P3-82 The Effect of Initial Almond Water Activity on Thermal Inactivation of Enterococcus faecium during Dry Heating

Wednesday, July 31, 2013
Exhibit Hall (Charlotte Convention Center)
Pichamon Limcharoenchat, Michigan State University, East Lansing, MI
Bradley Marks, Michigan State University, East Lansing, MI
Sanghyup Jeong, Michigan State University, East Lansing, MI
Introduction: Validating commercial thermal pasteurization processes for low-moisture food products can be very challenging, due to dynamic process/product conditions and bacterial adaptation.  In particular, changing water activity (aw) is the most critical factor, but it has not been well incorporated into inactivation models that can be easily applied to commercial processes.   

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.