Purpose: The study aimed to: i) validate RF heating treatment to inactivate Salmonella Enteritidis in corn flour, as a model low moisture food, and ii) study the effect of holding and cold shock in enhancing RF inactivation.
Methods: Corn flour (moisture content 10.8%) was inoculated with Salmonella Enteritidis PT30 (Salmonella PT30) at approximately 108 CFU/g. Inoculated samples (0.7 g) were heated in a water bath at 75, 80, and 85°C for predetermined time periods to determine D and z-values. Corn flour (seven pounds) was loaded into a polyetherimide (PEI) container (7 by 24 by 30 cm3) with prepackaged inoculated corn flour (five grams) placed in the geometric center. The samples were subjected to RF heating to reach temperatures of 75, 80, and 85°C, respectively, and held for 10 min after heating. The Salmonella PT30 survivors in heated samples with/without holding were enumerated. To study the effect of cold shock, inoculated samples (20 g) were loaded into small plastic bottles (height 6.3 by diameter 3.4 cm), which were subjected to RF heating to reach 80 and 85°C. The heated samples were kept in a freezer at -20°C for 48 and 96 h before the Salmonella survivors were evaluated.
Results: The D-values of Salmonella PT30 at 75, 80, and 85°C were 14.2, 6.1, and 2.03 min, respectively, and the z-value was 11.9°C. Heating only (without holding) to reach 75, 80, and 85°C temperatures had caused 0.9-, 2.02-, and 2.83-log reductions of Salmonella in the corn flour, respectively. Holding samples for 10 min in the container further resulted in 0.45 to 1.53 log-reduction. Freezing of the heated samples for 96 h caused around 0.6 to 1.8 log reduction.
Significance: Results demonstrated that RF heating treatments effectively inactivated Salmonella PT30 in packaged corn flour and that RF combined with holding and cold shock significantly enhanced the inactivation effect.