Combine high pressure and mild heat to kill microbes.

High pressure has been used successfully to extend the shelf life of high-acid foods, such as refrigerated fruit juices, jellies and jams. There is an increasing interest in the use of pressure technology to extend the shelf life of such low-acid foods as poultry meat, red meat and goat's milk cheese. The microbiological safety of such products is a concern, and processors need to choose appropriate treatment conditions to properly inactivate pathogens.

Many factors can influence the sensitivity of microorganisms to pressure. Gram-negative bacteria tend to be more pressure-sensitive than gram-positive bacteria. It has been suggested that as the cell membrane structure is more complex in gram-negative bacteria, it could be more susceptible to environmental changes caused by pressure treatment. A considerable variation in pressure resistance within strains of the same species has been seen in both gram-positive and gram-negative bacteria.

The nature of the substrate-the food itself-can also affect the response of microorganisms to pressure. Pressure treatment at 20 C in ultra-high-temperature-treated milk offers more protection than in poultry meat to E. coli O157:H7 and L. monocytogenes. Listeria innocua were found to be more pressure-resistant in liquid ultra-high-temperature treated dairy cream than in minced beef.

These various factors can result in a lower than expected, and often an inadequate, microbial kill in certain foods. Scientists at the Queen's University of Belfast (Department of Food Science, Newforge Lane, Belfast BT9 5PX, Northern Ireland, U.K.) tell us that one way to increase microbial inactivation is to combine pressure with another processing technology such as exposure to mild heat. This strategy is likely to be successful since microbial injury can occur at significantly lower pressures than are required for inactivation.

With this in mind, these university researchers investigated the simultaneous use of pressure and heat to inactivate relatively pressure-resistant strains of S. aureus and E. coli O157:H7 in poultry meat and ultra-high-temperature-treated milk. The investigators compared the results with microbial inactivation achieved by either treatment alone. They found that the simultaneous application of high pressure and mild heating was more lethal than either treatment alone.

Researchers found that the substrate has a significant effect on the survival of the pathogens during treatment. Moreover, different pressure-temperature conditions could be chosen to achieve a desired inactivation level.

Further information. A. Gilmour; phone: [phone]; fax: [phone]; email: [e-mail].