High-pressure processing continues to evolve commercially.

Since the turn of the century, food scientists have documented that extremely high pressures can destroy food microorganisms. In the 1980s, the food industry began to transfer these benefits to commercial food processes. High-pressure processing (HPP) is a nonthermal technique that can be used to preserve foods. In a typical process, a food material is prepackaged and processed under elevated pressures up to 130,000 psi, or nine times the highest ocean pressure.

HP-processed foods have better flavor, texture, nutrient retention and color compared with thermally-processed foods. Pressure can be applied at ambient temperatures, eliminating unwanted, cooked off-flavors. This especially benefits heat-sensitive products, such as citrus juices. The pressure is transmitted in a uniform, instantaneous manner throughout the product. Therefore, the process is independent of the volume and geometry of the product. This is another key advantage over conventional processes.

Currently, most commercially-available HPP systems use batch-type approaches. After the food is loaded and the vessel closed, mechanical pressure is exerted to achieve desired target pressures. Recently, Flow International introduced a semi-continuous high-pressure processor for liquid foods. Researchers have shown that pressures of 40,000 psi to 80,000 psi at ambient temperatures inactivate vegetative bacteria and enzymes, and destroy most food spoilage yeasts and molds. Researchers have also shown that gram-positive microorganisms resist high pressure better than gram-negative organisms.

Recent studies at the National Center for Food Safety and Technology (Illinois Institute of Technology, 6502 S. Archer Rd., Summit-Argo, IL 60501) and elsewhere suggest that spores cannot be inactivated using pressure alone. Combinations of pressures (up to 120,000 psi) and heat treatment (at or above 50 C) appear to have lethal effects on bacterial spores. It may be possible to combine HPP, either sequentially or simultaneously, with other processing methods. A food's pH and sugar, salt and water content, among other factors, determine the pressure, temperature and processing time.

At present, HPP is used mainly for high-value or novel products of superior quality. The Japanese are successfully using the technology to produce a variety of commercially-pasteurized products. In the United States, HPP is being used commercially to pasteurize foods such as guacamole. Other potential applications include processing dairy products, blanching, pressure-shift freezing and thawing.

Further information. V M. (Bala) Balasubramaniam; phone; 708-563-8175; fax: [phone]; email: [e-mail].