220 230 240 250 260 270 280 290 Temperature (°F)

Figure 13. Thermal death time (TDT) curve showing temperature dependency of D value (decimal reduction time of microorganisms) given by temperature change (Z) required for tenfold change in D value. Source: Ref. 3, reprinted with permission, copyright 1992 by Marcel Dekker, Inc., New York.

and temperature requirements for any idealized thermal process. For example, assume a process is required that will achieve a 6-log-cycle reduction in the population of bacterial spores whose kinetics are described by the TDT curve in Figure 13, and that a temperature of 235°F has been chosen for the process. The TDT curve shows that the D-value at 235°F is 10 min. This means that 10 min will be required for each log cycle reduction in population at that temperature. If a 6-log-cycle reduction is required, a total of 60 min is required for the process. If a temperature of 270°F had been chosen for the process, the D-value at that temperature is approximately 0.1 min, and only 0.6 min (or 36 s) would be required at that temperature to accomplish the same 6-log-cycle reduction.

Sterilization F-Value (Lethality). The example process calculations carried out using the TDT curve in Figure 13 showed clearly how two widely different processes (60 min at 235°F and 0.6 min at 270°F) were equivalent with respect to their ability to achieve a 6-log-cycle reduction for that organism. Therefore, for a given Z-value, the specification of any one point on the line is sufficient to specify the sterilizing value of any process combination of time and temperature on that line. The reference point that has been adopted for this purpose is the time in minutes at the reference temperature of 250°F (121°C), or the sterilizing F-value (lethality) for the process. Since the F-value is expressed in minutes at 250°F (121°C), the unit of lethality

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