Survival of foodborne pathogens on inshell walnuts

• Published in: International journal of food microbiology 2013-09, Vol.166 (3), p.341-348
• Main Authors: Blessington, Tyann, Theofel, Christopher G., Mitcham, Elizabeth J., Harris, Linda J.
• Format: Article
• Language: English
• Subjects: Anti-Bacterial Agents - pharmacology; Biological and medical sciences; Desiccation; Escherichia; Escherichia coli; Escherichia coli O157 - drug effects; Escherichia coli O157 - physiology; Food industries; Food Microbiology; Fruit and vegetable industries; Fundamental and applied biological sciences. Psychology; Inshell; Juglans; Juglans - microbiology; Listeria; Listeria monocytogenes; Listeria monocytogenes - drug effects; Listeria monocytogenes - physiology; Microbial Viability - drug effects; Nut; Salmonella; Salmonella enterica enteritidis; Salmonella enteritidis; Salmonella enteritidis - drug effects; Salmonella enteritidis - physiology; Sodium Hypochlorite - pharmacology; Temperature; Time Factors; Walnut
• ISSN: 0168-1605; 1879-3460
• DOI: 10.1016/j.ijfoodmicro.2013.07.016

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. •Foodborne pathogens survived for long periods on inshell walnuts.•Decline of Salmonella was slower on walnuts stored at 4°C than at 23°C.•Low initial bacterial levels (