High pressure processing of foods

Foods such as guacamole, whole shell oysters, salsa, ready-to-eat meats, jams and jellies, salsa, chopped onions and peppers can be found in the global marketplace and are processed to some extent using high hydrostatic pressure. Essentially, this means the products have been submersed in water and then subjected to compression pressures approximately six times greater than the highest pressures reached in the biosphere; that would be pressures found at the bottom of the Marianas Trench in the Pacific Ocean where, at 36,000 feet underwater, pressures reach 100 megaPascals (MPa) or approximately 15,000 pounds per square inch (psi). In comparison, packaged guacamole is pressure-treated at 580 MPa (100,000) psi for several minutes to preserve the product and deliver premium sensory quality.

Why high pressure processing?

It’s a means of “invisible processing”. Exposure to such pressure extremes kills microorganisms in foods, so high pressure processing (HPP), is a means to preserve food without thermally processing or cooking the product. The compression damages the cellular membranes of bacteria and other microorganisms, leading to their inactivation. Chemically, hydrostatic pressure has no affect on covalent bonds; for the most part, pressure disrupts hydrophobic bonds most commonly found in proteins. Hydrophobic bonds significantly affect the shape of the protein. Therefore, pressure can denature enzymes and proteins found in cell membranes and foods. High protein foods are usually animal-derived, and can appear “lightly cooked” as a result of pressure exposure and slight denaturation of surface proteins. Whole fruits and vegetables do not demonstrate protein denaturation but can show compression damage. For other foods, as listed above, sensory changes from pressure exposure are modest or nonexistent, so that the treated food tastes, feels and looks raw, while spoilage organisms and pathogens are destroyed and nutrient content of the food remains unchanged.

When applied, pressure acts instantaneously and uniformly throughout the mass of food, independent of shape; In other words, there is no pressure gradient. In thermal processing, there is a temperature gradient, so time is required for the product to come up to a uniform temperature throughout the product. During pressure treatment, foods do decrease in volume as a function of the imposed pressure; thus, packaging used for HPP-treated foods must accommodate up to a 15% reduction in volume and return to its original volume without loss of seal and barrier properties. Most flexible packaging works well.

This application of pressure in food processing is not a new concept. HPP of foods commenced in the late 19th Century at the West Virginia Agricultural Experiment Station (Hite, 1899). Bert Hite imported steam engine parts from New York City and built the first pressure unit for food processing in Morgantown. His body of work was quite thorough (Hite et al., 1914) and even included HPP examination of viruses (Giddings et al., 1929). HPP of foods received spotty attention through the 20th Century; however, interest resurfaced in the 1980s. An important contributing factor to its resurgence was the development of pressure equipment, capable of daily industrial use3,5,6.

In the 1980s and 90s, foods and food microbiota were examined for desired application of HPP. HPP as a preservative replacement for thermal processing was investigated and found lacking under certain circumstances, most notably for the production of commercially sterile, low-acid foods. Commercial sterility is desirable for shelf-stable products, but pressure alone cannot inactivate bacterial endospores as produced by Bacillus and Clostridium. Spores can be considered as anhydrous packaging mechanisms, that have evolved to allow sporeforming bacteria to survive within incredibly difficult environments for extended periods of time. As such, the internal structure of spores is vastly different from vegetative or replicating forms of bacteria, even though spores transform from and to bacterial cells easily enough. One way to deal with spores in pressure-treated foods is to elevate the treatment temperature when the products are pressure-processed (Black et al., 2007). Elevating the temperature during HPP enhances spore germination; once spores are activated and germinated they lose their inherent resistance and can be inactivated by pressure and other approaches. A problem is that spores germinate unreliably, so more traditional approaches are incorporated and combined as hurdle technology. For example, application of pressure to foods with an acidic pH (e.g., salsas, jams and jellies), along with refrigerated storage, or even the use of additives, such as nisin, that has specific effectiveness against spores (Chen and Hoover, 2003). These hurdle-type approaches can improve control problems, derived from spore outgrowth in pressure-treated foods, while maintaining the desirable sensory characteristics1,2.

HPP works well enough against bacterial cells (Hoover et al., 1989). Osmo-sensitive pathogens, such as Vibrio and Campylobacter, are also very sensitive to inactivation from exposure to pressure. For example, a 6 log10 CFU (colony-forming units)/mL suspension of Vibrio parahaemolyticus in clam juice is eliminated when exposed to only 173 MPa for 10 minutes. Hormel Foods Inc. uses a pressure of 580 MPa to process packaged prosciutto ham. Their process results in an inactivation of 5 log10 CFU/mL of Listeria monocytogenes. Proscuitto is a dry-cured ham and use of heat as a post-processing control method leads to off-flavours and soft textures that are unacceptable for the product. Although the treated prosciutto is not sterile, HPP inactivates any contaminating L. monocytogenes in the product without affecting sensory quality. Hormel now treats all of their packaged ready-to-eat (RTE) meat products to ensure product safety. Recall of RTE meats can be very damaging to a food company. Perdue Farms, Inc. also applies HPP to their packaged poultry products for the same reason. Occasionally, when HPP is applied in this fashion, it is referred to as “cold pasteurisation”8.

