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Scientific Principles Underlying Decontamination

The inactivation curve is done by plotting population against time at a specified temperature (Gazso & Ponta, 2005). But since the population is usually in very large numbers, it is always advisable to use the log of microorganisms. The ensuing issue is therefore that the D-value is read from the line of best fit. Afterwards the D – value of the thermal processes can be determined by considering the change in temperature over change in the log and is therefore an estimate of one log cycle at a specified temperature (Gera, 2011; Ogata, & Shibata, 2008).
For all the graphs, X axis are the plot for time (sec) whereas y axis represents microbial population (cfu/ml).

Inactivation Curves for each Set of Results


Explanation for the shapes of the inactivation curves

For all the curves, the population decreases as the time of decontamination. This is a clear indication that the bacteria population is reduced on further decontamination. However, the situation changes after a minute of heating as time increase but population for the Streptococcus faecalis and E.Coli almost remains the same. The population of Staphylococcus aureus changes at 2 minutes of decontamination.
This indicates that the bacteria became inactive and succumbed to the high temperatures long time.

Appropriate data to determine the D values* for each organism and reasons for the choice

The data selected for the three vegetative bacteria are as follows: 20oC for Staphylococcus aureus, 20oC for Streptococcus faecalis and 35oC for Escherichia coli. The reason is that from the representation of the curves, these will give best linear lines for ease of D-value calculation (Frascella, Gilbert, Fernandez, & Hendler, 2000).
The Graph showing D-value of Staphylococcus aureus at 20oC
The Graph showing D-value of Streptococcus faecalis at 20oC
The Graph showing D-value of Escherichia coli at 35oC

Determination of D values* for each organism and comments

The overall formula for D_value is
T2 – T1
D = ______________________
Log10 N1 – Log10 N2
For Staphylococcus aureus the D values* are:
D values* at 5oC
Dt5=5-1/log (4*10^7)-log(9*10^2)
D values* at 20oC
Dt20=5-1/log (4*10^7)-log (1.5*10^2)
D values* at 35oC
Dt35=5-1/log (4*10^7)-log(5.8*10^1)
The D values* for Streptococcus feacalis are:
D values * at 5oC
D values* at 20oC
Dt20=5-1/log (3.2*10^7)-log(3.0*10^1)
D values* at 35oC
Dt35=5-1/log (3.2*10^7)-log(4.6*10^1)
The D values* for E. coli are:
D values* at 5oC
Dt5=5-1/log (4.6*10^7)-log (4.4*10^2)
D values * at 20oC
D values * at 35oC
Dt35=log (4.6*10^7)-log(9.6*10^1)
It can be concluded that for Staphylococcus aureus, the highest inactivation rate was at 35oC, whereas the lowest was at 5oC. This shows it takes longer to reduce microbial load by 90% at low temperature this applies to the three organisms.
The highest inactivation rate for E.coli was 0.88020 cfu/ml/minute at 35oC, and the lowest rate was at 5oC (Block, 2000; Taniguchi, Stanley, H., & Ludwig, 2002; Mullan, 2007; Young & Reichert, 1997; Juneja, Huang, & Marks, 2001; Zhang, Zheng, & Zhi, 2007).
In conclusion, as the temperature increased the rate of inactivation increased, and for Staphylococcus aureus and E.coli, 35oC was the optimum temperature for inactivation by thermal-chemical disinfection.

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