Dry inoculation methods for nonfat milk powder

Carriers with inoculated microorganisms are often used to validate low-moisture food safety interventions. In this study, we evaluated dry inoculation methods using silicon dioxide (SiO2) and a small portion of nonfat milk powder (NFMP) as dry carriers for NFMP. Silicon dioxide was characterized by vapor sorption analysis. One milliliter of inoculum of a 5-strain Salmonella cocktail (serovars Agona, Reading, Tennessee, Montevideo, and Mbandaka) or Enterococcus faecium NRRL B-2354 was inoculated onto 1 g of SiO2 or 10 g of NFMP as carriers. Both inoculated carriers were air-dried for 72 h [22 degrees C, relative humidity (RH) similar to 30%], equilibrated to water activity (a(w)) 0.25 +/- 0.02 (24 h at 22 degrees C, RH 25%), and mixed with preconditioned NFMP (a(w) = 0.25 +/- 0.02) to reach an inoculation level of 8.2 +/- 0.2 log cfu/g. Inoculated NFMP was stored at 22 degrees C, RH 25%, and its bacterial populations were monitored for 30 d. Both sets in equilibrated NFMP were subjected to isothermal treatments in closed aluminum cells at 85, 90, and 95 degrees C. Silicon dioxide maintained moisture content (0.29 +/- 0.03%, dry basis) at different water activities. The NFMP inoculated with both carriers exhibited stable bacterial populations over 30 d at 22 degrees C. Strains in NFMP inoculated with SiO2 showed equal or higher D-values but equal z-values compared with those inoculated with a small portion of NFMP. Enterococcus faecium exhibited comparable thermal resistance to Salmonella under all tested conditions. This study supports E. faecium as a Salmonella surrogate in thermal processing of NFMP and the use of SiO2 to inoculate NFMP.