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CONCEPT #1: Logarithmic numbers
Observe this mathematical property (using your calculator)
Log (1) = 0
Log (10) = 1
Log (32) = 1.51
Log (100) = 2
Log (1000) = 3
Log (1000000) = 6
Log (1048576) = 6.02
No. of bacteria = Log10 (no. of bacteria)
Hence, on the left is the graph when the death curve is plotted using the value of no. of bacteria
on the right is the graph when the death curve is plotted using the Log10 (no. of bacteria), which is
the characteristic straight line of thermal death curves
CONCEPT #2: Logarithmic Reductions (Log Reductions)
Example,
Before heat treatment,
106 = 1,000,000 = 1 million bacteria cells
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After heat treatment,
105 = 100,000
How much is the reduction?
100,000 1,000,000 = 90% reduction = (
bacteria survive = 1D reduction (in English, it means 1
decimal reduction, which says that the numbers are reduced by one decimal, or reduced by the
factor of 10)
Observe the table below
1D followed by
2D followed by
3D, etc.
Actual no. of
bacteria reduced
How much of original 106
bacteria is killed by the
sequential log reductions
using heat
How much of original 106
bacteria survived the
sequential log reductions
using heat
1D 106 reduced to 105 = 90% of 106 killed = 10% of 106 survived
2D 105 further reduced
by one decimal = 105
reduced to 104
= 99% of 106 killed = 990000
killed
= 1% of 106 survived =
10000 survived
3D 104 further reduced
by one decimal = 104
reduced to 103
= 99.9% of 106 killed =
999000 killed
= 0.1% of 106 survived =
1000 survived
12D = 99.9999 9999 99% of 106
killed
= 0.0000 0000 01% of 106
survived
CONCEPT #3: The thermal death curves (and hence D-values) for different bacteria species are
different
Example,
Why? The "danger zone for different foods" or "optimal growth conditions for different species" are
different
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Example,
CONCEPT #4: Why do we need Z-values? The process of bacterial death is a function of both time
and temperature
Examples, in real world conditions: Heat transfer is not equal! Hence the temperature is not always
constant
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Hence, the thermal death curve graph below does not really inform you of the true log reduction by
heat treatment, as it is based on a constant temperature of 121
C
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Examples,
If you draw a graph comparing D value (time) versus Z-value temperature, you get this
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CONCEPT #5: A typical value for the parameter Z is 10C, meaning that if you change the
temperature by 10C, the time required to kill the same fraction of bacteria increases or decreases
by a factor of 10.
Examples
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*This Z-value of 10
C is in fact based on experience with canned foods where Clostridium
botulinum is important, and Z-value of 10
C should only be used as a reference for this bacteria
species. This Z-value of 10
C is commonly used to inform the heat processing of canned food. The
common solution is the 12D cook. Because if we consider the worst possible scenario a
can full of solid packed Clostridium botulinum spores; in such a situation we have about 1012
spores per gram. Thus putting the food through a cooking process which achieves 12
decimal reductions should destroy all the spores ofClostridium botulinum
in a gram in theworst possible case.
Clostridium botulinum spores is very important in canned food. Refer to your hygiene and
sanitation module.
CONCEPT #6: Real life applications of all these concepts
Different choices of times and temperatures can make huge differences in the appearance and taste
of the dish. You thus almost always have a choice: you can cook at high heat for a short time, or you
can cook at low heat for a longer period. Food safety, though, is paramount, hence you need to learnhow to apply the information from D-values and Z-values.
The shape of the thermal death curve varies with bacterial species and with environmental
conditions such as the pH and kind of food
In food, he presence of salt, sodium nitrite, or other additives can also make a big difference, as can
the presence of certain proteins or fats. Fats can either help shield bacteria from heat or make them
more sensitive to elevated temperatures.
Authorities differ on the proper reduction standards for specific contexts. For fresh food,
various sources recommend 4D, SD, 6D, or higher levels of bacterial reduction. Any bacterialreduction, even a 12D drop, can prove unsafe if the contamination is great enough.
