Concept, Determination Of Process Lethality Requirements And Importance


The foods, which we eat or drink are also excellent substrates (food) for
microorganisms, which are present in air, water, soil, utensils and even in raw
foods. Under suitable conditions of growth, particularly temperature and
moisture, the microorganisms multiply using these food items and produce
luxuriant growth.

Concept, Determination Of Process Lethality Requirements And Importance

Many foods serve as carrier of various pathogenic and nonpathogenic microorganisms, which may spoil the food by their growth, change
of chemical nature of food, release of unpleasant odour, production of various
harmful enzymes and toxins. 

Such foods are unfit for human consumption. For
these reasons, it is essential to prevent the entry and growth of microorganisms
in our food if present, by suitable processing. Before using a suitable process,
we should understand various factors which may influence the effectiveness of
a process.


Most foods are derived either from plants or from animals. In this course, we
are concerned with foods of plant origin and are known as vegetables or fruits
based on their use. These foods have different pH and are classified as low acid
foods, medium acid foods, acid foods and high acid foods.
a) Low acid foods
The foods having pH above 5.3 are called low acid foods. For example:
peas, corn, lima beans etc. 
b) Medium acid foods
The foods which have pH between 4.3 and 5.3 are called medium acid
foods. For example: asparagus, beets, pumpkin, spinach etc. 
c) Acid foods
Foods which have pH between 3.7 and 4.5 are called acid foods. For
example: pears, pineapple, tomatoes etc. 
d) High acid foods
Foods having pH 3.7 or lower are included in this category. For example:
Berries and sauerkraut. 
You must have noted that in general vegetables are low or medium acid foods
while fruits are acid or high acid foods.
Most foods are subjected to heat treatment or cooked before use. The heat
process is essential in the canning of foods to eradicate the microorganisms,
which may be present in the raw food or may enter from the environment
during processing; and may spoil the food if not eradicated. 
The effect of pH of the food is complicated as the heating at high temperature
causes decrease in the pH of low or medium acid foods. Higher the original
pH, the greater the drop of pH by heating. Foods artificially adjusted to more
alkaline pH give increasing protection to spores against heat as pH increases
towards 9.


The pH of the foods influence the heat resistance of microorganisms. In
general cells or spores are most heat resistance in a substrate that is at near
neutrality. An increase in acidity or alkalinity hastens killing by heat.
However, a change towards acidic pH is more effective than a corresponding
change in alkalinity. 
This will be more clear from the Table 5.1, which shows
the effect of pH on heat resistance of spores of Bacillus subtilis. Therefore low
acid foods are heated under pressure (i.e. temperature above 100o
C) while the
high acid foods are heated up to 100o
C for making free from microorganisms.

Effect of pH on heat resistance of spores of Bacillus subtilis in
1:15 M phosphate buffer (1000


The heat resistance of microorganisms varies widely within the species and
their forms: 
• Thermophiles are more resistance than mesophiles and psychrophiles are
least resistance. 
• Spores formers are more resistant than non-spore formers. Cocci are
usually more resistant than rods. 
• The bacteria that clump considerably or form capasules are more resistant
to heat than those which do not. 
• Cells high in lipid content are difficult to kill than cells having low lipid.
However, there are many notable exceptions to the above mentioned general
Higher the optimal temperatures for growth, the greater the resistance to heat.
Thermal death time of bacterial cells of a few microorganisms are exemplified
The heat resistance of microbial spores is much higher than the vegetative
cells, and vary with the species of microorganism and conditions during
sporulation. Resistance may vary from <1 min to 20 h at 100o
Similar to
non-spore forming species, the spore forming species which have higher
optimal temperature for growth are more resistant to heat than those spore
forming species having lower optimal growth temperatures. 
Simultaneous growth of two spores formers enhances the resistance of spores
having lower heat resistance, e.g. Clostridium perferingens growing with C.
sporogenes. Thermal death times of spores of a few microbial species are
given in Table 5.3.
Above examples of thermal death times of vegetative cells as well as of spores
are at various concentrations of cells or spores in different substrates. These
values may change to lower or higher under different conditions.

What happens to enzymes in food by heat treatments?

Most foods and microbial enzymes are destroyed at 79.4o
C, however some
may withstand higher temperatures, especially if high temperature for short
duration is employed. This is called pasteurization, which you will learn later
in this course.


