Ultrasound inactivates Listeria, Shigella.

An FDA rule requiring unpasteurized fresh juice to carry a warning label has been motivating juice producers to pasteurize their products. Although an overwhelming percentage of fruit juice products--about 98%--is pasteurized, the FDA estimates that 48,000 people each year suffer from foodborne illness caused by exposure to the bacteria in fruit juices.

But the cost of applying thermal pasteurization technologies, as well as alternative techniques, could be prohibitive for small and mid-sized juice producers. There is a need to develop a pasteurization technique that ensures product safety, provides better quality and which is affordable for small and mid-sized juice producers.

The objective of scientists at the University of Illinois was to determine the feasibility of using ultrasound to pasteurize juice. They first examined the bactericidal effects of sonication on L. monocytogenes, S. sonnei and S. boydii. An ultrasound generator was used with its power determined by a calorimetric method. Five Shigella strains and two strains of L. monocytogenes were tested to identify the strain most resistant to ultrasound treatment.

Investigators subjected the most resistant strains to sonication at an amplitude of 20 mm. A cooling system controlled temperatures during sonication and kept those temperatures at less than 35 C. Three power levels were used by changing cell suspension volumes: 50 ml, 100 ml and 200 ml.

Log reductions from 4.7 to 6.7 were obtained for Shigella and from 0.6 to 2.7 for L. monocytogenes after 20 minutes of sonication at less than 33 C. The most resistant strains were S. boydii 18 and L. monocytogenes Scott A. Maximum inactivation was achieved at a power level of 1.44 W per [cm.sup.3].

At this power level, researchers observed a two-sectional inactivation behavior for S. boydii 18, with a fast 5-log inactivation in the first 10 minutes and a less than 1-log reduction after an extended 20 minutes of sonication. L. monocytogenes exhibited a linear inactivation during the treatments.

The results indicate that high-power ultrasound might be an alternative pasteurization technology for use in juice processing.

Further information. Scott Martin, Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, 382 D Agricultural Engineering Sciences Building, MC-646, 1304 West Pennsylvania Ave., Urbana, IL 61801; phone: 217-244-2877; fax: 217-244-2517; email: se-martn@uiuc.edu.

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