Evaluation of Methods for Inoculating Powdered Milk and Soy Flour with Salmonella enterica Serovar Typhimurium LT2, Enterococcus faecium, and Cronobacter sakazakii

Introduction: Salmonella and Cronobacter are two microorganisms of concern in low water activity (a_w) powdered products; this is due to their ability to remain viable for long periods of time. In order to develop a method for sterilization of powdered products, first, a method for inoculating low a_w powders must be developed. There are a few existing methods, but most use a carrier molecule or have only tested one organism on powdered foods.

Purpose: The purpose of this study was to evaluate two low a_w inoculation methods and develop a methodology that does not utilize a carrier molecule and works for multiple bacterial species.

Methods: Salmonella enterica serovar Typhimurium LT2, Salmonella surrogate Enterococcus faecium, and Cronobacter sakazakii were inoculated onto both nonfat dry milk (NFDM) and soy flour using one of two inoculation methods. The first inoculation method involved adding bacterial pellets to powders, directly, and homogenizing. Inoculation method two utilized a chromatography reagent sprayer to inoculate powder.

Results: Method one achieved a significantly (P<0.05) higher inoculum level than method two in both NFDM and soy flour after four separate tests with each powder. Method one achieved inoculum log levels (CFU/g) of 9.8±0.45 in NFDM and 9.9±0.19 in soy for C. sakazakii; 7.4±0.35 NFDM and 8.2±0.56 soy for S. enterica; and 8.8±0.41 NFDM and 9.1±0.32 soy for E. faecium. Method two achieved inoculum levels of 9.9±0.19 in NFDM and 8.3±0.28 in soy for C. sakazakii; 6.3±0.22 NFDM and 6.9±0.19 soy for S. enterica, and 9.1±0.32 NFDM and 6.9±0.59 soy for E. faecium.

Significance: The methods evaluated showed that inoculation of low a_w powders with various microorganisms does not require a carrier molecule and are suitable for inoculation of various foodborne pathogen or surrogate organisms in powdered foods.