High Hydrostatic Pressure Processing - ACS Symposium Series (ACS

Chapter 10 High Hydrostatic Pressure Processing Dallas G . Hoover, Dongsheng Guan, and Haiqiang Chen Downloaded by UNIV OF GUELPH LIBRARY on July 7, 2012 | http://pubs.acs.org Publication Date: April 6, 2006 | doi: 10.1021/bk-2006-0931.ch010

Department of Animal and Food Sciences, University of Delaware, Newark, D E 19716-2150

This paper overviews the nonthermal food processing technology of high hydrostatic pressure processing. In brief segments, the fundamental chemistry of hydrostatic pressure applications, generation of adiabatic heat, and historical perspective of the pressure processing of foods are presented. The key process parameters of the technology are discussed with regard to some current commercial products. The most substantial portion of the article deals with the response of microorganisms to the process, focusing primarily on problematic varieties involved in spoilage and foodborne illness with specific examples highlighted among vegetative bacteria, bacterial endospores, human infectious viruses, animal viruses, fungi, protozoa and parasites. The paper concludes with a section presenting examples of hurdle technology incorporating high pressure processing in food products. Additional process factors that are presented in combination with pressure include temperature, bacteriocins, modified-atmosphere packaging, preservative enzymes, and use of pulsed electric fields.

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© 2006 American Chemical Society

In Advances in Microbial Food Safety; Juneja, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

Downloaded by UNIV OF GUELPH LIBRARY on July 7, 2012 | http://pubs.acs.org Publication Date: April 6, 2006 | doi: 10.1021/bk-2006-0931.ch010

141 According to Le Chatelier's principle, pressure enhances reactions leading to volume reductions, whereas processes involving a volume increase are inhibited by pressure application (/). This principle governs the structural rearrangements that take place for proteins upon pressurization. At ambient temperature high pressure usually disrupts relatively weak chemical bonds such as hydrogen bonds, hydrophobic bonds, and ionic bonds. In contrast, covalent bonds remain unaffected so primary structure remains intact during and after pressurization. In general, high pressure denatures proteins, solidifies lipids and destabilizes biomembranes. It is this destabilization or leakage of membranes that is the primary mechanism in which vegetative microorganisms are inactivated. Applications of high hydrostatic pressure induce the generation of heat from compressed fluids. The heat of compression is also called adiabatic heating. The temperature increase during compression under adiabatic conditions can be described by the following equation (2):

dT aT dP~pC =

p

where Τ = temperature (Κ), Ρ = pressure (Pa), a = thermal expansion (1/K), ρ = density (kg/m ), Cp = heat capacity (J/kg»K). This equation indicates that relatively high initial temperature can lead to a relatively large temperature increase rate (°C/MPa). For example, temperature increases of water due to compression heating are 2.8, 3.8, and 4.4°C/100 M P a at initial temperatures of 20, 60, and 80°C (5). Bert Hite was the first to use high hydrostatic pressure processing (HPP) as a food preservation method. He pressure-processed a variety of foods and beverages in the late 1890s and early years of the 20 Century (4, 5). Since those initial efforts, occasional attempts by others were made through the century to study the pressure treatment of foods, but it was not until the early 1980s that the potential of HPP as a food process came to be realized. With improvements in the technology and design of pressure-generating equipment, HPP research resumed in the U.S. and Japan and proliferated elsewhere. With continued demand for minimally processed, high-quality foods, HPP emerged as a very promising method to reliably deliver safe foods that lacked the undesirable changes to sensory quality and nutrient content so often characteristic of foods receiving excessive thermal treatments. In Japan, pressure-treated jams and jellies were the first commercialized food products that employed pressure for preservation. These fruit products were initially marketed in 1991 and they continue to be sold in Japan with the addition 3

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of salad dressings and a wide range of fruit juices. In 2001, pressure-treated guacamole entered the U.S. marketplace, followed by H P P salsa. Pressureprocessed chopped onions are anticipated for sale as an ingredient in premium salad dressings in 2004. Also anticipated in 2004 are applesauce and applesauce/fruit blends packaged as eat-on-the-go single-serve flexible tubes from a Canadian venture and fruit "smoothie" products from Mexico for North American distribution, respectively.

