Low-energy x-rays reduce Salmonella numbers on the surface of almonds.

California almonds were implicated in two outbreaks of salmonellosis in the past few years that were traced to S. enteritidis PT30. This prompted the recall of 6,000 tons of raw almonds and the issuance of the final rule for mandatory pasteurization of all California almonds--a greater than 4-log reduction for Salmonella. This rule was published in the Federal Register in 2007, as proposed by The Almond Board of California.

Because raw almonds were not previously pasteurized, the rule has created an urgent, industry-wide demand for technologies that can achieve the mandated reduction in Salmonella numbers without adversely affecting the sensory and quality characteristics of the product.

Low-energy irradiation--less than 300 keV--is effectively absorbed on the surface of foods with minimal penetration. It is particularly well-suited for surface pasteurization, especially for food materials that have radiation-sensitive components, such as the unsaturated free fatty acids in nuts. So, Michigan scientists wanted to determine how effective low-energy x-rays would be in inactivating S. enteritidis PT30 found on the surface of almonds.

In experiments, raw almonds were surface-inoculated with S. enteritidis PT30 at about [10.sup.8] CFU per g. They were dried to an aW of 0.7. Inoculated samples of about 5 g were bagged in a single layer and irradiated with low-energy x-rays at a surface dose of 0.5, 1.0, 1.5 or 2.0 kGy. The irradiated samples were then homogenized and serially diluted in 0.1% peptone water and plated on xylose lysine deoxycholate agar, which had been overlayed with trypticase soy agar containing 0.6% yeast extract. The samples were incubated for 48 hours at about 35 C before numbers of bacteria were counted.

At 2.0 kGy, a total kill--a greater than an 8.8-log reduction--was achieved, giving a D10 value of 0.27 kGy. The D10 value represents the irradiating dose required to reduce the bacterial population by 90%. Based on these findings, low-energy irradiation is a feasible nonthermal alternative for almond processors to use to achieve a minimum 4-log reduction of Salmonella.

Further information. Sanghyup Jeong, Visiting Assistant Professor, Pilot Plant Manager, Department of Biosystems and Agricultural Engineering, 220 Farrall Hall, Michigan State University, East Lansing, MI 48824; phone: 517-432-3463; fax: 517-432-2892; email: jeongsa1@msu.edu.

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