Bill Byrne 2/16/12
For Food Safety the importance of the amount of heat (Temperature) and the duration (Time) that this heat is applied while
cooking is extremely important, particularly for those Home Chefs who are experimenting with Low temperature/Long Time
(Sous Vide) cooking processes. This leads to the concept of Decimal Reduction Time and how it relates to Thermal Death (Kill
Curves) of microorganisms which effects Food Safety.
Historical experiments gave rise to the concept Of Decimal Reduction Time (DRT). The Decimal Reduction Time (DRT) is the
amount of Time it takes in Minutes for heating a Microorganism solution at a constant lethal to cause a reduction by a fact of 10
times (one log factor) the number of living cells. A lethal temperature is a temperature at which Microorganisms begin to die.
Bottom line is Decimal Reductio Time is a temperature to kill 90% of the Spores ofa specific Specimen (in this case Food) at a
specific Temperature.
Thus after a food goes under a 1 DRT reduction, only 10% of the original Spores remain.The term D75 = 5.48 min; indicates that
a specific specimen when heated for 5.48 Minutes at 75 Deg C, kills off 90% of its spores.
The Table below shows a Specimen which at the beginning had 10,000,000 Spores and the specimen has a DRT of 1 min. at
75Deg C. i.e. D75 = 1. How many Spores will remain after the Specimen is heated at 75 Deg C for 5 min?
Answer is 100 Cells
# of Cells at
Beginning
# of Cells
at End
Time heated
(mins)
Log(10) Beginning
10,000,000
7
1,000,000
6
1
1
1,000,000
6
100,000
5
2
1
100,000
5
10,000
4
3
1
10,000
4
1,000
3
4
1
1,000
3
100
2
5
1
100
2
10
1
6
1
10
1
1
0
7
1
Log(10) End
L0g(10) Begin
- Log(10) End
There is a significant amount of experimental data to support the conclusion that if a suspension of a
Food Based Vegetative spores/cells is heated above a certain lethal temperature; death of microorganisms occur. Further it
has been shown that heating a homogeneous suspension of microorganisms, at a constant lethal temperature; that the
decrease in the number of Living Organisms is nearly Logarithmic with respect to time.
Thus it follows that Log (N/N0) =-t/D
The equation for calculating DRT is: ____________Heating Time (Mins)____________
Log(10) 0f Original
Log(10) of Surviving
No. of Spores
No. of Spores after heating Time
1 Log Cycle = a 10 times reduction
In Microorganisms
Log of Survivors
0
D
X –Axis Minutes
The Z value is defined as the change in temperature required to yield a 10 times increase or decrease
In the rate of thermal destruction of Microorganisms. For many Spore forming bacteria; Z is in the range of 8 to 12 Deg C