Packaging Challenges with High Pressure Processing

Ultra high pressure (UHP) or high pressure processing (HPP) is essentially a nonthermal process for food preservation. The process has been known and studied since the nineteenth century, but only during the past 30 years has it entered the mainstream of food science and technology.

In this process, food is subjected to elevated pressures of up to 6,000 atmospheres—or 87,000 psi—with or without any heat to achieve a microbiological inactivation mechanically by disrupting the cells. Additional effects of ultra high pressure include destruction of insects and their eggs, and occasionally a very small retardation of some enzymes. 

Advantages of ultra high pressure processing of foods include instant uniform treatment of the food independent of package size or shape (with some restrictions such as metal-containing package structures discussed below), food product composition, structure, or particulate size and shape. Because of the minimization of heat, much benefit is accomplished at ambient or even lower temperatures; the flavor, color, and nutritional characteristics are not adversely affected, and some major food preservation effects are achieved. Those benefits are fully recognized, but despite all the advantages, a perfect food preservation system has not been achieved.

Microbiological spores including the dreaded pathogens are not eliminated by UHP alone. For practical purposes, enzymes are not really affected by UHP but, of course, may be controlled by heat accompanying UHP. Cell permeability and structure may be affected; temperature control may be required to keep the foods cool during processing because of compression heating. To date, the process is batch—intermittent batch—which is awkward, although some semi-continuous operations are extant. Batch processing is not exactly unknown in food preservation. After all, retort processing is often batch, and some drying and freezing processes are hardly continuous.

From a packaging perspective, when operated as a pre-fill process, rigid packaging, e.g., metal and glass and even semi-rigid plastic that does not transmit pressure, is not suitable. Some UHP processing has been set up as post-fill, meaning the product is UHP processed continuously and after treatment is aseptically filled into pre-sterilized packages. It is not an easy task, but still possible with liquids and is applicable to high acid products. Some operations are even established with the product in an open container to achieve the pressure levels desired, and the package is hermetically closed after treatment.

Capital equipment and its operational variable costs are not inexpensive, and to date, that has been a significant deterrent to more widespread application of the technologies associated with ultra high pressure processing.

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During the UHP process, water, and therefore the aqueous product, may be compressed up to 15% and compression heating is about 3° C per 100 MPa. Thus, temperature control is desired to avoid adverse heating effects. High pressure may lead to a shift in water pH toward higher acidity, which may be a desirable result. 

High pressure causes proteins to unfold as a result of pH, temperature, and water activity of the surrounding liquid, thus altering some of the food’s characteristics or delivering some benefits such as easy opening of shellfish, which is one of the major commercial applications of this technology.

During ultra high pressure processing, in general the product is inserted into a semi-rigid or flexible package structure, which is sealed and immersed in water, which transmits the pressure through the package structure into the product and simultaneously produces an inherent adiabatic heating. The next step is holding the high pressure for a finite period followed by decompression, which results in adiabatic cooling. Thus, the package structure must maintain hermetic closure—gas, moisture, and microbiological barrier—throughout the time frame of the process and the subsequent distribution. The package structure must be resistant to water, especially under pressure, and also must be able to resist the volume change during compression. If the process is one of pulsed treatments as is sometimes suggested for greater preservation, the package structure is subjected to more tortuous conditions.

Packaging and Processing Interactions
“Many [package] structures have been tested using aluminum foil or polymer films coated with aluminum, silicon dioxide (SiOx), or aluminum oxide (AlOx). Most of the packaging structures have been tested with no relevant changes caused by the HPP treatment. The packages showed sufficient degree of flexibility and resilience to compensate for the reduction in volume, no significant changes in tensile strength being observed between control and HPP treated samples,” according to Gavala et al. (2009).

“Heat seal strength is a critical point in packages because if any void is present or generated by the treatment along the seal of the package, the safety of the packaged food will be seriously compromised. In general, thermoplastic materials have been proven to withstand HPP without significant losses in seal-ability,” the authors continue.

“High-pressure induced delaminating has been observed in flexible packages made of composite materials and having relatively high headspace volumes. Gases reduce their volume during HPP treatment much more than food or packaging materials promoting tensions and failures,” according to the authors. “Headspace volumes should be minimized to reduce chances of having seal damages, delaminations, and large deformations. The presence of large headspace volumes increases largely the time to obtain the required pressure since more pressuring fluid has to be pumped into the vessel….”

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Structural design with diverse materials can present changes in materials properties. “Aluminum and metal oxides present lower compressabilities than do polymeric materials. The difference can be responsible for delamination and blistering observed in barrier structures containing aluminum layer or coatings, which increase in delamination with exposure time and temperature,” the authors continue.

Package Barrier Properties
Gas and moisture barrier properties are usually the most important attributes that package materials and structures must exhibit for product protection and so must be the most closely monitored, particularly since they have been demonstrated to be subject to adverse change during some HPP operations. “In general, the permeability to oxygen, carbon dioxide, and water vapor do not change relevantly after the HPP treatment. Nevertheless, a large decrease in barrier has been observed in metalized films apparently due to the damage caused in the aluminum layer,” report Gavala et al.

“It is relevant to mention that HPP treatments produce much less damage than do conventional thermal [retort] treatments in the barrier properties of hydrophilic materials such as EVOH. The concurrence of low temperatures of treatment, shorter treatment period, and the presence of high pressure produces a much lower water sorption in those polymers, which results in the reduction of package plasticization and the subsequent loss of barrier. The volume reduction of the voids present in the polymer [free volume] which takes place during the high pressure treatment could be responsible for a decrease in solubility and a much slower molecular diffusion through the polymer matrix. These two parameters contribute to reduce the extent of mass transport through the packages and the food-package interactions. … Migration studies showed that HPP treatment does not alter the migration values of flexible packaging structures,” write Gavala et al.

Because UHP is not yet a sterilization process, products such as guacamole, salsa, processed cured meats, and even prepared entrees must be distributed under refrigeration to retain their fine qualities and safety. Extension of refrigerated shelf life is a major benefit of the application of UHP.

Recently, as has been the direction for combined food preservation processes, UHP has been linked to other processes to reduce the thermal inputs. Pressure assisted thermal sterilization (PATS) is a method that is delivering some tangible results with products.

Thus, although a single process—such as UHP—on its own might be only a partial solution, when processes are used in combination with other, less severe processes, significant product preservation benefits result but, as usual, being careful of the properties of packaging that must be retained.

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

References