Survival of foodborne pathogens on inshell walnuts

Abstract

The survival of Salmonella enterica Enteritidis PT 30 or five-strain cocktails of S. enterica, Escherichia coli O157:H7, and Listeria monocytogenes was evaluated on inshell walnuts during storage. Inshell walnuts were separately inoculated with an aqueous preparation of the pathogens at levels of 10 to 4log CFU/nut, dried for 24h, and then stored at either 4°C or ambient conditions (23–25°C, 25–35% relative humidity) for 3weeks to more than 1year. During the initial 24-h drying period, bacterial levels declined by 0.7 to 2.4log CFU/nut. After the inoculum dried, further declines of approximately 0.1log CFU/nut per month of Salmonella Enteritidis PT 30 levels were observed on inshell walnuts stored at 4°C; at ambient conditions the rates of decline ranged from 0.55 to 2.5log CFU/nut per month. Rates of decline were generally greater during the first few weeks of storage, particularly at lower inoculum levels. The survival of the five-strain cocktails inoculated at very low levels (under 400CFU/nut) was determined during storage at ambient conditions. The pathogens could be recovered by either enumeration or enrichment from most samples throughout the 3-month storage period; reductions in bacterial levels from the beginning to end of storage were 0.7, 0.2, and 2.3log CFU/nut for Salmonella, E. coli O157:H7, and L. monocytogenes, respectively. For 6% of all nut samples (14 of 234 samples), pathogens were isolated from the second but not first 24-h enrichment, suggesting that bacterial cells were viable but not easily culturable. Salmonella-inoculated walnuts were exposed for 2min to water or a 3% solution of sodium hypochlorite (to mimic commercial brightening) either 24h or 7days after inoculation; treated nuts were dried for 24h and held at ambient conditions. Salmonella levels were reduced by less than 0.5log or 2.4 to 2.6log CFU/nut on water– or chlorine– treated walnuts, respectively, regardless of postinoculation treatment time. Additional reductions of 2.6 and 2.1log CFU/nut were observed for water- and chlorine-treated walnuts, respectively, after storage for 2weeks at ambient conditions. Bacterial foodborne pathogens are capable of long-term survival on the surface of inshell walnuts even when initial levels are low.