Standardizing an Oregano Inoculation Procedure for Use in Challenge Studies on Reduction of Salmonella in Dry Spices

Introduction: Spices have attracted interest among food safety professionals after being found to be the vehicle of Salmonella causing foodborne outbreaks. As for other low moisture foods, the FDA has recommended validating lethality treatments to ensure achievement of target log reductions in Salmonella. However, standardization of methods to conduct such validation studies is needed and there is little information about the best methods.

Purpose: To standardize a procedure for inoculating oregano as part of challenge studies on decontamination of dry spices.

Methods: Oregano was inoculated with a cocktail of Salmonella Rissen and Salmonella Montevideo using liquid and dry inoculation procedures. Liquid inoculation (LI) involved two inoculation steps, each mixing 10 ml of the Salmonella cocktail (10.2 log CFU/ml) with 50 g of oregano. After each inoculation, oregano was dried at 35°C until it reached its original a_w (0.45). Dry inoculation (DI) was conducted by mixing the oregano with silica or talc powder (TP) containing (8.4 and 6.3 log CFU Salmonella/g, respectively). Salmonella was then recovered by plating on TSA, incubating at 35°C for 3 h and then overlaying with XLT4 before continuing incubation up to 24 h.

Results: For LI, counts of Salmonella in the oregano were 3.8 log CFU/g after the first step and 6.6 log CFU/g after the second step, whereas for DI the counts were 6.7 log CFU/g and 6.0 log CFU/g when using silica and TP, respectively. The mean concentration of Salmonella in oregano when using only the first LI was significantly different from the second LI and the DI procedures (P < 0.05).

Significance: These methodologies can potentially be extended to other spices for evaluating decontamination treatments in the laboratory, or conducting in-house validation studies using surrogate organisms instead of Salmonella.