Thermal death point is the lowest temperature at which all microorganism in a
liquid suspension are killed in 10 minutes


The thermal death time is defined as the time required, at a given temperature,
for heat killing of a population of a single species of microorganism in aqueous
suspension. tD depends on the size of the population and on the pH of the
suspension. It is an important factor for controlling the microorganisms by heat
treatment or to determine the heat resistance of a microorganism.


The description of all the procedure and equipments/apparatus, used in the
determination of thermal death time is beyond the scope of this course.
However, a simple glass-tube method, used in canning industry is discussed
Glass Tube Methods
A known population of cells of an axenic culture in a small volume (1 ml) of
buffer solution is sealed in small glass tube. The tubes are heated in a
thermostatically controlled bath to a selected temperature. The tubes are
selected periodically, cooled immediately to 0o
C and the population of viable
cells is determined. 
In case of spores, the suspension is first pasteurized to kill
the vegetative cells, if present, before subjecting the spore suspension to dT
test. This is necessary as the lysed vegetative cells may have protective effect
on spore-population.
Care is also taken to break up the clumps and remove the growth medium by
centrifuging and washing. 
The volume of microbial suspension added to the
buffer is kept 1-2 percent to avoid the change in the composition of heating
substrate and the vials containing the suspension are brought to constant
temperature, usually 0o
C before subjecting to heat treatment. 
If temperature
above 100o
C is selected, oil bath instead of water bath is used. The test is
always made in multiple tubes. Viability of the surviving organism after heat
treatment should be checked on appropriate medium containing all the
nutrients, which support maximum growth of that organism.
Decimal Reduction Time

When a microbial population is heated, the cells die at a constant rate. For
example, suppose a population of 1 million (106
) cells has been heated to a
high temperature for 1 minute and 90% has died. We are now left with 100,000
) cells. If the leftover population is heated for another 1 minute, 90% of the
population leaving 10000 (104
) survivers. 
Thus the each one minute of heat
treatment will reduce 90% of the remaining population. This is shown in Table
5.4 and is known as decimal reduction time (DRT) and represented by D. It
can be defined as the time of heating at a temperature to cause 90% reduction
in the population of viable cells or spores.

Microbial death rate at constant temperature

The D value (Decimal reduction time) may also be defined as the ‘time at
given temperature for the surviving population’ to be reduced by 1 log cycl.
Please refer Figure 5.1, if we extrapolate the times from 103
and 102
, the time
difference is (3.5 – 2.5 = 1min is D). It means within 1 min initial population
will decrease by 90 per cent (from 1000 to 100, Difference 1000 – 100 = 900).

Thermal Death Time Curve (TDT Curve)/Kinetics

The methods to construct TDT curve are: (i) The growth – no growth method
(ii) classical end point method and (iii) based on D values. We shall here
discuss the method based on D-values. D values can be calculated at different
temperatures (refer 5.7.2).
 As the temperature is increased, the D value
decreases. It means if we heat the sample at high temperature, it will take less
time to kill the microorganism in a given food sample. If we plot log D values
against temperature, we will get a straight line. From this we can derive another important parameter in heat processing Z, the temperature change
which results in a ten fold (1 log) change in D.
Z = (T2 – T1)
D value for a known population of cells or spores of a microbial species at
several temperatures can be estimated. By plotting D values on the logarithmic
scale against temperature TDT curves can be constructed (Figure 5.2).   
TDT curve for spores of flat sour bacteria; 115,000 spores per ml in corn at
pH 6.1 (z = 19).

If we are interested to process the food item so that it may be free from any
spore or microorganisms, first we have to calculate D, Z and F values. F is the
time in minutes required to destroy the organism in a specified medium at
C. These values vary with the heat resistance and concentration of the test
organisms and with the medium in which it is heated. From the Z and F values
process times can be calculated.
12D Concept

Canned foods are susceptible to the spores of the organism Clostridium
botulinum, this organism causes botulism. As a safety measure, the canning
industry use the 12D heat treatment for low acid foods. In this process enough
heat is provided to reduce 1012 spores of C. botulinum to 1 spores per ml. It can
be explained as follows. 
Assuming that D has a value of 0.21 minutes for spores of C. botulinum at
C and that out of 12 cans of food contains 1 spore. A heat process at
C for 2.52 min would reduce the spores to 1 spore in 1012
cans. The value
of 2.52 min has been arrived by the following formula:
Fo = D121 (log a – log b)
= 0.21 (log 1 – log 10−12)
= 0.21 × 12
= 2.52

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