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Growth of Salmonella varies in high-water activity pecans.

There is increasing concern that foods with low water activity (Aw) may carry foodborne pathogens, particularly Salmonella. Low-Aw foods involved in outbreaks of salmonellosis include almonds, coconut, chocolate and potato chips. Salmonella has been isolated from several types of tree nuts. With the exception of almonds, however, tree nuts have not been implicated in outbreaks of salmonellosis.

Exposing pecans to pre- and post-harvest environments imposes some risk of contamination. This risk exists at each step in the pre- and post-harvest environments.

Recalls of pistachios and hazelnuts contaminated with Salmonella make us want to know more about the behavior of the pathogen in tree nuts. Salmonella is able to grow at Aw as low as 0.93. But wetting pecans can cause water uptake and create kernel Aw values exceeding 0.93. Over time, populations of the pathogen on in-shell pecans or pecan nutmeats could potentially increase to high levels.

Scientists at the University of Georgia determined the survival and growth characteristics of Salmonella in high-Aw, 0.96 to 0.99, pecan nutmeats, in-shell pecans, and in the inedible components (shuck, shell and middle septum) of in-shell pecans. They also determined if the survival and growth characteristics of Salmonella are affected by its exposure to a water extract of shucks and to pecan orchard soil containing the extract.

Salmonella can grow on high-Aw, nutrient-rich pecan nutmeats, shucks and shells, but is sensitive to antimicrobials in the septum tissue and aqueous extract of shucks. The ability of the pathogen to grow in high-Aw pecan nutmeats and in inedible components emphasizes the importance of limiting the time these components are exposed to high-moisture pre-harvest and post-harvest environments.

Salmonella did not grow on high-Aw nutmeat halves, pieces or granules stored at 4 C for up to 48 hours. Growth did occur, however, at 21 C, 30 C and 37 C. Increases of 1.77 to 5.87 log CFU per gram occurred within 48 hours at 37 C. The granules provided more readily available nutrients for the pathogen. The pathogen grew on high-Aw pecan shucks and shells, but died on middle septum tissue that had been stored at 21 C, 30 C and 37 C for up to six days.

Further information. Michael Doyle, Center for Food Safety, University of Georgia, Griffin Campus, Griffin, GA 30223; phone: 770-228-7284; fax: 770-229-3216; email:
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Publication:Emerging Food R&D Report
Date:Feb 1, 2012
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