ABSTRACT
The standard methods of calculating microbial mortality and spore inactivation are based on the assumption that these processes follow a first-order kinetics. Consequently, assessment of the safety of a preservation process or of the efficacy of a disinfection method has usually been done on the basis ofD values: the time needed to reduce the number of targeted microbial cells or spores by a factor of 10. Similarly, traditional methods to establish the safety of thermal preservation processes for low-acid foods, which is mandated by regulating authorities, is based on ‘‘thermal death time’’: the time needed to reduce a population of C. botulinum spores by 12 orders ofmagnitude at a given temperature. Becausemost experimental survival data only cover five to eight orders of magnitude reduction, the thermal death time is calculated by extrapolation, assuming that a first-order inactivation kinetics holds over the entire 12-decade range. Since commercial sterilization usually involves heating and cooling and, hence, nonisothermal conditions, the degree of inactivation achieved is calculated by integration of the temporary effects over the pertinent temperature history or ‘‘profile.’’ The process lethality is assessed in terms of F0 values, calculated by (Teixeiva 1992, Jay 1996, Holdsworth 1997, Toledo 1998):
F0 ¼ ðt 0
Z dt ð1Þ
where Tref is a reference temperature and Z is the temperature span at which D itself is reduced by a factor of 10. Traditionally, D has been 250jF, or 121.1jC, for low-acid foods (see below).
