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

38 Thermal Processing Principles 729

pathogen that produces the deadly botulism toxin. It has been

generally recognized thatC. botulinumdoes not grow and pro-

duce toxin below a pH of 4.5. Hence, the dividing pH between

the low-acid and acid groups is set at 4.5. There may be other

microorganisms that are more heat-resistant thanC. botulinum.

Thermal Resistance of Food Microorganisms

As indicated in the above sections, the success of thermal pro-

cessing depends on the destruction of all pathogenic and most

spoilage-causing microorganisms in a hermetically sealed con-

tainer and creating an environment inside the package that is

not conducive to the growth of spoilage-type microorganisms

and their spores that resist the thermal process. The microorgan-

isms vary in terms of heat resistance and require different degree

of thermal processing. In order to establish a thermal process,

first, the target heat-resistant microorganisms of concern need

to be identified.C. botulinumis the principle target microorgan-

ism of concern in low-acid canned foods. The second step is

evaluating the thermal resistance of target organism. The ther-

mal resistance data should be well described by an appropriate

and reliable mathematical model. Data on the temperature de-

pendence of the microbial destruction rate are also needed to

integrate the destruction effect through the temperature profile

under processing conditions.

Kinetics of Microbial Death

An understanding of the mechanism and kinetics of thermal

death of food microorganisms would be helpful in the practical

use of heat in processing of foods. Thermal destruction of bacte-

ria is generally accepted to be exponential with time, and process

calculations used in thermal processing are generally based on

this assumption (Stumbo 1973). However, there are also nonex-

ponential thermal death patterns indicated in different literatures

(Stumbo 1973, Toledo 2007). The theory that thermal death of

bacteria follows the first-order kinetics has been well recognized

(Stumbo 1973). A first-order reaction is one in which the rate is

proportional to the number of microorganisms present.

Decimal Reduction Time and Thermal Death Time

When a suspension of microorganisms is heated at constant

temperature, the decrease in number of viable organisms follows

logarithmic order of death. In other words, the logarithm of the

surviving number of microorganisms following a heat treatment

at a particular temperature plotted against heating time will give

a straight-line curve (Fig. 38.1). These curves are commonly

called survivor curves. The microbial destruction rate is defined

as a decimal reduction time orD-value (minutes), which is the

heating time in minutes at a given temperature required to result

in one decimal or log unit reduction in the surviving microbial

population. In other words,D-value represents a heating time

that results in 90% destruction of the microbial population from

the initial number. The rate of microbial destruction with time

at constant temperature expressed as

dN

dt

=−k[N]^1 (17)

0.1

1

10

100

1000

10000

0 10 20 30 40

Survivors

Time at a constant temperature (min)

D

Log100- log 10= 1

Figure 38.1.Survivor curve that depicts the number of survivor

microorganisms versus time at a constant temperature.

wheredN/dtis the change in microbial population with timet,k

is the reaction rate constant, and 1 is to indicate the order of re-

action (first order). After separation of variables and integration,

the above equation can be written as

lnN=lnN 0 −kt (18)

whereN 0 is initial population of microorganisms.

Dis based on common logarithms, in contrast withk,which

is based on natural logarithms. ButDandkare related as

ln

(

N

N 0

)

=ln( 10 )log

(

N

N 0

)

=−kt (19)

From this

log

(

N

N 0

)

=−

k

ln(10)

t (20)

Thus, theDvalue is the negative reciprocal of the slope of a

plot of log (N)againstt, andDandkare related as

−

1

D

=−

k

ln(10)

; D=

ln(10)

k

or D=

2. 303

k

(21)

Therefore, theDvalue from survivor curve can be expressed

as

D=

(t 2 −t 1 )

(logN 0 −logN)

(22)

whereN 0 andNrepresent the survivors following heating fort 1

andt 2 (minutes), respectively.

Commonly in thermal processing applications, survivor

curves are plotted on specially designed semi-log papers for easy

handling and interpretation data. The survivor number of mi-

croorganisms is plotted directly on the logarithmicy-axis against

time on the linearx-axis. From this graph, theDvalue can be