title: 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 4 log CFU/nut, dried for 24 h, and then stored at either 4 °C or ambient conditions (23-25 °C, 25-35% relative humidity) for 3 weeks to more than 1 year. During the initial 24-h drying period, bacterial levels declined by 0.7 to 2.4 log CFU/nut. After the inoculum dried, further declines of approximately 0.1 log 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 2.5 log 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 400 CFU/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 2.3 log 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 2 min to water or a 3% solution of sodium hypochlorite (to mimic commercial brightening) either 24 h or 7 days after inoculation; treated nuts were dried for 24h and held at ambient conditions. Salmonella levels were reduced by less than 0.5 log or 2.4 to 2.6 log CFU/nut on water- or chlorine- treated walnuts, respectively, regardless of postinoculation treatment time. Additional reductions of 2.6 and 2.1 log CFU/nut were observed for water- and chlorine-treated walnuts, respectively, after storage for 2 weeks at ambient conditions. Bacterial foodborne pathogens are capable of long-term survival on the surface of inshell walnuts even when initial levels are low.
authors.affiliation: Department of Food Science and Technology, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA; Department of Plant Sciences, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA; Western Center for Food Safety, University of California, Davis 1477 Drew Ave., Suite 101, Davis, CA 95618, USA.
authors.forename: Tyann
authors.initials: T
authors.lastname: Blessington
authors.forename: Christopher G
authors.initials: CG
authors.lastname: Theofel
authors.forename: Elizabeth J
authors.initials: EJ
authors.lastname: Mitcham
authors.forename: Linda J
authors.initials: LJ
authors.lastname: Harris
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D003890:Desiccation; D019453:Escherichia coli O157 / Q000187:drug effects / Q000502:physiology*; D005516:Food Microbiology; D031324:Juglans / Q000382:microbiology*; D008089:Listeria monocytogenes / Q000187:drug effects / Q000502:physiology*; D050296:Microbial Viability / Q000187:drug effects; D012477:Salmonella enteritidis / Q000187:drug effects / Q000502:physiology*; D012973:Sodium Hypochlorite / Q000494:pharmacology; D013696:Temperature; D013997:Time Factors
chemical_list: D000900:Anti-Bacterial Agents; D012973:Sodium Hypochlorite
keywords: Escherichia; Inshell; Listeria; Nut; Salmonella; Walnut