22-3 |
How can the efficacies of thermal and non-thermal preservation methods be compared? |
M. PELEG, Dept. of Food Science & Nutrition, Univ. of Massachusetts, Chenoweth Bldg., Box 31410, Amherst, MA 01003 The semilogarithmic survival curves of microbial cells and spores exposed to a lethal agent are frequently nonlinear, indicating that their inactivation is not a process which follows a first order kinetics. Consequently, a process calculation based on D and Z values cannot be considered as admissible. What can be meaningfully compared is the actual final survival ratio, or number of decades reduction. To calculate a survival curve, of any thermal or non-thermal process, one can assume that its local slope is that of the survival curve at the momentary temperature, pressure or chemical agent concentration, etc., at a time which corresponds to the momentary survival ratio. If this assumption is correct, and the isothermal, isobaric or iso-concentration survival curves and the process's temperature, pressure or concentration profile can be expressed algebraically, all that is needed is to express the survival parameters' temperature, pressure or concentration dependence algebraically too. As long as the model is not used for extrapolation, the mathematical format of the three kinds of algebraic expressions is unimportant. The calculated survival curve is the numerical solution of a differential equation whose coefficients are terms derived from the experimental survival data and process's profile. It can be computed and plotted with standard mathematical software. This is shown with simulated survival curves of food pathogens and bacterial spores of safety concern. In principle, the method can be extended to processes where several factors that affect the inactivation rate change simultaneously.
Session 22, Establishing processing criteria in nonthermal food preservation
|