Process Parameters Most pressure units used to process foods or food ingredients generate pressures in the range between 100 and 800 M P a . A pressure of 580 M P a (85,000 psi) has been used to commercially process guacamole. O f course, the shortest length of time at pressure as possible is preferred; foods are exposed to the set pressure point from milliseconds to over 20 min, although times of 5 to 7 min are usually more common (6, 7). The product temperature during pressurization can be controlled and maintained below 0°C or above 100°C; however, current industrial units normally use ambient temperatures. H P P systems can be used semi-continuously for pumpable fluid foods or, as is usually the case, in a batch manner for pre-packaged solid or semi-solid foods. The major critical process factors for HPP include treatment pressure, holding time at pressure, come-up time to achieve pressure, decompression time, initial temperature of food materials, process temperature, temperature distribution in the pressure vessel as a result of adiabatic heating, characteristics of the product (e.g., p H , composition and water activity), the packaging material and types of microorganisms found in the foods (8). As long as food packages fit into the treatment chamber, package size and shape are not critical factors because pressure acts instantaneously and uniformly throughout the chamber and food mass. There is no pressure gradient in the food. If pressure pulsing is used, additional process factors include pulse shape (i.e., the waveform), frequency and pulse-pressure magnitudes.

Response of Microorganisms to Hydrostatic Pressure The key value for use of hydrostatic pressure in food processing is the inactivation of microorganisms contained in the food. A few microbial life forms, such as bacterial endospores and viruses (i.e., poliovirus) are unaffected by pressure alone and require pressure treatment at elevated temperatures or some other action to realize a feasible level of inactivation; however, for most other microorganisms of concern in foods pressure will usually deliver a level of



143 inactivation that has sometimes been referred to as "pasteurization". Most types of detrimental food microorganisms, e.g., vegetative bacteria, most human infectious viruses, fungi, protozoa and parasites, can be considerably reduced or eliminated by exposures to high pressure. The yardstick for microbial inactivation of H P P usually starts with the approximation that gram-negative bacteria are usually more sensitive to pressure inactivation than gram-positive vegetative bacteria and then proceeds up the evolutionary ladder in biology to note that fungi are more susceptible to pressure inactivation than bacteria while protozoa and parasites are more sensitive than fungi, and the higher the organisms on the ladder, the greater the sensitivity to pressure (6). Viruses are notable in that a broad range of sensitivities (or from another point-of-view, resistances) is evident from studies that have been done. As might be expected with any generalization, exceptions to the rule are not uncommon. A range of pressure sensitivities are found within most microbial groups. For example, most fungal conidiospores and ascospores can usually be inactivated at pressures between 300-450 M P a at ambient temperature (9, 10, 11, 12, 13), but in a study on dormant ascospores of Talaromyces macrosporus, treatment over a pressure range of 200 to 500 M P a and 20°C activated dormant ascospores and caused little or no inactivation of the fungi. Higher pressures of 500 to 700 M P a (20°C) were required to inactivate the ascospores; however, application of 700 M P a for 60 min only reduced the spore population by less than 2 logio units, indicating the high resistance of some ascospores to pressure (14). Some general examples or common responses of vegetative bacteria to inactivation by pressure include more than a 5- l o g reduction in viable Staphylococcus aureus counts after pressure treatment of 600 M P a for 15 min at 20°C in ultra-high temperature (UHT) milk (15), and a 6-logio inactivation of a pressure-resistant strain of Escherichia coli 0157:H7 ( N C T C 12079) after exposure to 550 M P a for 5 min and 20°C in orange juice over the p H range 3.4 to 5.0 (16). Bacterial endospores are the most difficult life-forms to eliminate with hydrostatic pressure. Application of pressure alone will not inactivate bacterial endospores. In 1932, Bassett and Macheboeuf (17) very capably demonstrated this fact by detecting viable spores of Bacillus after a 45-min exposure to > 1,724 M P a (250,000 psi) at ambient temperature. Thus, a hurdle approach that utilizes pressure in combination with other processes or factors is required to inactivate spores (18). Usually pressure treatment with mild heat (e.g., 40 to 55°C) is used for substantial reduction of spore levels (18, 19). Presently, successful commercial preservation of foods utilizing HPP largely incorporate refrigerated storage of the product or a product p H below 4.5 in order to prevent the germination of spores of C. botulinum and other sporeforming bacteria. Production of commercially sterile low-acid foods employing H P P must overcome the high degree of resistance by bacterial spores. 10


