copolymer resin, which has a low activation temperature, so that the metallized laminate barrier is not diminished in the sealing process. For information about the packaging film, contact Huntsman Corp., 500 Huntsman Way, Salt Lake City, UT 84108 (phone 801-584-5700, or www.huntsman.com) —or circle 329. For information about Elvax resin, contact DuPont Packaging and Polymers, P.O. Box 80026, Wilmington, DE 19880-0026 (phone 800-438-7225, fax 302-992-3495, www.DuPont.com/packaging) —or circle 330.

by AARON L. BRODY
Contributing Editor