Purpose: Using wheat flour and peanut butter as separate model food products inoculated with either Salmonella Enteritidis or E. faecium: A) Assess inactivation kinetics with aw=0.45±0.05 at 80 ℃; B) Determine and compare D80℃, 0.45 values by using two types of test cells (TDT and TAC cells); C) Explore and find factors (water activity, food types) that influence thermal resistance; D) Identify desorption isotherms (aw changes with temperature) which influence thermal resistance.
Methods: Organic wheat flour (carbohydrate rich) and peanut butter (oil rich) were inoculated separately with Salmonella Enteritidis and E. faecium culture (1010 CFU/ml), and conditioned to aw 0.45 with final bacterial populations 108 CFU/g after equilibration. Inactivation studies were conducted at 80℃ using TAC and TDT cells in parallel. The TAC cell was able to maintain a constant aw, while the TDT cell maintained a constant sample moisture content, allowing the elevation of aw during the heating treatment.
Results: In wheat flour, D80℃, 0.45 values (4.25±0.45 min for Salmonella Enteritidis, and 5.56±0.49 min for E. faecium) by using TDT cells were significantly lower than those (7.28±1.17 min for Salmonella Enteritidis, and 9.96±1.06 min for E. faecium) by using TAC cells; in peanut butter, D80℃, 0.45 values were similar irrespective of the test cells used (TDT: 6.84±0.46 min, TAC: 8.03±0.68 min for Salmonella Enteritidis; TDT: 36.12±1.17min, TAC: 37.42±1.68 for E. faecium).
Significance: Water activity (aw) and food type are essential factors influencing the thermal resistance of bacteria. Such factors should considered in further thermal inactivation studies and validation for industrial applications.