Food Biochemistry and Food Processing (2 edition)

BLBS102-c38 BLBS102-Simpson March 21, 2012 14:17 Trim: 276mm X 219mm Printer Name: Yet to Come

732 Part 7: Food Processing

of microbial kinetic data and reliable measurement of tempera-

ture profile of the product at the worst case or slowest heating

point within the container are important inputs to determine or

evaluate the reliability of a given thermal process.

The desired degree of lethality in terms of minimum equiva-

lent time at a given reference temperature is generally preestab-

lished for a given product considering kinetic data of target mi-

croorganism, and processes are designed to deliver a minimum

of this preset value at the thermal center. There are essentially

two widely accepted categories of methods for using these data

to perform thermal process calculations. The first of category

is the General Method of process calculation based on Bigelow

et al. (1920) method of calculation, and the second is the For-

mula Method of process calculation based on concepts of Ball

(1923) or Stumbo (1973).

General Method

The General Method developed by Bigelow et al. (1920) is a

graphical procedure to calculate thermal processes. It is a simple

method and can be applied to any product and process situation.

The method is used to determine sterility equivalents at known

product temperatures measured directly by the use of thermo-

couples positioned within the slowest heating test container at

the slowest heating spot in the container. It is the most accurate

method for evaluating under a given set of heat penetration data

gathering. It is universally applicable to essentially any type of

thermal processing situation and used to get the baseline target

value upon which all other experimental and formula calculation

method performances are compared.

However, the time–temperature data is at the coldest spot

and the method is only used to calculate process times for the

given heat profile data. Because of this main limitation of the

method, it lacks predictive power for different processes. There

are two General Method approaches: Original General Method

and Improved General Method.

Original General Method

The Original General Method uses the time–temperature data

at slowest heating point of the product and converts this data

to destruction rate graph to determine sterilization value of the

process (area under the curve in the graphical procedure). The

procedure is to plot the inverse of TDT against time to produce

a destruction rate curve. The area beneath the curve corresponds

to the accumulated sterilization value delivered by the process

during heating and cooling (Fig. 38.4). Conversion of available

time–temperature data to destruction rate is done using Equation

27. From gathered time–temperature data, TDT is calculated and
    then the reciprocal of TDT (TDT−^1 ) is computed for correspond-
    ing temperature (Table 38.4 columns 3 and 4). The reciprocal
    of TDT is known as the destruction rate achieved at that par-
    ticular time–temperature combination. Using destruction rate
    versus time (1/TDT vs. Time), the graph is drawn and the area
    under the curve is determined by counting the squares, or using
    planimeter. A unit of sterilization area on the curve is equal to
    the product of destruction rate and time of unity, which is the
    minimum processing requirement to achieve desired degree of
    commercial sterilization value (SV) with respect to target mi-
    croorganism. The sterilization value of the overall process can
    be calculated by considering the area under the curve divided by
    the unit sterilization area.
    Graphical method of process calculation is summarized in
    Table 38.4 forC. botulinumwithD 0 valueof0.21minuteto
    achieve TDT value of 12D 0 (F 0 ) withzvalue of 18◦F.
    In order to get a target sterilization value, the process (steam
    off) time is appropriately increased or decreased, as the case
    may be by shifting cooling curve to right or left direction on

Unit SV = Unit area = 0.05x20 Total SV= Total area under the curve

Lines drawn to the left parallel to

the cooling line to reach desired

sterilization value

0.45

0.4

0.3

0.2

0.1

0

020

Time (min)

Lethality (1/TDT)

40 60

0.35

0.25

0.15

0.05

Figure 38.4.Graphical approach of process calculation. Broken lines are parallel lines drawn to the left after carrying out over-processing in

order to get desired SV.