Handbook of Meat Processing

342 Chapter 19

(Guerrero Legarreta 2001 ). D and F are

related through the equation:

F=−Da b()log log (19.4)

where:

a = initial viable cell load

b = fi nal viable cell load

In extreme cases, when the presence of C.

botulinum is presumed or complete spore

destruction is necessary, a “ botulinum cook ”

is applied. This is a process, generally applied

only to low - acid foods such as meats, fi sh, or

dairy products, where cell number is reduced

from 10^1 to 10^0 , a 12 log - cycle reduction or

12D. Heat treatment ensures the probability

of fi nding 1 spore in 10^12 cans. Processing

parameters in botulinum cook are (for C.

botulinum ): D 120 ° C = 2.52 minutes and

z = 10 ° C.

Process Lethality Calculations

When a new thermal process is designed or

applied for the fi rst time to a food, F - values

are analyzed using thermocouples at various

positions on the container, mainly at the cold

point. Recently, the use of thermocouples has

been substituted by radiotelemetry.

The rate at which a microbial population

is destroyed and the total processed severity

can be calculated by several methods; the

easiest ones use the area under a curve that

represents lethality versus time. Lethality is

calculated by the equation:

log t F()=−() 250 T z (19.5)

where:

t = time at any given minute

T = temperature in the process

Another method for calculating the overall

process lethality is by adding the F - value at

every moment during the heating and cooling

phases. Manev (1983) describes in detail the

process for a tropical preserve expected to

have a shelf life up to 1 year at 40 ° C; in this

moment of the process: heating up, tempera-
ture holding, and cooling down. A hypotheti-
cal situation occurs assuming F = 1; this is
the lethality effect when heating at 120 ° C for
1 minute. F s is the sum of all F values in each
point of the container
Heating in not homogenous in a food
material, even less in canned meats or meat
products, where fat, connective tissue, and
other compounds such as carbohydrates
and other additives are present, each with a
wide range of heat - transfer capacities.
Calculations, therefore, are carried out in ref-
erence to the point where heating occurs at
the slowest rate or the “ cold point ” ; at this
point, the sum of all lethal effects is denoted
as F c. The cold - point position depends, there-
fore, on the food composition and the leading
heating mechanisms. If convection is the
main mechanism, the cold point is located
along the vertical can axis. Can agitation, for
instance when cans are rotated, increases the
heat - transfer rate; in this case, the cold point
is located approximately one - third up from
the can bottom. This is the case with meat
chunks in brine or canned sausages. If con-
duction is the main mechanism, the cold
point is in the geometric can center. F c is
lower than F s , as heating in the center is
always lower than in the rest of the container

10 o

106

105

104

103

102

101

number of survivors per mL or g

0 10 20

110°C

D=19 min

120°C

D=1 min

Figure 19.1. Thermal death time curve. (Adapted
from Stiebing 1992 .)