Dry inoculation methods for nonfat milk powder

摘要

Carriers with inoculated microorganisms are oftenused to validate low-moisture food safety interventions.In this study, we evaluated dry inoculation methodsusing silicon dioxide (SiO_2) and a small portion of nonfatmilk powder (NFMP) as dry carriers for NFMP.Silicon dioxide was characterized by vapor sorptionanalysis. One milliliter of inoculum of a 5-strain Salmonellacocktail (serovars Agona, Reading, Tennessee,Montevideo, and Mbandaka) or Enterococcus faeciumNRRL B-2354 was inoculated onto 1 g of SiO_2 or 10g of NFMP as carriers. Both inoculated carriers wereair-dried for 72 h [22°C, relative humidity (RH) ~30%],equilibrated to water activity (a_w) 0.25 ± 0.02 (24 h at22°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°C,RH 25%, and its bacterial populations were monitoredfor 30 d. Both sets in equilibrated NFMP were subjectedto isothermal treatments in closed aluminumcells at 85, 90, and 95°C. Silicon dioxide maintainedmoisture content (0.29 ± 0.03%, dry basis) at differentwater activities. The NFMP inoculated with both carriersexhibited stable bacterial populations over 30 d at22°C. Strains in NFMP inoculated with SiO_2 showedequal or higher D-values but equal z-values comparedwith those inoculated with a small portion of NFMP.Enterococcus faecium exhibited comparable thermal resistanceto Salmonella under all tested conditions. Thisstudy supports E. faecium as a Salmonella surrogatein thermal processing of NFMP and the use of SiO_2 toinoculate NFMP.