On calculating sterility in thermal and non-thermal preservation methods

There is growing evidence that the mortality of microbial cells, and the in activation of bacterial spores, exposed to a hostile environment need not f ollow a first order kinetics. Consequently microbial semi-logarithmic survi val curves are frequently non-linear, and their shape can change with tempe rature or under different chemical agent concentrations, for example. Exper imental semi-logarithmic survival curves under unchanging conditions, can b e described by an equation whose coefficients are determined by the particu lar temperature, agent concentration, etc. If the dependency of these coeff icients on temperature, agent concentration, etc., can be expressed algebra ically, then in principle one can construct the survival curve for the chan ging or transient conditions that exist in industrial thermal and non-therm al treatments. This is done by incorporating the lethal agent's mode of cha nge, e.g. the heating or pressure curve into the survival curve equation pa rameters. The result is a mathematical model that would enable the calculat ion of the time needed to achieve any degree of microbial survival ratio nu merically, without the need to assume any mortality kinetics. Such a model can be used to assess, or compare, the efficacy of different preservation p rocesses where the intensity of the lethal agent changes with time. The con cept is demonstrated with a special simple case using simulated thermal tre atments. The outcome of the simulations is presented as planar log survival vs time relationships and as curves in a three-dimensional log survival-te mperature-time or log survival-concentration-time space. (C) 1999 Canadian Institute of Food Science and Technology. Published by Elsevier Science Ltd . All rights reserved.