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.

Modeling and Statistical Issues Related to Salmonella in Low Water Activity Foods

“…The influence of temperature began to show around day 30, after which a significant decline was observed for the data at 22°C, while no significant decline was observed at either −24 or 4°C (Figure 10.1). These observed survival kinetics for Salmonella on raw peanuts are in line with other published literature, where survival curves for Salmonella in low water activity foods do not follow log-linear kinetics and show significant asymptotic tails (Uesugi, Danyluk, and Harris, 2006;2010, Blessington, Mitcham, andKimber et al, 2012;Blessington et al, 2013a;Blessington et al, 2013b;Keller et al, 2013;Santillana Farakos, Frank, and Schaffner, 2013). The data presented in Figure 10.1 indicate that the best descrip tion of Salmonella survival under these conditions requires a model that includes a nonlinear inactivation rate and the ability to incorporate tailing.…”

Growth and Survival of Enterobacteriaceae and Inoculated Salmonella on Walnut Hulls and Maturing Walnut Fruit

“…Salmonella is capable of survival and growth on moist walnut hulls and in hull slurries. When inoculated onto inshell walnuts or kernels, Salmonella, E. coli O157:H7, and L. monocytogenes survive for several months under cool and ambient conditions (5)(6)(7)15). Nut-specific guidance for Good Manufacturing Practices (16) is available, and the walnut industry has published best practices for both growers and processors (23).…”

“…After hulling and before storage, in-shell walnuts are dried at #43uC for 12 to 48 h (to moisture levels of less than 8%) (37). Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes can survive for extended periods on both in-shell walnuts (7,15) and walnut kernels (5,6) under typical storage conditions.…”

Bacterial community diversity on in-shell walnut surfaces from six representative provinces in China

“…Blessington et al . also found that foodborne pathogens could survive not only in walnut kernels but also on in-shell walnut surfaces for a long time during storage 19 , 20 . In China, since there are a few reports about nut-induced disease outbreaks, it is necessary to investigate the existence and distribution of bacterial pathogens on nut shells.…”