Examine the nature of high-pressure inactivation.

Improving food quality involves inactivating the microorganisms that cause food spoilage. Currently, the most widely used method for inactivating these bacteria, yeasts and molds is thermal processing. However, high pressures can also be effective. The extent of inactivation depends on a number of factors, including the magnitude and duration of pressure, the type of microbial species, processing temperature and the type of substrate.

Harnessing high pressures to achieve microbiological safety is the subject of much ongoing research, according to an article recently published in Chemistry and Society. The kinetics of high-pressure inactivation are significantly different than those for thermal and other processing methods. For example, a small but significant number of microorganisms can survive high-pressure treatment. This requires further study. Reliable data are needed if pressure-sterilization is to be used in a wide range of foods.

Several research groups are studying the destruction kinetics and mechanisms of inactivation of E. coli and Listeria species. They also are examining combinations of high temperature and pressure treatment. In addition, the microbial stability of pressure-treated foodstuffs is important because accurate predictions of shelf life are needed for commercial products.

When microorganisms are subjected to high pressures, molecules in the cell membrane are modified, affecting the membrane function and permeability. Key enzymes, including those involved in DNA replication, are inactivated. Some cellular proteins are denatured. These changes may not completely inactivate the cells but only injure them.

Storage conditions after pressure-treating a product can dictate the fate of these injured cells. A food's composition can have a substantial effect on the pressure resistance of microorganisms. Both milk and meat protect against pressure, and solutes, such as sugars, can offer similar protection. Acid conditions may sensitize bacteria to pressure, and acidity can kill pressure-injured cells. This makes such acid foods as fruit juices ideal candidates for this type of processing, as relatively mild pressure treatments can be used successfully.

The growth phase of the cell is also important. Cells that are actively multiplying and growing tend to be more pressure-sensitive. Pressures in the 300 MPa to 700 MPa range can kill bacteria. But some bacteria form spores that are much more resistant. Still, pressure can cause the spores to germinate so they lose their resistance to heat and pressure. Significant kills are possible when combining about 400 MPa and 60 C to 900 C, or using pressure cycling with pressures between 50 MPa and 400 MPa. Although most yeasts and molds are sensitive to these treatments, some, such as Byssochlamys nivea, produce spores that are particularly resistant.

Further information. Stephanie Grant, U.K. High Pressure Club for Food Processing, Department of Food Science, The Queen's University of Belfast, Newforge Lane, Belfast BT9 5PX, Northern Ireland, U.K.; phone: [phone]; fax: [phone]; email: [e-mail].