The When and Why of Aseptic Packaging

One event must be last on every IFT Annual Meeting program. In 2000, this privilege fell to the symposium, “Aseptic Packaging to Extend Refrigerated Shelf Life,” sponsored by IFT’s Food Packaging Division and Food Microbiology Division and moderated by Gerber/Novartis’ Jairus David and me.

Members who stayed were treated to some remarkable practical insights into clean room operations and the application of aseptic techniques for solid foods. The eight industry presenters who braved the sparse audiences of the last meeting day The When and Why of Aseptic Packaging offered insights into developments of recent food packaging technology—the use of techniques that are usually associated solely with ambient-temperature shelf stability to resolve issues of chilled foods.

The symposium highlighted the innovations that have helped to propel not-from-concentrate juice, milk, pasta, and even sprouts to lead positions in consumer purchases. Why is it engraved into the teachings and texts of food science that the technologies of Ball, Martin, and Rausing cannot be expanded beyond soup and milk with a nod to juice? These giants intended their research results to reach into solids and into the realm of refrigerated foods: just read the literature of the 1950s and ’60s for the revelations that industry pragmatists saw immediately. Aseptic technologies have been employed for most gable-top paperboard cartons and blow-molded polyethylene bottles of not-from-concentrate juices in refrigerated display cases for two decades. The product contents are not sterile, but are “clean” enough that biochemical shelf life has become for these products more important than microbiological.

The program was divided into two components, the first on fruit beverages and milk-based liquids, and the second on clean room with emphasis on solid food delivery.

Tetra Rex’s Ron Swank provided a detailed overview of extended shelf life (ESL), which translates (but not legally) into safely extending the shelf life of refrigerated products beyond that of traditional pasteurized products. Shelf life is extended by reducing defects for low-acid foods to below 0.5–2%, with the target for year 2005 to be below 0.5%. Because ESL products are not commercially sterile, products will spoil if not properly handled. Few competitive microorganisms are present to suppress pathogens during the long time available for their growth. Defects arise from inadequate processing, operating, and maintenance.

Sanitary requirements for traditional pasteurized products are not satisfactory for ESL products, so the levels of aseptic control must be increased. For example, for high-acid foods, process temperature/time for ESL is 75–95ºC for 4–15 sec, compared to 95–121°C for conventional aseptic for ambient-temperature, shelf-stable foods. Protocol is 100–138°C for 2–15 sec for ESL low-acid products and 135–150°C for 4–15 sec for aseptic. Conventional pasteurization at 63°C for 30 min or 90°C for 0.5 sec results in about 28 days of refrigerated shelf life (provided that all other process and packaging elements conform), compared to up to 90 days for ESL processed and packaged products.

To achieve these extensions, equipment must be sterile, packaging equipment must be enclosed, and packages themselves must be hermetic. Prior to operation, the machinery must be disinfected/sterilized to a defect rate of less than 0.1–1% for high-acid products and 0.2–2% for low-acid products. The viability of ESL has been demonstrated by its commercial operation in dozens of sites around the world.

Tropicana’s Kurt Deibel discussed bulk aseptic storage of not-from-concentrate orange juice. The objective is to store sufficient high-quality, single-strength juice during the harvest season to package into retail or hotel/restaurant/institutional units and fill channels during off-season. Critical steps to the operation are deaeration, in-line blending, and aseptic transfer into sterile vessels that are maintained at 33°C. Each pod of tanks is capable of holding up to 36 million lb of orange juice. It is critical to validate and revalidate the process to ensure against loss.

Minute Maid’s Sevugan Palaniappan made a presentation entitled, “ Aseptic Packaging for ESL Chilled Juice.” Options available to achieve ESL for retail orange juice include hot fill, clean fill, and ultra clean. Deterioration mechanisms include oxidation caused by dissolved, headspace, and diffused oxygen; and Maillard and vitamin C degradation. In ultra-clean techniques, the product is heated and cooled before filling—aseptic techniques without reaching the levels required for ambient temperature shelf stability. The defect rate in conventional pasteurization is 0.1–0.01, compared to less than 0.001 in ultra-clean and less than 0.0001 in aseptic.

Equipment maker Serac’s Art Guenter estimated that ten United States dairies were operating ESL to achieve up to 60 days of refrigerated shelf life for fluid milk products. Products such as orange and apple juice, iced tea, coffee, flavored water, and infant formula are packaged on Serac “aseptic” lines. For milk to reach 30 days of refrigerated shelf life, product is subjected to 90°C for 15 sec and packaged in an ultra-clean environment maintained clean by laminar air flow. A refrigerated shelf life of 60 days is achieved by 137°C exposure for 0.1 sec, followed by packaging in an environment under ultrahygienic conditions. Serac equipment employs Oxonia, a 4% mix of hydrogen peroxide plus peracetic acid, at 60°C to reduce the microbiological load on the surfaces of glass or polyester bottles. Closures are aluminum foil, pretreated by gamma radiation, followed by UV radiation after die cutting prior to closure in the machine; screw cap treated with Oxonia; or aluminum-foil-lined screw cap also treated by Oxonia and applied to the bottle finish and heat sealed.

