Drdf dy J

aAt 2 rAr

In this way, the temperature at the can center can be calculated after each time interval to produce a heat penetration curve upon which the process lethality or F- value can be calculated. When the numerical computer model is used to calculated the process time required at a given retort temperature to achieve a specified lethality, the computer follows a programmed search routine of assumed process times that quickly converges on the precise time at which cooling should begin in order to achieve the specified F-value. Thus, the model can be used to determine the process time required for any given set of constant or variable retort temperature conditions. The next section describes the method for calculating the process time required at any given retort temperature to deliver a specified process sterilizing value or lethality.

Process Calculation

The numerical integration of equation 3 is the most versatile method of process calculation because it is universally applicable to essentially any type of thermal processing situation. It makes direct use of the product temperature history at the slowest heating point of the container obtained from a heat penetration test or predicted by a computer model for calculating the process sterilizing value delivered by a given temperature-time history. This method is particularly useful in taking maximum advantage of computer-based data acquisition systems used in connection with heat penetration tests. Such systems are capable of reading temperature signals received directly from thermocouples monitoring both retort and product center temperature and processing these signals through the computer. Both retort temperature and product center temperature are plotted against time without any data transformation. This allows the operator to see what has actually happened throughout the duration of the process test. As the data are being read by the computer, additional programming instructions call for calculation of the incremental process sterilizing value at each time interval between temperature readings and summing these over time as the process is under way (numerical integration of equation 3). As a result, the accumulated sterilizing F-value is known at any time during the process and can be plotted on the graph along with the temperature histories to show the final value reached at the end of the process. An example of the computer printout from such a heat penetration test is shown in Figure 18.

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