144 A phenomenon observed with other food processing methods is the protection food offers to microorganisms. As demonstrated in many laboratories, most foods are more protective to microorganisms when compared to inactivation of microorganisms in water, buffer or microbiological media. This is also true in pressure processing (20, 21). For example, Chen and Hoover (22) compared the resistance of Y. enterocolitica to high pressure in ultra-high temperature (UHT) milk and sodium phosphate buffer (0.1 M , p H 7.0). In buffer, pressurization of Y. enterocolitica at 350 M P a for 26 min, at 400 M P a for 11 min, and 450 M P a for 7.5 min reduced the counts of Y. enterocolitica by more than 8 l o g CFU/mL, while in milk these same processing conditions only reduced Y. enterocolitica counts in milk by less than 2.5 logio CFU/mL. Giddings and his coworkers (23) first examined the sensitivity of viruses to pressures and reported that inactivation of tobacco mosaic virus ( T M V ) required pressures as high as 920 MPa. From more recent investigations it now appears that most human viruses are substantially less resistant to pressure than T M V . Most of viruses of food safety concern can be inactivated at pressures of 450 M P a or less. A n exposure to pressures between 400 to 600 M P a for 10 min eliminates 10 to 10 viable particles of human immunodeficiency virus (24). Certain viruses can be inactivated at even lower pressure magnitudes. Jurkiewicz et al. (25) showed that pressurization of simian immunodeficiency virus (SIV) at 250 M P a for 1 h at 21.5°C reduced its infectivity by 5-log units, while pressurization at 200 M P a for 3 h or 150 M P a for approximately 10 h was needed to obtain the same level of destruction. A 10-min exposure to 400 M P a eliminates 8-logi plaque-forming unit (PFU) population of herpes simplex virus type 1 (HSV-1), and a 10-min exposure to 300 M P a inactivates 5-log PFUio populations of human cytomegalovirus ( H C M V ; 26). A 10-min exposure to 400 M P a was shown to eliminate 5.5-logi tissue culture infectious dose of H I V type 1 at 25°C (27). It appears that the sensitivities of viruses to pressure are not correlated to genetically related taxonomic groups or even between strains as from the same group. Kingsley et al. (28) investigated pressure inactivation of viruses that contaminate raw shellfish. Five-min treatments at 275 M P a completely inactivated 7-logi tissue culture infectious doses of feline calicivirus, a surrogate for norovirus. Five-min exposures to >450 M P a reduced 7 logio PFU/ml of hepatitis A virus ( H A V ) in tissue culture medium to nondetectable levels. Interestingly, it was found that suspension of hepatitis A in seawater increased the pressure resistance of the virus as compared to treatment in culture medium. Five-min treatments at 600 M P a had no effect on poliovirus, which agreed with the work of Wilkinson et al. (29). Apparently low temperature treatment at pressure promotes inactivation of viruses due to enhanced dissociation and denaturation of viral proteins (30, 31).


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145 The explanation for this phenomenon is the specific and strongly temperaturedependent interaction of protein nonpolar groups with water. Low temperature under pressure promotes interaction of non-polar side chains to water decreasing the hydrophobic effect resulting in cold denaturation of proteins. Non-polar interactions are more affected by pressure because they are more compressible, which results in an additive effect of high pressure and low temperature that reduces the entropy of the system (52). Oliveira et al. (33) examined the combined effect of pressure and low temperature on the stability of foot-and-mouth disease virus ( F M D V ) , an animal virus that is of great concern to the meat industry. F M D V was found to be sensitive to pressure, pressurization at 240 M P a for 2 h caused a reduction of infectivity of 4-logi units at room temperature and 6-log units at -15°C. Exposure to 550 M P a for 30 sec inactivated Cryptosporidium parvum oocysts suspended in apple and orange juices by at least 3.4 logio, and 60-sec treatments efficiently rendered the oocysts nonviable and noninfectious (34). A n exposure to 200 M P a for 10 min completely inactivated all Anisakis larvae isolated from fish tissues and suspended either in distilled water or in a physiological isotonic solution between 0 and 15°C. A l l larvae were killed when exposed to 140 M P a for 1 h (35). Anisakis simplex larvae inoculated into king salmon and arrowtooth flounder fillets were completely killed by treatments of 414 M P a for 0.5-1 min, 276 M P a for 1.5-3 min, and 207 M P a for 3 min (36); however, application of HPP to raw fish fillets was of limited success because of the significant whitening of the flesh caused by pressure treatment. 0

