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High-pressure processing of foods is an active and growing area of research and development, judging from the quantity and variety of research reported at IFT’s Nonthermal Processing Division Workshop in September 2005 at the USDA Eastern Regional Research Center (ERRC) in Wyndmoor, Pa.
HPP involves subjecting foods to pressures on the order of hundreds of mega Pascals—e.g., 400 MPa, equivalent to about 4,000 times atmospheric pressure or about 60,000 psi—usually at close to room temperature but sometimes at temperatures up to 121°C. The objective is to extend the shelf life of foods and to reduce pathogens while retaining most of the food’s “fresh” flavor, color, and texture.
Because of the high pressures, the process is conducted in sturdy vessels and usually as a cyclical batch operation. The producers of commercial equipment, several of whom were represented at the workshop, include Avure, Kent, Wash.; Elmhurst Systems, Albany, N.Y.; Engineered Pressure Systems, Inc., Haverhill, Mass.; Stansted Fluid Power, Essex, UK; and NC Hyperbaric, Burgos, Spain.
Ed Ting (phone 253-813-3346), Chief Technology Officer at Avure, explained in a breakout session that his company’s equipment grew out of the technologies of its parent companies, ABB Pressure Systems and Flow International, in high-pressure compaction of ceramics, powder metal, and diamonds, and water-jet cutting, in which a thin stream of water is pumped to about 50,000 psi up to 1,800 times a minute, using pistons. These experiences gave Avure the know-how to manufacture large HPP systems with the duty cycles required for the food industry.
Most HPP vessels use an external frame or yoke and usually have a non-threaded closure. Because water and foods compress under pressure, pumps must supply up to 16% additional water during a cycle, according to Ting. This water is usually captured when pressure is released, then screened and re-used. The temperature of the water is usually controlled. There is a substantial adiabatic heat of compression which generally raises temperature about 3°C for each 100 MPa when water is the pressure-transferring medium. This temperature change, however, is reversed upon decompression.
Bala Balasubramaniam (phone 614-292-1732), Assistant Professor of Food Science at Ohio State University, refers to pressure-assisted thermal processing (PATP) in his research because he finds that high pressure alone is not sufficient to inactivate spores. He pre-heats foods, then uses the adiabatic heat of compression to raise the temperature further. When pressure is released, the temperature drops very quickly, helping to minimize the adverse effects of heating, he said.
High pressure is believed to preserve foods by inactivating vegetative cells. High pressure alone does not appear to kill significant populations of spores, but it does appear to induce germination. Spores appear to be inactivated by combinations of both pressure and heat. Balasubramaniam and his colleagues J. Ahn, A.E. Yousef, and S. Rajan investigated spores of Bacillus amyloliquefaciens and selected surrogates for pathogenic Clostridium and Bacillus species based on resistance to pressure and temperature.
Patrick Dunne (phone 508-233-5514), Senior Advisor at the U. S. Army Natick Soldier Center, Natick, Mass., pointed out that both pulsed electric fields (PEF) and HPP appear to have little effect on most enzymes that affect quality of foods and that enzymes may then determine the acceptable shelf life of nonthermally treated foods. Other researchers have found that high pressure might even stabilize some enzymes.
R. Buckow, V. Heinz, and D. Knorr of the Berlin University of Technology, Berlin, Germany, found that several hydrolytic reactions catalyzed by enzymes proceeded more rapidly at elevated pressures than at atmospheric pressure and the same temperatures. Their research showed that the effect was due to retarding of the inactivation of the enzymes by heat, permitting the reaction to occur at high temperature, which increased the reaction rate. Pressure actually delayed the reaction, but the ability to use higher temperature more than compensated for the delay. The net effect was an increase of 20–100% in substrate conversion at optimum pressures and temperatures, which varied for each enzyme and substrate combination. Optimum pressures were 106–318 MPa, and optimum temperatures were 55–84°C.
These results suggest that the mechanism of HPP is not inactivation of enzymes. Other research is inconclusive on whether there are physical changes to cells under HPP.
C.N. Jordan, A.M. Zajac, D. Holliman, G.J. Flick, and D.S. Lindsay of Virginia Tech found no morphological changes in pressure-treated spores of Encephalitozoon cuniculi in apple cider.
In contrast to noting no morphological change, E. Black, M. Linton, M.F. Patterson, G.F. Fitzgerald, and A.L. Kelly of University College Cork and the Dept. of Agriculture and Rural Development, Belfast, Ireland, observed significant physical damage by HPP to spores of Bacillus subtilis treated in milk. The group combined HPP with use of the bacteriocin nisin, a natural fermentation product that is a mild antibiotic approved for certain foods by the Food and Drug Administration.
