Study of Enterococcus faecium NRRL B-2354 as a surrogate for Salmonella in low-moisture foods and its use in the validation of radio frequency pasteurization
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Salmonella outbreaks associated with low-moisture foods (LMFs) have become a food safety concern for the food industry, government and the public. A qualified Salmonella surrogate and procedures of applying it are needed for microbial validation of thermal processing of LMFs. The objectives of this research were to 1) validate potential surrogate Enterococcus faecium NRRL B-2354 (E. faecium) by comparing its heat resistance with Salmonella in different low-moisture environments, 2) evaluate dry carriers for inoculating powdered foods, and 3) validate radiofrequency pasteurization of wheat flour using E. faecium. These objectives were accomplished by isothermal inactivation studies of Salmonella and E. faecium using thermal-water activity (TAC) and thermal-death-time (TDT) cells. TAC cells control relative humidity while TDT cells fix moisture content in heat treatments. Via TAC cells, both Salmonella enterica Enteritidis PT 30 (S. Enteritidis) and E. faecium in silicon dioxide (SiO2) were treated under relative humidity 11-68% at 80°C. In the TDT cells, heat resistance parameters of Salmonella and E. faecium in wheat flour and non-fat milk powder inoculated by different carriers were obtained. Lastly, inoculated pack procedures for microbial validating radiofrequency pasteurization of wheat flour was developed. E. faecium showed equal or higher thermal resistance than Salmonella at tested conditions. D-and zT-values of both strains in wheat flour decreased with increased moisture content. D80°C values of S. Enteritidis and E. faecium in SiO2 increased exponentially with decreasing relative humidity. SiO2 is a good carrier for transmitting bacterial cells into powdered foods. Salmonella and E. faecium in non-fat milk powder inoculated by SiO2 were observed with high thermal resistance. An inoculated pack procedure was developed for validating radiofrequency pasteurization of wheat flour. This dissertation compares thermal resistance parameters for Salmonella and E. faecium in food and non-food systems under designated treatments. Results support E. faecium as a valid Salmonella surrogate in tested conditions. Water activity, moisture content, and bacterial carriers were proved to be key factors influencing bacterial thermal resistances in LMFs. Radiofrequency heating technology showed potential for pasteurizing wheat flour. The new information generated from this research aids in the development of thermal processes to ensure food safety of LMFs.