BLBS102-c38 BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come
38 Thermal Processing Principles 731Mathematically, using reference temperature (Tr) and reference
Dvalue (D 0 ),zvalue (◦C) is expressed asz=Tr−T
logD−logD 0(23)or in between two temperature ranges ofT 1 andT 2 ,z=T 2 −T 1
logD 1 −logD 2(24)whereD 1 andD 2 areDvalues atT 1 andT 2 , respectively.
A largerzvalue indicates that the rate of destruction of mi-
croorganism is less temperature sensitive. A smallzvalue in-
dicates higher temperature sensitivity and a small change in
temperature will result in a significant change on microbial pop-
ulation. Most commonly, quality parameters like nutrients and
color have largezvalues as compared to microorganisms that
have smallzvalues. Again, microbial spores will have a higher
zas compared to vegetative bacteria.
TheDvalue at any given temperature can be obtained from a
modified form of the above equation using the referenceDvalue
(D 0 at a reference temperature,Tr,usually 121.1◦C (250◦F) for
thermal sterilization and 82.2◦C (180◦F) for pasteurization).D=D 010 (Tr−T
z ) (25)Concepts of Process LethalityLethality (F 0 value) is a measure of the heat treatment or steril-
ization processes. To compare the relative sterilizing capacities
of heat processes, a unit of lethality needs to be established. For
convenience, this is defined as an equivalent heating time of 1
minute at a reference temperature, which is usually taken to be
121.1◦C (250◦F) for the sterilization processes. The criteria for
the adequacy of a process must be based on two microbiolog-
ical considerations: (i) destruction of the microbial population
of public health significance and (ii) reduction in the number of
spoilage-causing bacteria.
In the canning industry, theF 0 value is often used for the low-
acid canned foods and refers to the lethality value (FTzrefvalue)
with azvalue of 10◦C(18◦F) and a reference temperature (Tref)
of 121.1◦C (250◦F). Thezvalue of 10◦C is used as the thermal
characteristic for the pathogenic microorganism,C. botulinum
spores.
Experimental work and experience have shown that a process
that achieves 12 log reductions ofC. botulinumcan be considered
commercially sterile (Tucker 2008). If such a process is correctly
applied to a product, then the health risk to the consumer will
be insignificant. Applying a 12Dprocess reduces the probability
of spore survival by a factor of 10^12. If cans contain one initial
spore, then for trillion cans produced and given a 12Dprocess,
only one can would contain a surviving spore.
TheD 1 21.1◦C(D 250 ◦ForD 0 ) value can be used for establishing
such a process. TheD 0 forC. botulinumis taken as 0.23 minute.
Thus, 12Dvalue would represent 12×0.23 or 2.76 minutes.
The minimumF 0 value should be 2.76 minutes to accomplish
the 12Dprocess. In general, ifN 0 is the initial spore concen-
tration andNthe target spore concentration need to achieve,
then for exposure to a theoretical constant process temperatureof 121.1◦C at the coldest or slowest heating point, the target
process would be:F 0 =D 0 log(
N 0
N)
(26)However, several low-acid foods are processed beyond the
minimumF 0 value of 2.76 minutes in order to deal with spoilage-
causing bacteria of much greater heat resistance. For these or-
ganisms, acceptable levels of spoilage probability are usually
dictated by economic considerations. Most food companies ac-
cept a spoilage probability of 10−^5 (5D=TDT) from mesophilic
spore formers (organisms that can grow and spoil food at room
temperature). The organism most frequently used to characterize
this classification of food spoilage is a strain ofC. sporogenes,
known as PA 3679, with a maximumD121.1◦Cvalue of 1.00
minute. Therefore, the 5Dminimum value would be 5 min-
utes and hence aF 0 of 5 minutes is more commonly used for
these foods. Where thermophilic spoilage is a problem, more
severe processes may be necessary because of the high heat re-
sistance of thermophilic spores. Fortunately, most thermophiles
do not grow readily at room temperature and require incubation
at unusually high storage temperatures (> 38 ◦C) to cause food
spoilage.
Sterilization time required at temperature (T) for the same
value target microorganism to deliver equal degree of sterility is
mathematically expressed asFTz=F 0 ∗ 10(Tref−T
z)
(27)The same general relationships as were discussed under ster-
ilization apply to pasteurization. A combination of temperature
and time that is sufficient to inactivate the particular species
of bacteria must be used. Fortunately, most of the pathogenic
organisms, which can be transmitted from food to the person
who eats it, are not very resistant to heat. For thermal process
calculation of high-acid canned foods with an extended refrig-
erated shelf life, for shelf-stable low pH<3.9 products (such as
fruit juices) and acidified foods under normal storage conditions,
food industries commonly use the pasteurization value (Powith
zvalue of 10oC andTrefof 82.2oC) instead of the sterilization
value (Fovalue), since foods generally are processed below or
around 100◦C.THERMAL PROCESS DETERMINATION
METHODSThermal process evaluation is a process of determining the de-
sired processing time at a given heating condition to achieve
desired process lethality or vice versa. Determination of process
lethality (F 0 ) can be done in two ways: using calculation method
(Equation 27) and microbiological data (Equation 26). Different
calculation methods have been used to determine the required
processing time to achieve the desired lethality. The reliabil-
ity of the method depends on how accurately it integrates the
lethality effect of transit temperature response of the food un-
dergoing thermal process, with respect to test microorganism of
public health concern. This implies that accurate determination