Purpose: To quantify the effect of initial product aw on the inactivation rate of Enterococcus faecium (a non-pathogenic surrogate for Salmonella) inoculated on the surface of almonds, and the predictive ability of a modified thermal inactivation model.
Methods: Almonds were inoculated with Enterococcus faecium (~108 CFU/g), equilibrated to three water activities (~0.34, 0.55, 0.85), placed in a steel mesh rack (15 almonds), and heated in a computer controlled oven (~149°C dry air, ~11°C dew point) for 0 to 15 min (1 min intervals; triplicate runs). After thermal treatment, aw of each sample was measured (4 almonds). The rest of the sample (11 almonds) was immediately cooled in peptone water (~4°C), plated on MRS, and enumerated. A previously published, modified inactivation model, accounting for process dew point, was used to predict E. faecium inactivation based on measured almond surface temperature and process dew point.
Results: The accuracy of the model prediction was reasonably good (RMSE = 0.57 and 0.33 log for low and high aw, respectively). Also, the model parameters were re-estimated based on the present data, in order to compare responses at the different initial aw levels. All the parameters (Dr, ZT, and ZM) of the model were significantly (P < 0.05) different for each group, which implies that the initial aw significantly affected the pathogen response.
Significance: Therefore, model-based validations of thermal pasteurization processes will be improved by incorporating product aw, in addition to process humidity, into thermal inactivation models.