Handbook of Meat Processing

Canned Products and Pâté 341

instance, 90% of a C. progenies population

is destroyed if heating at 110 ° C is maintained

for 10 minutes; D value in this case is

D 110 ° C = 10 min. At 115 ° C, the necessary time

for the same microbial reduction is 3 minutes

(i.e., D 115 ° C = 3 min); at 120 ° C, 1 min is neces-

sary to obtain the same population reduction

(D 120 ° C = 1 min). On the other hand, at differ-

ent heating times, the number of destroyed

microorganisms increases with time of expo-

sure. For example, in order to destroy 6 log

cycles (6D) of a given microbial population,

the required heating times are: 120 ° C for 6

minutes (D 120 ° C = 6), or 115 ° C for 18 minutes

(D 115 ° C = 18), or 110 ° C for 60 minutes

(D 110 ° C = 60) (ICMSF 1996 ). D - values allow

comparisons of the necessary heat treatment

severity among different microorganisms or

among temperatures.

z - Value

Heat resistance of a given microbial popula-

tion is given by a z - value, which indicates the

required temperature to reduce D - values by

1/10. The thermal death - time curve gives the

heating time (x - axis) versus log number of

survivors per ml or per g (y - axis); in this

curve, the slope is the z - value, showing the

temperature (in ° C) required to reduce a

given microbial population by 1 log cycle.

For instance, if z = 10 ° C, then D 100 ° C = 50

minutes, therefore D 110 ° C = 5 minutes, and

D 120 ° C = 0.5 minutes. This is shown in Figure

19.1.

F - value

F - value represents the thermal death extent

of a given microbial strain, as well as treat-

ment severity. It allows predictions con-

cerning the product shelf life, as well as

comparisons between different heat treat-

ment conditions. As it is impossible to instan-

taneously reach the processing temperature

in every part of a container or a can, the

F - value is the sum of heat treatments in every

canning, condensed steam is the most effi -
cient heating medium through a barrier (the
can) to the cold fl uid inside the can.
Heating mechanisms may change in prod-
ucts that modify their physical characteristics
during processing. For instance, in canned
emulsions, such as luncheon meats and p â t é s,
where the product changes from semifl uid to
solid, the heating mechanisms change from
convection to conduction. Process calcula-
tions must be carried out accordingly.
Finally, in products such as soups contain-
ing meat pieces or in sausages in brine, a
combined heat and mass transfer mechanism
takes place: heat transfer between fl uids
within the meat and between the product and
the heating medium, and mass transfer as
water and nutrients diffuse within the can.

Thermal Inactivation Parameters

In order to calculate the time - temperature
relationship for a given heat process, several
parameters have been developed that accu-
rately describe the necessary time at a given
temperature to achieve a given microbial
destruction. Severe heat treatment can destroy
all viable cells, and possibly all spores,
although other food characteristics such as
physicochemical (texture, color, water reten-
tion) and sensory quality can also be altered.
Therefore, there must be a compromise
between destruction of most undesirable
microorganisms (pathogens and spoilage -
related) and food quality.

D - Value

According to Equation 19.1 , 90% of the
microbial population is destroyed (1 log
cycle) at a given time interval, provided a
constant temperature is applied. This time
interval differs from one microbial strain to
another and is called decimal reduction time
(D). It is given by the time in minutes neces-
sary to destroy 90% of a given microbial
population at a constant temperature. For