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Examples of Microbial Inactivation in Food Products Studies to investigate the use of HPP on fruit juices has been extensive. In early product development work, pressures of 200 M P a effectively killed yeasts and molds in freshly squeezed orange juice at ambient temperature (9). Neither freshly squeezed orange juice nor juice inoculated with yeasts and molds showed an increase in total counts after 17 months of storage at 4°C following a 400M P a pressure treatment at 23°C (70). When apple, orange, pineapple, cranberry and grape juices were inoculated with ascospores and vegetative cells of Zygosaccharomyces bailii and pressurized at 300 M P a for 5 min, the populations of vegetative cells and ascospores were reduced by almost 5-logi units and 0.5-1 logio units, respectively (75); the ascospores proving more difficult to eliminate. Significant variations in bacterial pressure resistance were demonstrated for different types of fruit juices. For example, a three-strain cocktail of E. coli 0157:H7 was found to be most sensitive to pressure in grapefruit juice (8.3-logi reductions) and least sensitive in apple juice (0.4-logi reductions) when 0

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146 pressurized at 615 M P a (2 min and 15°C; 37). The obvious difference in pressure resistance is unclear. Some fruits or vegetables were examined for the potential of HPP treatment. A pressure of 340 M P a and 15 min extended the shelf-life of fresh-cut pineapple (77). Pressures of 300 and 350 M P a reduced the populations of gram-negative bacteria, yeasts and molds by at least one logio in lettuces and tomatoes; however, the tomato skins loosened and peeled away, and lettuce browned (72). The potential of H P P to reduce the microbial loads of certain seeds were also investigated. Garden cress, sesame, radish, and mustard seeds were immersed in water and exposed to different levels of pressures (250, 300, 350, and 400 MPa) at 20°C for 15 min (38). Seed germination on water agar was recorded up to 11 days after HPP. Radish and garden cress seeds were the most pressure-sensitive and pressure-resistant types, respectively. For example, after a 250-MPa treatment, radish seeds displayed 100% germination nine days later than untreated controls, while garden cress seeds attained 100% germination one day after the controls. Garden cress seeds were inoculated with suspensions of seven different kinds of bacteria (starting inocula 10 CFU/g). Treatment at 300 M P a for 15 min and 20°C resulted in 6-logi reductions of Salmonella Typhimurium, E. coli MG1655, and Listeria innocua, > 4-logi reductions of Shigella flexneri and the pressure-resistant strain E. coli LMM1010, and a 2-logio reduction of Staphylococcus aureus, but Enterococcus faecalis was not inactivated to a significant extent. The effect of pressure processing on microorganisms in mechanically recovered poultry meat was investigated by Yuste and coworkers (39, 40)). Aerobic mesophiles in the meats were susceptible to HPP. Addition of nisin and meat acidification significantly enhanced pressure inactivation of both mesophilic and psychrotrophic microorganisms. Treatment of freshly ground raw chicken at 408, 616, and 888 M P a for 10 min resulted in microbiological shelflives of 27, 70, and >98 days, respectively, when the pressured samples were stored at 4°C; unprocessed chicken samples had a microbiological shelf-life of 3 to 4 days (41). 7

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Examples of HPP Combined with Other Approaches It is frequently observed that high pressure, in combination with other preservation factors, enhances bacterial inactivation and results in longer shelflife of treated foods. Since capital costs of high pressure equipment increase exponentially with operating pressures, process costs are related to operating pressures (8). Therefore it is economically beneficial to use lower levels of pressure in combination with other processing techniques in order to obtain the


147 desired target levels of microbial inactivation while maintaining a maximum degree of sensory and nutrient qualities for the product. It is well-established that elevated temperatures promotes pressure inactivation of microorganisms. Chen and Hoover (42) found that a 5-min treatment of 500 M P a at 50°C resulted in a more than 8-logi reduction of L. monocytogenes in milk, while at 22°C a 35-min treatment was needed to obtain the same level of inactivation. Patterson and Kilpatrick (75) found that simultaneous application of high pressure and mild heat was more lethal to E. coli 0157:H7 and S. aureus than either treatment alone. A 5-min treatment of 500 M P a at 50°C resulted in a 6.0-logio reduction of S. aureus in U H T milk, while a

High Hydrostatic Pressure Processing - ACS Symposium Series (ACS

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