HPP inactivates other microorganisms besides bacteria, and just like bacteria, different types within a group of microorganism can display a range of sensitivities to pressure inactivation. Viruses, such as hepatitis A and norovirus are inactivated from exposures to pressure of 450 to 500 MPa, but other viruses such as Aichi virus, poliovirus and some coxsackieviruses are resistant to pressures above 600 MPa (Grove et al., 2006)6. Fungi and parasites also show a diversity of responses. Generally, the primary factors that affect HPP inactivation rates are:

• The type of microorganism
• The culture or growth conditions and age of the microorganism
• The composition, pH, and water activity of the food
• The temperature, magnitude and time of pressurisation (Hoover, 2002)7

More specifically with these trends, it is usually the case that:

1. Increasing the pressure magnitude or time of pressure treatment increases the number of microorganisms inactivated (with spores a notable exception).
2. Pressure inactivation rates are enhanced by exposure to acidic pH or an increase in the pressure treatment temperature above ambient (e.g., 40 to 60°C).
3. Some foods are more pressure-protective than others (e.g., high-fat or high-protein foods).
4. Gram-positive bacteria are usually more resistant to pressure than gram-negative bacteria (with some notable exceptions).
5. Younger (i.e., actively growing) cells are more pressure-sensitive than old (i.e., dormant) cells.
6. Incomplete inactivation of microorganisms by HPP, can result in injured cells possessing temporary nonculturability, yet have the capability to recover in optimal growth conditions and replicate.
7. The more developed the life form on the evolutionary scale, the more sensitive it is to pressure (Hoover, 2002)7.

Not all HPP applications have a part in the killing of problematic microorganisms. HPP can also be used to shuck oysters and other types of shellfish. When whole shellstock oysters are exposed to 275 MPa (40,000 psi) for 1 to 2 minutes, nearly 100% of the oysters open and the meat can be removed from the shell by a flick of the wrist. Pressure exposure kills the oyster and denatures the protein attachment to the shell, thus releasing the meat. This process makes mechanical shucking with short, sharp knives no longer necessary; the HPP oysters are plumper, having suffered no damage from mechanical shucking. Money is also saved as the HPP treatment is faster, safer, and does not require a relatively large number of skilled shuckers. Once shucked, further processing at 415 MPa for several minutes can be carried out to extend the refrigerated shelf-life to three weeks. Such a pressure exposure does result in some observable protein denaturation of the product; otherwise, the sensory characteristics of the oyster are unaffected. As a bonus, the product is safer to consume, with the microbiota of the oyster reduced by the additional HPP treatment. HPP is also commercially used in Canada, in lobster processing, to extract a higher yield of lobster meat from the shell than conventional methods6,7,8.

Enzymes found in the food also contribute to spoilage (e.g., browning enzymes). The effects of HPP on enzymes are not as predictable as for the inactivation of microorganisms. Enzymes are highly variable in their response to high pressure; there can be partial or total inactivation, no effect, or promotion of activity after HPP treatment. Given this situation, HPP products need to be evaluated individually for overall effectiveness versus spoilage enzymes. In some cases for fruit and vegetable products, a traditional blanching step is required to avoid premature browning of the product.

Conclusion

HPP for food and beverages has come of age. It has now matured to the point that it is a recognised commercial food process technology. It does not work for all product applications, but for some categories of products, as noted in this article, it does work very well. Unlike the use of ionising irradiation for food preservation, HPP is not prohibited for use with organic foods. HPP is also being examined for nonfood products, for example in vaccine production and the preservation of biomedical products. HPP research can be expected to continue in food process and product development.

References

1. Black, E.P., P. Setlow, A.D. Hocking, C.M. Stewart, A.L. Kelly and D.G. Hoover. 2007. Response of spores to high-pressure processing. Comp. Rev. Food Sci. Food Safety 6(4):103-119.
2. Chen, H., and D.G. Hoover. 2003. Bacteriocins and their food applications. Comp. Rev. Food Sci. Food Safety 2(3): 81-100.
3. Giddings, N.J., H.A. Allard and B.H. Hite. 1929. Inactivation of the tobacco mosaic virus by high pressure. Phytopathology 19:749-750.
4. Grove, S.F., A. Lee, T. Lewis, C.M. Stewart, H. Chen, and D.G. Hoover. 2006. Inactivation of foodborne viruses of significance by high pressure and other processes. J. Food Prot. 69: 957-968.
5. Hite, B.H. 1899. The effect of pressure in the preservation of milk. Bull. W. Va. Univ. Agr. Exp. Stn. 58:15-35.
6. Hite, B.H., N.J. Giddings and C.E. Weakly. 1914. The effects of pressure on certain microorganisms encountered in the preservation of fruits and vegetables. Bull. W. Va. Univ. Agr. Exp. Stn. 146:1-67.
7. Hoover, D.G. 2002. Microbial inactivation by high pressure. In Control of Foodborne Microorganisms (Juneja, V.K. and J.N. Sofos, eds.) Marcel Dekker, Inc., New York. pp. 419-449.
8. Hoover, D.G., C. Metrick, A.M. Papineau, D.F. Farkas and D. Knorr. 1989. Biological effects of high hydrostatic pressure on food microorganisms. Food Technol. 43(3): 99-107.