Thermal Food Processing

88 Thermal Food Processing: New Technologies and Quality Issues
Temperature (°C)
150
120
65
10
Retort
Time (min)
Product center
0 25 50 75 100 125 150
FIGURE 3.7 Computer-generated plot of retort temperature, can center temperature, and
accomplished lethality (F o) over time for thermal processing of a conduction-heated food.
(From Datta, A.K. et al., J. Food Sci., 51, 480–483, 507, 1986. With permission.)
processing these signals through the computer. Through programming instructions,
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 lethality (∆F i) at each time interval between
temperature readings and summing these over time as the process is under way.
As a result, the accumulated lethality (F) 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 3.7. Another test can be repeated quickly for a longer or shorter
process time, with instant results on the F o achieved. By examining the results
from both tests, the desired process time for the target F value can be closely
estimated and then quickly tested for confirmation. The results of two such heat
penetration tests are shown superimposed on each other in Figure 3.8. These
results show that test 1, with a process time of 68 min, produced an F value of
6, and test 2, with a process time of 80 min, produced an F value of 8, suggesting
that a target F value of 7 will be achieved by an intermediate process time. This
can be confirmed by running a test at the suggested process time, and examining
the resulting F value.
F o
25
20
15
10
5
0
Accomplished F o (min)