On calculating sterility in thermal and non-thermal preservation methods

Abstract

There is growing evidence that the mortality of microbial cells, and the inactivation of bacterial spores, exposed to a hostile environment need not follow a first order kinetics. Consequently microbial semi-logarithmic survival curves are frequently non-linear, and their shape can change with temperature or under different chemical agent concentrations, for example. Experimental semi-logarithmic survival curves under unchanging conditions, can be described by an equation whose coefficients are determined by the particular temperature, agent concentration, etc. If the dependency of these coefficients on temperature, agent concentration, etc., can be expressed algebraically, then in principle one can construct the survival curve for the changing or transient conditions that exist in industrial thermal and non-thermal treatments. This is done by incorporating the lethal agent's mode of change, e.g. the heating or pressure curve into the survival curve equation parameters. The result is a mathematical model that would enable the calculation of the time needed to achieve any degree of microbial survival ratio numerically, without the need to assume any mortality kinetics. Such a model can be used to assess, or compare, the efficacy of different preservation processes where the intensity of the lethal agent changes with time. The concept is demonstrated with a special simple case using simulated thermal treatments. The outcome of the simulations is presented as planar log survival vs time relationships and as curves in a three-dimensional log survival–temperature–time or log survival–concentration–time space.