Extended-Shelf-Life Packaging

Just 25 years ago when aseptic packaging—in consumer-size paperboard lamination and plastic packages—burst upon the United States scene, hundreds of processors, purveyors, packagers, and assorted hangers-on hooked into the concept with all manner of variants—a few valid, but most distortions that could endanger consumers.

Notions prevailed, such as that high-acid foods could not be aseptically processed and packaged (even though the earliest and most successful commercial entities were and are high-acid fruit beverages); peroxide sterilants were hazardous to health (even though billions of packages worldwide had been sterilized by hydrogen peroxide); aseptically packaged foods could not move through refrigerated distribution channels; aseptically processing and packaging of large particulates was ready-to-go (even though the Food and Drug Administration consortium just "approved" the system in 2002); and on and on.

In the midst of all of the misinformation that probably retarded some of the inevitable growth came the counter-rhetoric—from your humble servant as well as his supporters that aseptic processing and packaging meant statistical sterility and that an operator could not be half or 90% sterile. With low-acid foods, the failure of sterility could mean the presence and ultimate growth of pathogenic microorganisms. With high-acid foods, absence of sterility could lead to commercial spoilage. And to confound the facts came the concept of extended-shelf-life (ESL) packaging, which did not mandate sterility. Recitation of this history might be irrelevant except that the garble continues to be heard today.

Defining Terms
So many words and phrases abound in this realm that I shall recite some of the realities of pasteurization and aseptic processing and packaging:

• Pasteurization. Pasteurization is the thermal destruction of microorganisms of public health significance and others capable of spoilage in "normal" channels of distribution. The word has been adopted for ionizing radiation and other preservation processes.

The official definitions of pasteurized recognize the equivalency of several time–temperature combinations in destroying public health microorganisms: 212°F for 0.1 sec is equivalent to 145°F for 30 min, and all time–temperature protocols between. The longer low-temperature treatments are more damaging to the product than the higher-temperature, shorter-time treatments.

Equipment is sanitary, and packages—formerly predominantly polyethylene-coated-paperboard gable-top packages but now moving toward high-density-polyethylene or polyester bottles—are clean but not sterile. Generally, pasteurized dairy products offer a 10- to 28-day shelf life under refrigeration. Pasteurized fruit beverage products are ambient-temperature shelf-stable from a microbiological perspective; other products labeled "pasteurized" may offer any shelf life defined by the offerer.

• Aseptic. Although the word aseptic has meaning in non-food contexts, it is generally confined to independent sterilization of fluid products and their packaging and assembly of the two under sterile conditions. Sterilization means statistical absence of microorganisms of concern—with different standards for high-acid (spoilage) and low-acid (some pathogenic) microorganisms. For dairy products, the process calls for an F value greater than 5.0, coupled with sterile package materials and sterile packaging operations.

The most common aseptic packaging is brick-shaped cartons fabricated from paperboard/polyethylene/aluminum foil lamination, but not far behind are barrier plastic cups, aluminum foil pouches, barrier plastic bottles, and even metal cans. Aseptic processing and packaging imply ambient-temperature distribution because sterility has been achieved. Aseptic products may be distributed under chilled conditions to retard biochemical deterioration.

• Processing. An indispensable element in aseptic, ESL, or cold-fill processing is that product processing and packaging are performed separately from each other. Each is conducted to reduce the microbiological load, and linkages are under near-microbiologically controlled conditions. Usually, processing is of fluids and accomplished in a heat exchanger to accelerate the heat transfer and thus minimize the adverse thermal damage to the product.

Processing operations follow a protocol of sterilizing the equipment, ensuring entry of the raw product into the sterilization area, heating, rapid cooling to arrest further thermal damage, filling sterile product into the package, and closing and sealing the package.

• Packaging. The general protocol for packaging—for any of the several technologies described—is to begin with steam, steam plus hydrogen peroxide, hydrogen peroxide, hot water (for high-acid product packaging only), or other chemical-plus-heat treatments engineered to sterilize the equipment before any product or package enters the system. Subsequent to equipment sterilization, the equipment must be maintained sterile by, for example, constant flow of sterile air produced by HEPA filtration, incineration, or even chemical treatment.

Package treatment to achieve sterility or ultra-clean condition may be thermal—such as steam, hot-water immersion for high-acid products, hot air or radiant heat for low- or high-acid products—or chemical, usually H₂O₂, usually hot. The concentration and temperature of H₂O₂ are dictated by the product pH and the intent: 30+% and 180+°F for low-acid-product ambient-temperature distribution. For high-acid ESL products, a much lower concentration of H₂O₂ at ambient temperature is appropriate.

For many ESL systems—which, by definition, can tolerate a lower microbiological kill—the chemical applied to reduce microbiological count is often peracetic acid (sometimes mixed with H₂O₂).

