EDITOR’S NOTE: This article is an excerpt from FSNS’ “Bringing the Pressure to Food Processing” located at www.food-safetynet.com.
Food processors have a variety of novel food processing technologies at their disposal to help meet consumers’ expectations for quality and convenience. One such technology is high pressure processing (HPP), also described as high hydrostatic pressure processing or Pascalization. Several high pressure-based food processing technologies exist, including high pressure processing pasteurization, high pressure assisted thermal sterilization (HPATS), and high pressure homogenization. However, among these, high pressure processing pasteurization has come to the commercial forefront due to its nature as a non-thermal processing capability. As such, it helps retain many of the qualities of the fresh food product that would otherwise be altered by traditional thermal processing. Thus, for those products like guacamole and fruit juices, where preserving the initial state of the fresh food product is key to consumer acceptance, HPP has gained considerable traction.
While there are several derivatives of the technology, the basic premise of HPP pasteurization involves pressure-processing foods that are sealed in their final packages. These packages are placed into a mechanical chamber (such as a steel cylinder), the chamber is then filled with a low compressibility liquid (such as water), and compression is applied to create pressure. To process the products, the chamber is then either held static at a given pressure for some period of time, or is subjected to pressure oscillation. After processing, the chamber is decompressed. Thus, HPP is primarily operated in a batch-based manner, but several chambers can be run in parallel to make the process semi-continuous in nature. No extreme amounts of heat are created in the process, and in most cases, colors, shapes, and nutrient contents of most foods are preserved (though some protein denaturation can occur in the 400 to 600 MPa range). This makes HPP ideal for processors that desire to maintain the appearance and sensory appeal of minimally processed food products, like those mentioned above, while also achieving the microbial reduction standards that they desire or are required to meet.
Though HPP helps maintain the appearance and sensory appeal of a minimally processed food product, the technology also produces substantial reductions in the levels of certain spoilage and pathogenic microorganisms, and can inactivate certain degradative enzymes. A good case-in-point of the application of HPP is that seen for one food processor, who has found success in reducing Listeria by at least 5 log cycles in avocados and ready-to-eat meat products. However, despite its effectiveness, the mechanics of microbial inactivation via HPP are still dependent on several variables including the type of organism being targeted, the physiological state of the organism, and intrinsic factors of the food product.
With regard to organism type, it has been learned that eukaryotic cells (e.g. protozoa like Toxoplasma), viruses, and Gram-negative bacteria (especially Vibrio), are generally more sensitive to HPP treatment. In contrast, Gram-positive bacteria (especially Staphylococcus and lactic acid bacteria) and bacterial endospores are generally more resistant. Inactivation of some bacterial endospores can require greater than 1000 MPa of pressure, which is outside of the range produced by most commercial HPP units. However, HPATS, where 200 to 600 MPa of pressure is combined with temperatures of 90 to 121 degrees Celsius, has shown utility in spore inactivation. Using HPATS, spores are inactivated in a shorter amount of time as compared to a conventional thermal process, and sensory characteristic changes are reduced. As a result, HPATS is a food processing technology that will likely continue to develop in the future.
In addition to organism type, the physiological state of an organism also plays a role in its response to HPP treatment. In general, cells in the stationary phase of growth tend to be more resistant to HPP than cells in the logarithmic growth phase. Likewise, barotolerance, or resistance to pressure, can be affected by the degree of fatty acid unsaturation in an organism’s membrane, its ability to express stress response proteins, and accumulation of osmoprotectants such as trehalose and glycerol. Furthermore, within certain microorganism species, barotolerant strains have been identified, which tend to be more resistant to high-pressure inactivation than other species members. For example, Escherichia coli O157:H7 strain C490 and Listeria monocytogenes strain NCTC 11994 have been identified as pressure-tolerant strains of those particular species.