This paradox is resolved by making use of a relationship which shows that the rate of change in the thermal destruction of bacteria (i.e. the rate of change in their D values) is logarithmic around temperatures commonly used in heat sterilisation. This means that the lethal rate of destruction at any temperature can be related to that at a reference temperature. This relationship is graphically represented .in Figure 2 which shows a thermal death time curve passing through 1 min at 121.1 ºC. This "phantom" curve shows that relative to the lethal rate of unity at 121.1°C the lethal rates at 91.1, 101.1, 111.1, 131.1, 141.1 and 151.1 ºC are 0.001, 0.01, 0.1, 10, 100 and 1 000, respectively.
The sterilising effect of a thermal process (the process Fo value) can therefore be computed by integrating the combined lethal effect of exposure at all time/temperature combinations throughout the process. This means that a process that delivers an Fo value of 2.8 min (the so called 12D process for Clostridium botulinum) is equivalent in . sterilising effect to heating the contents of the can to 121.1 ºC instantly, holding it at that temperature for 2.8 min, and then cooling it instantly. Similarly, a process for solid style canned tuna packed in 84 x 46.5 mm cans may have a target Fo value of 10 min, which can be achieved by processing for 74 min at 116 ºC or 50 min at 121.1 ºC. With each process, however, the sterilising effect is the same as, and equivalent to, holding the can of tuna at 121.1 ºC for 10 min under conditions of instantaneous heating and cooling.
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