Salmonella Survival Kinetics on Pecans, Hazelnuts, and Pine Nuts at Various Water Activities and Temperatures

The impact of temperature, water activity (a_w), and nut composition on Salmonella survival on tree nuts has not been thoroughly examined. The aim of this study was to determine the effect of temperature, a_w, and nut composition on the survival of Salmonella on tree nuts and develop predictive models. Pecans, hazelnuts, and pine nuts were chosen based on differences in their typical fat content. Nuts were inoculated with a cocktail of five Salmonella serotypes (11 log CFU/mL) and then were dried and stored at 4, 10, and 25°C at 0.41 ± 0.06 and 0.60 ± 0.05 a_w for 1 year. Ten-gram quantities were removed at different intervals up to 364 days to test for surviving Salmonella populations (plating on selective and nonselective media) and a_w. Experiments were carried out in triplicate. Salmonella populations were relatively stable over a year at 4 and 10°C at both a_w levels with <1.5-log CFU/g decline. The best predictive model to describe Salmonella survival at 4 and 10°C was a log-linear model with a D-value for each tree nut and a_w combination. Significant declines in Salmonella levels were observed at 25°C, where the best fit was a Weibull model with a fixed ρ for all tree nuts (ρ = 0.86), a δ value for each tree nut and a_w combination, and a random factor to account for variability among replicates. The time for the first log reduction at 25°C and 0.37 ± 0.009 a_w was estimated at 24 ± 2 weeks for hazelnuts, 34 ± 3 weeks for pecans, and 52 ± 7 weeks for pine nuts. At the same temperature, but with 0.54 ± 0.009 a_w, the mean estimated time for the first log reduction decreased to 9 ± 1 weeks for hazelnuts, 10 ± 1 weeks for pecans, and 16 ± 1 weeks for pine nuts. Tree nut, a_w, and temperature were shown to have a statistically significant effect on survival ( P < 0.05). No apparent influence of fat content on survival was observed. The results of this study can be used to predict changes in Salmonella levels on pecans, hazelnuts, and pine nuts after storage at the different temperatures and a_w values.