The key is to expose the entire interior surface of the package—and its closure, for plastic bottles—to the antimicrobial for long enough at the temperature for the microbiological kill required for the objective (low- or high-acid; ambient-temperature or chilled distribution) to be achieved.

When chemical sterilization is applied to reduce the microbiological load on the package structure, it is important to remove the residual chemical (except for steam or hot water) to comply with regulations and because some chemicals such as H₂O₂ can be harmful to product and/or sensory quality.

Subsequent to filling and closing, the package exits the system without compromising the microbiological integrity of the package or the packaging machinery.

• Extended Shelf Life. Even with common industry usage, no official or even generally accepted definition of extended-shelf-life products exists. It seems to be the prolongation of shelf life beyond that of traditional pasteurized products. The objective is to safely increase shelf life, usually by reducing defects, in particular those with public health significance. Because ESL products are not commercially sterile, they contain few competing microorganisms, and the distribution times are long—up to 90 days, during which time pathogenic microorganisms might grow—they present a challenge. According to industry sources, ESL products may contain 0.5–2% defects, largely in plastic-coated-paperboard gable-top cartons.

ESL, sometimes referred to as ultrapasteurized, applies an F = 1.5 or 280°F for about 2 sec, which can be regarded as a sterilization process for the product. ESL packaging operations are conducted under conditions that do not sterilize the enclosure, and the packages are neither sterilized nor necessarily maintained sterile during operations. Because the packaged low-acid products, such as milk, cream, pudding, etc., are not sterile, the product must be distributed under refrigerated conditions, to times of perhaps 30–90 days. This distribution control obviates microbiological growth and prolongs retention of biochemical quality.

• High-Acid Products. When high-acid products are subjected to approximately the same protocols, the product and its internal packaging environment can be sterile, so ambient-temperature distribution is often possible. Today, however, the more common procedure does not necessarily sterilize the package interior or the packaging environment, forcing chilled distribution to retard growth of the few remaining spoilage microorganisms and to retard biochemical oxidations that would otherwise be damaging to fruit beverages. Such protocols, especially when integrated with oxygen-barrier packaging, have generated an entirely new and highly successful category of chilled juice products. Among the original beneficiaries of this system was not-from-concentrate orange juice, which has largely captured the juice business from frozen concentrated and canned.

• Hot and Cold Fill. Hot fill is the filling of hot high-acid fluid food products into packages so that the hot (e.g., 160–200°F) sterile product sterilizes the package and the packaged product is microbiologically stable under ambient-temperature conditions.

In cold filling, the fluid product is treated external to the package to reduce the microbiological load; the package may be treated to reduce the microbiological load; and the product is filled at ambient temperature or below. Both aseptic and ESL packaging employ cold filling. Cold-filled low-acid products must be distributed under refrigerated conditions; cold-filled high-acid products may be distributed under refrigerated or ambient-temperature conditions.

• Ultra-High Temperature and Ultra-Clean. The terms ultra-high temperature (UHT) and ultra-clean are often associated with ESL. UHT signifies that the fluid product is heated in continuous heat exchangers at temperatures well above 212°F for less than a few seconds and immediately cooled to chill temperatures for cold filling. Ultra-clean packaging is performed in environments in which the microbiological load has been very markedly reduced—approaching but usually not attaining sterility. ESL may be either hot or cold fill but does not connote sterility, thus dictating chilled distribution.

Biochemical Deterioration
Although microbiological growth may be retarded by the various pretreatments of product and package, other quality changes are frequent. Application of UHT processes can lead to enzyme regeneration, which results in rapid biochemical deterioration. Even if all biological actions are retarded, biochemical reactions such as oxidation lead to color, flavor, and even mouthfeel problems. Although not directly responsible, ESL, through its downstream chilled distribution, retards the inevitable biochemical deterioration—one major rationale for the application of ESL. Furthermore, the coupling of aseptic and ESL technologies—i.e., incorporating chilled distribution—results in significantly better quality retention for the product delivered to the consumer.

Packaging Systems
A variety of packaging systems have been developed for ESL:
• Systems for gable-top paperboard cartons, from such suppliers as International Paper and Pure Pak (e.g., www.evergreenpackaging.com); Elopak (www.elopak.com); and Tetra Pak (www.tetrapak.com).

• Systems for plastic bottles, from such suppliers as Serac (www.serac-usa.com); Remy-Sidel (now part of Tetra Pak); Procomac (www.procomacusa.com); Krones (www.kronesusa.com); Stork (www.stork-usa.com); AVE (www.avegroup.com); and KHS (www.KHS-inc.com).

• Systems for plastic cups, from such suppliers as Hamba (www.hamba.de); Bosch (www.boschpackaging.com); and ERCA (www.erca-formseal.com).

