Thermal Food Processing

78 Thermal Food Processing: New Technologies and Quality Issues
121°C, or the point in time where the equivalent process curve crosses the vertical
axis drawn at 121°C, and is known as the F value for the process. This is often
referred to as the lethality of a process, and since it is expressed in minutes at
121°C, the unit of lethality is 1 min at 121°C. Thus, if a process is assigned an
F value of 6, then the integrated lethality achieved by whatever time–temperature
history is employed by the process must be equivalent to the lethality achieved
from 6 min of exposure to 121°C, assuming an idealized process of instantaneous
heating to 121°C followed by instantaneous cooling after the 6-min hold.
All that is required to specify the F value is to determine how many minutes
at 121°C will be necessary to achieve the specified level of log cycle reduction.
The D 121 value is used for this purpose, since it represents the number of minutes
at 121°C to accomplish one log cycle reduction. Thus, the F value is equal to
D 121 multiplied by the sterilizing value (number of log cycles required in
population reduction):
F = D121(log a−log b)
(3.1)
where a is the initial number of viable spores, and b is the final number of viable
spores (or survivors).
In the example given earlier, assume the value D 121 = 1.5 min was taken from
the TDT curve in Figure 3.2 and multiplied by the required sterilizing value (six
log cycles). Thus, F = 1.5 (6) = 9 min, and the lethality for this process has been
specified as F = 9 min. This is normally the way in which a thermal process is
specified for subsequent calculation of a process time at some other temperature.
In this way, proprietary information regarding specific microorganisms of concern
or numbers of log cycles reduction can be kept confidential and replaced by the
F value (lethality) as a process specification.
Note also that this F value serves as the reference point to specify the equivalent
process design curve discussed earlier. By plotting a point at 9 min on the vertical
line passing through 121°C on a TDT graph, and drawing a line parallel to the TDT
curve through this point, the line will pass through all combinations of process time
and temperature that deliver the same level of lethality. The equation for this straight
line can be used to calculate the process time (t) at some other constant temperature
(T) when F is specified.
F = 10
[( T−121)/ Z] t
(3.2)
The following equation becomes important in the general case when the
product temperature varies with time during a process, and the F value delivered
by the process must be integrated mathematically,
F = ∫10 t
0
[( T−121)/ Z] t
(3.3)