Stability of Salmonella Inoculum on Hazelnuts

Introduction:  Due to a number of recent outbreaks associated with tree nuts, industries are in need of evaluating and validating effective processing steps to reduce Salmonella.  The hazelnut industry desires the validation of effective processes for reducing Salmonella that could be easily implemented without being cost-prohibitive.  Effective inoculation methods and inoculum stability has not been determined for hazelnut products.

Purpose:  Develop consistent inoculation procedures for Salmonella on hazelnut products and determine stability under storage conditions. 

Methods:  Salmonella strains previously associated with nuts (n = 5; various serovars) were lawn harvested from tryptic soy agar (10 log CFU/ml).  Harvests (25 ml) were mixed with in-shell hazelnuts (400 g; Corylus Americana var. Barcelona).   Nuts were dried in a biological safety cabinet at ambient temperature for 24 h prior packaging and storage at 4°C.  Inoculated hazelnuts were enumerated after 1 and 5 days of storage using standard dilution and plating methods on Xylose Lysine Desoxycholate (XLD) Agar following incubation (37°C, 24 - 48 hours).

Results: Immediately following inoculation, Salmonella levels on hazelnuts were 9.08 ± 0.23 log CFU/g.  A 24-h drying time led to a modest decrease in the inoculum to a level of 8.56 ± 0.43 log CFU/g.  Inoculum levels remained high (7.93 ± 0.33 log CFU/g) throughout 5 days of storage at 4°C.  Salmonella Anatum and Salmonella Enteritidis PT9C levels were the most negatively impacted by storage.

Significance:  Inoculation methods and subsequent storage at 4°C (up to 5 days) achieved and maintained suitably high concentrations for planned process validation studies.