Thermal Food Processing

Simulating Thermal Food Processes Using Deterministic Models 87
Product temperature at can centre (°C)
110
100
90
80
70
10 20 30 40
Time (min)
50 60 70
FIGURE 3.6 Temperature history at center of canned food during thermal process for
calculation of process lethality by general method.
short duration, as shown in Figure 3.6, the temperature T i at each time interval
can be read from the curve and used to calculate the incremental lethality (∆F i)
accomplished during that time interval. Then the sum of all these incremental
sterilizing values equals the total lethality, F o, delivered by the test process. To
determine the process time required to deliver a specified lethality, the cooling
portion of the curve in Figure 3.6 is shifted to the right or left and the integration
is repeated until the delivered lethality, so what is calculated agrees with the value
specified for the process.
When first introduced in 1920, this method was sometimes referred to as the
graphical trial-and-error method because the integration was performed on specially
designed graph paper to ease the tedious calculations that were required.
The method was also time consuming, and soon gave way in popularity to the
historically more convenient (but less accurate) Ball formula method. With the
current widespread availability of low-cost programmable calculators and desktop
computers, these limitations are no longer of any consequence, and the general
method is currently the method of choice because of its accuracy and versatility.
The general method is particularly useful in taking maximum advantage of
computer-based data logging 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