The filling equipment functions in a Class 100 sterile zone with laminar flow to control the environment. An important feature of the filler is an absence of dynamic seals or membranes in the product circuit to eliminate drip. An externally actuated magnetic valve is used on the filler nozzle. Another French equipment maker, Sidel/Remy, was represented by Chris Hoemeke, who offered information on his company’s entry into the aseptic ESL business. Sidel specializes in equipment to blow mold injection-molded polyester preforms into bottles. Remy’s emphasis has been on filling equipment, particularly for plastic bottles. Marrying the two has led to Combi, in which bottles are blown and filled in line to achieve ultraclean operations—the equivalent of the application of aseptic technologies for ESL.

In the Combi system, which is producing PET bottles of Nesquik milk beverage with up to 90 days of refrigerated shelf life, freshly made preforms are exposed to HEPA-filtered air to remove “dust.” The bottles are treated with hydrogen peroxide plus hot sterile (HEPA-filtered) air and obtain a five-decimal-log reduction in microbiological count. Preforms are heated and blown with sterile air into finished sterile bottles. Prior to operation, the filling area is sterilized with hot water under pressure. The filler uses an electromagnetic actuator to prevent recontamination. Closure is with gamma-radiation-presterilized aluminum foil that is treated with UV radiation at the machine. Failure rate is less than 0.0001. Output of the rotary system is about 600 bottles/min.

Hoemeke presented information about Sidel’s new Actis technology that enhances the oxygen barrier of polyester bottles. Excitement has been generated over the incorporation of oxygen scavengers into the core layer of polyester bottles, a result that requires multilayer preform injection molding. But one family of beer bottles coinjects the water-sensitive oxygen barrier, ethylene vinyl alcohol (EVOH), as the core layer. There are external surface coatings such as thermoset epoxy amines offered by PPG and used commercially for beer by Amcor in Australia, and the external thermoplastic coatings from Tetra Pak.

But the most challenging are the internal coatings such as glass from Tetra Pak and Actis from Sidel. Actis is described as a 0.1-micron-thick amorphous carbon deposition on the bottle interior. It is deposited by injecting acetylene gas into the bottle interior and subjecting it to a microwave plasma to reduce the gas to a hydrogenated carbon. Output speeds of the application equipment is about 10,000 0.6-L bottles/hr. Actis-treated polyester bottles experience oxygen barrier improvements of up to 30 times over untreated polyester, to the extent that independent testing by Netherlands’ TNO could not distinguish Actis-treated polyester bottle oxygen barrier from that of glass bottles. Although carbon dioxide barrier was not a developmental objective, the bottles met the criterion of less than 1% loss/month, about six times better than that of untreated polyester.

Although clean room operation was engineered for packaging of solid foods, its initial applications were for beverage packaging. Peter Consitt, principal of Operational Innovations, a specialist in clean room technologies, discussed this topic in detail. In classifying clean rooms, particulates less than one micron size per cubic foot are the measure. Lower numerical descriptors depict cleaner rooms. Normal air contains about one million particulates/cu ft, not all of which are microorganisms. The cleanest rooms operate with positive pressure from the center of the environment. Air may be sterilized with HEPA filters. Just cleaning the environment is not sufficient, since the area must be maintained clean in operation.

Gerber/Novartis’ Jairus David suggested that minimally processed foods such as might be processed in clean room environments be renamed as “appropriately processed foods.” In his discussion on clean rooms, he stated that a room with fewer than 0.1 particulate/cu ft was commercially viable. In the food industry today, processed cheese and yogurt rooms are Class 10000 to help control mold. Other products packaged in clean-room environments include dry milk, wet pasta to achieve 70 days of refrigerated shelf life, and beverages. A process isolator is a small clean room without humans present, since people are “loaded with microorganisms and pathogens.” A process isolator is, in effect, a glorified Class 100 glove box around the processing and packaging within a clean room to eliminate defects.

The final speaker at the Annual Meeting was a courageous Tom Evans of Pacific Pre-Cut Produce, a producer of clover, alfalfa, broccoli, and radish sprouts. The company grows sprouts that would conventionally experience a shelf life of just a few days and be vulnerable to pathogenic microbial presence. Both GMP and HACCP programs are active with third-party oversight. Within the growing room containing closed tanks, ceilings, and walls are all food-grade materials, and the floor is a non-shed aggregate. The air is changed three times each hour, seeds are soaked in chlorinated water, and sprouts are harvested into chilled chlorinated water.

The symposium speakers contributed rare insights into the extensive protocols needed to function in nonsterile foods intended to be distributed under refrigeration. Quality is better retained using aseptic technologies that do not attain sterility and so might be, in some instances, subject to pathogenic microbiological problems. The public health issues have been recognized and are combated with very low probability of ultimate problems in commercial practice. Nevertheless, the specter of a pathogenic incident is always present.

In the future, as the quality retention is enhanced, we will learn more about controlling pathogens, as well as spoilage microorganisms, in “appropriately processed” foods. The information that will improve the aseptic operations for refrigerated foods will probably come from all those industry organizations that have delivered these benefits and continue to invest heavily in understanding how they might make further progress in the realm of ESL.

by AARON L. BRODY
Contributing Editor