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Survival of salmonellae in orange juice

In March 1997 Food Safety & Hygiene discussed the survival of enterohaemorrhagic E. coli (EHEC) and Salmonella species in fruit juices. We concluded that EHEC and salmonellae can survive in fruit juices and some other acid foods with a pH below 4.5 for many days especially at refrigeration temperatures. Actual reported incidents associated with acid foods remain rare however.

In a recently published paper (Journal of Food Safety 17 1997 273-281), Parish, Narciso and Friedrich describe experimental studies which add weight to this conclusion. Four Salmonella serovars were inoculated into pasteurized orange juice adjusted to pH 3.5, 3.8, 4.1 and 4.4 and incubated at 0° and 4°C. Inoculated salmonellae (10⁶ colony-forming units/ml) survived in detectable numbers up to 27 days at pH 3.5, 46 days at pH 3.8, 60 days at pH 4.1 and 73 days at pH 4.4.

The authors believe that the wide variability of results from different studies in this field published earlier can be at least partly explained by the acid tolerance phenomenon. This phenomenon occurs when bacterial cells challenged at sublethal pH levels develop remarkable tolerance to lower pH levels indicated by production of acid shock proteins (Foster and Hall, Journal of Bacteriology 172 1990 771-778). In the present study the four salmonellae strains were preconditioned at pH 5 before inoculation into orange juice at the lower pH values. The authors note that such preconditioned cells represent a 'worst case' scenario in which pH tolerant organisms might contaminate juice in a processing environment due to inadequate cleaning/sanitation procedures. They recommend that juice processors reduce risk to public health through adherence to strict cleaning/sanitation protocols and stringent standards for fruit quality and grading.

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Prepared by Keith Richardson and Beverley George
Food Science Australia
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