BLBS102-c38 BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come
734 Part 7: Food ProcessingTable 38.5.Example of Improved General Method for Process
Evaluation at Reference Temperature of 250◦F andzValueof18◦FTime
(min)Temperature
(◦F)Lethality Rate (min)
L= 10 (T−Tr/z)Accumulated
lethality (min)
L∗t0 62 3.59E− 11 0.0E+ 00
4 71 1.14E− 10 4.5E− 10
8 99 4.08E− 09 1.6E− 08
12 134 3.59E− 07 1.4E− 06
16 171 4.08E− 05 1.6E− 04
20 202 2.15E− 03 8.6E− 03
24 231 8.80E− 02 3.5E− 01
28 241 3.16E− 01 1.3E+ 00
32 247 6.81E− 01 2.7E+ 00
36 248 7.74E− 01 3.1E+ 00
40 250 1.00E+ 00 4.0E+ 00
44 250 1.00E+ 00 4.0E+ 00
48 231 8.80E− 02 3.5E− 01
52 181 1.47E− 04 5.9E− 04
56 137 5.27E− 07 2.1E− 06
60 97 3.16E− 09 1.3E− 08Accumulated lethality=15.80value during the entire heating and cooling phases (Equation 32;
Table 38.5).Accumulated lethality=F 0 =∫t010 (T−Tr
z )dt=∫t0L=∑t0Lt (32)Note that general methods do not have any specific re-
quirement about the shape of the time–temperature curve.
They are therefore accurate and they should be the meth-
ods of choice for handling “complex” temperature data (Ball
1928). In 1980s, computerized mathematical models for pre-
dicting time–temperature data using improved general method
were developed by different researchers (Teixeira and Manson
1982, Datta et al. 1986, Sastry 1986, Chandarana and Gavin
1989, Chang and Toledo 1989, Lee et al. 1990) forF 0 value
calculations.
Even though the General Method is the base for comparing
the accuracy of other formula methods, the process is critically
dependent on product, container, and process delivery system
conditions as studied at the time of the test. Extrapolation out-
side the given process condition yields undefined, invalid lethal-
ity results. Additional testing must be performed to verify the
sterilizing value delivered under the altered conditions.Formula MethodsThe formula method of process calculation simplifies the te-
dious steps associated in calculating process time using gen-eral methods. However, it requires characterization of the non-
linear heat penetration data to compute the key heat penetration
parameters.Characterization of Heat Penetration DataHeat penetration data are time–temperature combination data
that are collected after exposing a given food for a given period
of time under a given heating condition. For products in metal
cans, which are mainly heated by conduction, the slowest heating
point is the center of the container, and for products whose mode
of heating predominated by convection, the slowest heating point
is 1/10thabove the bottom of the can along the central vertical
axis. Accurate determination of heating profile of a given system
is very important for accurate establishment of process schedule.
The profile should be collected under conditions that simulate
the actual process situations. Typical heat penetration profiles
for conduction and convention heating products is shown in
Figure 38.5.
Because of the limitations of the General Method, a more
robust Formula Method was developed and proposed by Ball
(1923). It makes use of the fact that the difference between retort
and cold spot temperature decays exponentially over process
time after an initial lag period. Therefore, a semi-logarithmic plot
of this temperature difference over time appears as a straight line
that can be described mathematically by a simple formula and
related to lethality requirements by a set of tables that must be
used in conjunction with the formula. The originally developed
method was later modified by Ball and Olson (1957), which has
been still widely used in the canning industry.