Utilize alternatives for preserving low hydrated food products.

High hydrostatic pressure (HHP) has the potential to preserve products, retain their original fresh quality, improve their shelf life and offer better microbial security.

What has limited the commercial use of HHP has been the high cost of high-pressure processing equipment and the low capacity associated with batch-type processes. Another restriction of this process involves the high water content of some foods. At low water activity, aw > 0.9, microorganisms cannot be inactivated even at high-pressure levels of greater than 700 MPa or by combining pressure and heat. The resistance of bacterial spores could also be related to the hydration of the product. Moreover, high solute concentrations reduce medium compressibility, which is related to the effects of pressure.

Although microorganisms are not active in dry food products, they can remain viable, and grow and contaminate the products again when they are rehydrated. This could happen with food powders used in formulating products. Treating these powders usually involves their partial rehydration, treatment with high temperatures, and then dehydrating them, which is a waste of energy. Ionization is an alternative treatment but this treatment but might leave free radicals in the treated products.

Using gas as a pressurization medium might be an alternative approach. Supercritical fluids, such as carbon dioxide, can be used at relatively low pressures, less than 300 MPa, and at temperatures less than 60 C. Results would be partly related to the reversible acidification of the medium and would be more efficient in a hydrated medium. But the extraction of important compounds, such as aroma, might occur with this process.

Using a neutral gas like nitrogen would require pressure levels greater than 150 MPa, but would be efficient at low water activity, aw > 0.3. The process would have a sorption-desorption mechanical effect on cellular structure. It is efficient on vegetative cells as well as on bacterial spores, but requires specific pressure levels, type of gas, and certain pressure release kinetics. But it doesn't seem to affect the quality of food products.

Industrial applications would require an effective recovery of gas compression energy to be profitable and attractive. Otherwise, only food products of high value would be worth the investment.

Further information. Jean-Marie Perrier-Cornet, AgroSup Dijon, 26 boulevard Petitjean, BP 87999, 21079 Dijon cedex France; phone: [phone]; fax: [phone]; email: [e-mail].