Most of these systems operate under the technical principles outlined above, (i.e., sterilize the product; treat the package; assemble in a clean or sterile environment; and close the package). Note that the filling with sterile product is not an absolute with ESL, but the heat exchange for fluids is almost always a sterilizing protocol. Although package materials are generally not major sources of microorganisms, they may not be sterilized by most of the protocols, thus leading to the regulatory differentiation of ESL and aseptic.

Typical systems for the various types of packages are as follows:

• Form in Line. Some more-recent systems reflect engineering more complex than for perform-bottle clean/fill/seal systems. For example, the Italian SIPA begins with polyester resin and injection blow-molds the bottles at temperatures that destroy any microorganisms within an enclosed chamber. The sterile bottles are conveyed through a sterile tunnel directly to a sterile filling machine, usually a sterile Italian Procomac, where the bottles are filled with sterile product. This system has been applied commercially for high-acid products in 1.5-L bottles in Europe. Distribution is at ambient temperature, so technically it is not an ESL system. The Procomac system may also be linked to any preformed plastic bottle–making equipment.

• Preformed Bottles. Sidel is a French maker of polyester-bottle injection blow-molding equipment. It offers a system that produces polyester bottles in line and transfers them to the French Remy ESL packaging equipment. The Sidel equipment may form preforms from resin or may begin with preforms that are injection blow-molded in line with the packaging equipment. All components are regarded as excellent representations in their categories, and the integrated system is reported to be operating well commercially for both ESL and aseptic, at least in Europe.

Although the transfer may be clean from the Sidel bottle maker at the Remy machine, the newly blown empty bottles are inverted and sprayed with a mixture of peracetic acid and H₂O₂, then sprayed with hot sterile water to remove the residual sterilant prior to filling with sterile product.

Analogous packaging systems are offered on Serac, Krones, Stork, KHS, and AVE equipment that inputs preformed bottles—from remote factories or from injection blow molders in proximity to the packaging equipment—as the starting structures.

With plastic bottles for chilled distribution, closure should be hermetic, generally translating into heat sealing. Aluminum foil lamination from precut material presterilized using ionizing radiation or cut directly from roll stock after on-line treatment such as ultraviolet radiation or chemical is employed. Some vendors offer aluminum foil built into semi-rigid plastic closures that are treated with peracetic acid/H₂O₂ just prior to induction sealing to the plastic bottle finish.

Most but not all of the ESL packaging systems are rotary turret and therefore capable of producing up to 600 1-L bottles/min at relatively high efficiencies. The filling units are contained within enclosures that are presterilized and maintained under sterile conditions.

The differences between the systems from any of the above organizations are sometimes subtle and sometimes profound, and must be carefully evaluated before selection since all are relatively costly—$5 million or more.

• Plastic Bottles. Depending on the contents and the distribution channels as well as marketing considerations, the bottles may be polyester, barrier polyester, or coextrusions of polyethylene and ethylene vinyl alcohol to enhance oxygen barrier. A challenging issue with polyethylene-interior bottles (and with polyethylene-coated paperboard cartons) is flavor scalping in the prolonged distribution channels.

• Gable-Top Paperboard Cartons. These cartons are received from the converter knocked down and erected on the ESL machine, which is engineered as an enclosed chamber. The open-top cartons are spray treated with the selected chemical to reduce the microbiological load, rinsed and/or heated in some manner to evaporate the residual, then filled and hermetically heat sealed prior to exiting the enclosure.

Gable-top paperboard cartons—all with plastic coating and seams that are skived, i.e., folded to bury the raw paperboard edge, and some laminated with aluminum foil—are used to contain dairy products such as whipping cream, juices, and liquid eggs—all comparatively early applications of ESL technologies.

Offers Further Promise
Without in any way meaning to diminish the importance of cup ESL systems, to address their array and intricacies in this space would not serve them well. Most have been derived from aseptic equipment engineered specifically for less than sterility and therefore for chilled distribution of packaged product. ESL systems for cups have been commercial for many years for products such as refrigerated puddings and fruit beverages.

All ESL systems, regardless of type—plastic bottles, gable-top paperboard cartons, or cups—require some control over distribution. Temperature control in channels, especially at the retail level and in moving from the grocer to the home, is especially important for low-acid products, most of which are not sterile.

We have entered an era of trying to offer consumers a product that is of better quality than can be obtained through conventional canning or aseptic packaging and that is certainly more convenient than frozen product. But as with so many chilled foods, we have increased the risk because the products are generally not sterile. Despite being engineered for long shelf life, the products are under refrigeration, and extreme disruption of this parameter might generate a problem.

Nevertheless, ESL and the total systems integrated with it have been an essential contributor to the major reinvention of the fluid milk, flavored milk, flavored coffee lightener, and fruit beverage businesses around the world. All three industries grew dramatically as part of the chilled foods revolution permeating our food supply chain today. And the technologies that stemmed from the basic aseptic technology that spawned ESL promise to suggest means to deliver particulate and solid foods safely in the future.

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