A. Mathys, V. Heinz, and D. Knorr of Berlin University of Technology reported on the pressure and temperature effects on pH of various buffers. Some buffers are less sensitive than others to pressure, but since there is almost always a temperature change due to high pressure, the sensitivity to temperature is also a factor. Perhaps a pressure-induced pH change in foods contributes to inactivation in certain foods?
Another hint as to a possible mechanism for HPP was presented by A.L. Kelly, T. Huppertz, P.F. Fox, and C.G. de Kruif of the University College Cork and NIZO Food Research, Ede, the Netherlands. They studied the effect of high pressure on dairy proteins and products. Kelly said that some of the effects of HPP on milk include a reduced size of micelles (the small aggregates of protein and calcium suspended in milk), increased calcium solubility, denatured whey protein, and a change in color. HPP also appears to accelerate ripening of Mozzarella cheese. He implied that HPP might be applied to milk as much for its impact on functional properties as for its preservative effect.
There are no HPP dairy products yet, according to Kelly, but one possibility was described by M. Linton, A.B. Mackle, and M.F. Patterson of the Dept. of Agriculture and Rural Development, Belfast. They pointed out that cheese made from raw milk is popular, especially in France, but that such cheese carries a risk of Listeria monocytogenes. They found that HPP eliminated L. monocytogenes from soft cheese made from inoculated raw milk. Prototype extended-shelf- life fruit yogurt products for use on the NASA space shuttle have been prepared by Oregon State University and Avure working with the U.S. Army Natick Soldier Center.
M.L. Bari, M. Mori, D.O. Ukuku, S. Kawamoto, and K. Yamamoto of ERRC studied inactivation of Escherichia coli O157:H7 in tomato juice and liquid whole egg. Liquid whole egg, of course, is vulnerable to heat and was found to coagulate at the highest pressure and temperature used (600 MPa, 50°C). Treatments were for relatively long times (up to 60 min) at near room temperature. Cycling the pressure up to four times for a total exposure of 40 min was more effective than using continuous pressure for the same time. This raises the possibility that pressure changes have an influence on inactivation. Reductions of 3–5 log cycles were achieved.
D. Guan, K. Kniel, K.R. Calci, D.T. Hicks, L.F. Pivarnik, and D.G. Hoover of the University of Delaware and University of Rhode Island studied inactivation of four types of coliphages to evaluate their potential as human enteric viral surrogates. The coliphages differed from each other in their sensitivity to high pressure at room temperature. Pressures up to 600 MPa for 5 min were required to get significant reductions in the most-resistant phages.
D. Maslak, V. Heinz, and D. Knorr from Berlin University of Technology presented interesting work showing that HPP actually seemed to protect cells from PEF under some conditions. However, at other conditions, there was a synergistic effect, suggesting that both processes may affect cell membranes.
R. Buckow, V. Heinz, and D. Knorr from Berlin University of Technology studied the effect of short exposure times on inactivation of microbes in beverages. They found significant reductions (7–9 log cycles) in milk and fruit juices at times as low as 10 sec. Some organisms required up to 60 sec and temperatures of 75°C for inactivation, but these are still fast processes compared to many others cited by other researchers.
What’s Next for HPP?
The most successful pressure-treated food product, so far, is guacamole. However, red and poultry deli meats have been successfully introduced into many retail stores. All food examples to date have been refrigerated foods. The lack of success against spores has meant that HPP cannot yet produce a shelf-stable low-acid food.
Most foods now are treated in flexible or semi-rigid packages, in batches, and in cycles measured in double-digit minutes. There is interest in treating fluids. One scheme pumps the fluid into a holding vessel and uses several such vessels to approximate continuous flow. Another scheme for fluids would use large bags, such as those used for bulk aseptic storage. This would still be a batch process and would require a larger chamber than has been used so far.
Another area of application for HPP is in treating shellfish. Oysters are simultaneously shucked and pasteurized without fully cooking, while lobster meat is removed from its shell by HPP.
Research is active on HPP at many sites, and notable progress is likely to be presented at the next workshop in Ireland in September 2006.
Virginia Tech’s High Pressure Processing Laboratory maintains a database at www.hpp.vt.edu, and Ohio State has a briefing on HPP at http://grad.fst.ohio-state.edu/hpp/.
by J. Peter Clark,
Consultant to the Process Industries, Oak Park, Ill.