An in-depth discussion of TDT calculations is not possible here. Readers are referred to works such as Ref. 5. The following formula is used to calculate the D value:
where t is the heating time (corrected for the come up time) at temperature T, A is the initial number of organisms, and B is the number of survivors. Alternatively, the D value can be determined by plotting the number of survivors at several time intervals against time on semilogarithm paper. (The D value is calculated from the slope of the bestfit line.) D values determined at several temperatures are plotted on semilogarithm paper against temperature (called a phantom TDT curve), and the z value (the change in the thermal death rate with temperature) is calculated from the slope of the line. Knowing the D and z values for an organism allows the calculation of the D value at any other temperature. The F value, or the number of minutes to destroy a given number of organisms at a specific temperature, can also be calculated. The minimum health sterilizing value for a low-acid food is generally a 12D value for C. botulinum (12 times the D value determined for C. botulinum spores in the specific food); the commercial sterility value is generally 5D for C. sporogenes (PA3679) spores.
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