High pressure imparts sterility; has little impact on flavor, color, texture.

High-pressure processing is in commercial use for pasteurizing such products as guacamole, meats and seafood, but has yet to find sterilization applications. However, advances in equipment design and construction have brought down the processing costs to make them competitive with retorting and freezing process costs.

The equipment is available to produce high-pressure sterilized entrees such as macaroni and cheese, salmon fettuccine and ravioli, that have a freshly prepared flavor, color and texture. In the United States a company would have to petition the FDA if it wants to process shelf-stable foods with the technology. A petition is not required for frozen or refrigerated products.

According to scientists at Washington Farms Inc. (3813 E. 80th St., Tacoma, WA 98443), Unilever and Basic American Foods, researchers have achieved sterility in low-acid foods using pulsed high pressure along with heat. An initial temperature, prior to sterilization, of 90 C combined with 690 MPa for 1 minute was followed by an ambient pressure pause for 1 minute, then another 690 MPa pressurization for 1 minute.

Using this approach, researchers were able to sterilize macaroni and cheese that had a 10(6)/g spore load of C. sporogenes PA 3679. Investigators repeated the test with a 10(6)/g spore load of B. cereus and were also able to sterilize the product. Further testing using a variety of pressures, initial temperatures, pulses and pulse-pause times found that there were several variables needed for achieving sterility. These include the need for two or more pulses, an end temperature (under pressure, caused by adiabatic heating) of at least 105 C, and a uniform initial product temperature.

The period of time at each pressure pulse or the period of time at the pause period between pulses had little effect, except with a marginal end temperature (below 105 C). The same held true for more than two pulses. The primary variables affecting quality include: the initial temperature; the time period needed to achieve initial temperature (preheating step); vessel pressurization time; initial product quality, the time period to depressurize; and the time period to cool after depressurization.

It is possible to achieve a better-quality product by using a higher pressure level and a lower initial temperature. This will minimize the preheating and post-processing heat damage. High pressure seems to protect the product against excessive heat damage during sterilization.

To ensure that a product is sterilized, you must measure and record certain variables, including: the temperature in the high-pressure vessel prior to processing; product temperature and its uniformity throughout the product, the ratio of pressurizing fluid to product in the vessel chamber, package integrity and various processing times.

The cost to modify an existing line and produce product is approximately $0.05/lb based on certain assumptions, including the type of equipment, cycle time, capacity, depreciation and capital costs. The investigators expect that the cost of pressurizing vessels will decrease, and that cycle time can be reduced. This may help make a cost of $0.03/lb possible in the future.

Further information. Richard Meyer; phone: [phone]; fax: [phone]; email: [e-mail].