Fish Microbiology and Quality Assurance (2+2)

Unit 4 - Food preservation techniques and microorganisms

4.2.4. Thermal destruction of microorganisms
The preservative effect of high temperature treatment depends on the extent of destruction of microorganisms. Certain basic concepts are associated with the thermal destruction of microorganisms.These include;

• Thermal death time (TDT)
• D- value
• Z- value
• F- value
• 12D concept

Thermal death time (TDT)
TDT is the time required to kill a given number of organisms at a specified temperature. Here, temperature is kept constant and the time necessary to kill all cells is determined. Whereas, thermal death point is the temperature necessary to kill given number of organisms in a fixed time, usually 10 min. But it is of less importance.
TDT is determined by placing a known number of bacterial cells/spores in sealed containers, heating in a oil bath for required time and cooling quickly. The number of survivors from each test period is determined by plating on a suitable growth media. Death is defined as the inability of organism to form viable colonies after incubation.
D-value (Decimal reduction time
D-value is the time in minutes required at specified temperature to kill 90% of microorganisms thereby reducing the count by 1 log units. Hence D – value is the measure of death rate of microorganisms. It reflects the resistance of an organism to a specific temperature and can be used to compare the relative heat resistance among different organisms/spores. D-value for the same organism varies depending on the food type. D -value is lower in acid foods and higher in presence of high proteins.
Example: D 250^oF (121.1^oC) for B. stearothermophilus: 4-5 min
C. botulinum: 0.1 – 0.2 min.
D 95^oC for B. coagulans: 13.7 min
B. licheniformis: 5.1 min.

Z - Value
Z-value refers to degrees of Fahrheit required for the thermal destruction curve to drop by one log cycle. Z value gives information on the relative resistance of an organism for different destruction temperature. It helps to determine equivalent thermal process at different temperature.
Example: If adequate heat process is achieved at 150^oF for 3 min and Z -value was determined as 100F, which means the10^oF rise in temperature reduces microorganisms by 1 log unit. Therefore, at 140^oF , heat process need to be for 30 min and at 160^oF for 0.3 min to ensure adequate process.

F - Value
F- value is the better way of expressing TDT. F- is the time in minutes required to kill all spores/vegetative cells at 250^oF (1210^oC). It is the capacity of heat process to reduce the number of spores or vegetarian cells of an organism.
F – Value is calculated by
F_o = Dr (log a-log b)
Dr = Decimal reduction time (D value)
a = initial cell numbers
b = final cell numbers

12D concept
12D concept is used mainly in low acid canned foods (pH >4.6) where C. botulinum is a serious concern. 12D concept refers to thermal processing requirements designed to reduce the probability of survival of the most heat resistant C. botulinum spores to 10-12. This helps to determine the time required at process temperature of 121^oC to reduce spores of C. botulinum to 1 spore in only 1of 1 billion containers (with an assumption that each container of food containing only 1 spore of C. botulinum).