Thermo-Chemical Disinfection Process is Tested Using Three Cultures of Vegetative Bacteria - Essay Example

For all the graphs, X-axis is the plot for temperature whereas y-axis represents time in seconds

An inactivation curve of Staphylococcus aureus bacteria

An inactivation curve of Streptococcus faecalis

An inactivation curve of Escherichia coli

• The explanation for the shapes of the inactivation curves.

For all the curves, the temperature increases as the time for inoculation increase at the start. This is a clear indication that the bacteria survived the low temperatures. However, the situation changes after the temperature reaches 1 degree as the temperature increase but the time decrease for the Streptococcus faecalis and E.Coli while that of Staphylococcus aureus changes at 2-degree temperature. This indicates that the bacteria became inactive and succumbed to the high temperatures. Thus, the inactivation was a success. The resistance to heat temperature observed at first especially for Staphylococcus aureus could be related to the ground beef although it was eliminated by increased temperature as time went on.

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

The D - value is the time taken to reduce initial microbial numbers at a specific temperature (Seymour 2005, p. 85). The area under the curve is calculated to obtain the D - value. The shape of the curves then is determined using the trapezoid rule (Rikimaru 2002, 297). The data most appropriate is that at 35oc because that is where maximum inactivation was achieved for all three bacteria.

D= Log 10 N1 – Log 10 N0 /  T2 – T1

Staphylococcus aureus

At 35oc=-log10 5.8*101 - log10 4.0*106/ 5-1=1.8-6.6/4=1.2

Streptococcus faecalis

At 30oc D= log10 4.6*101-log 10 1.2*106   /5-1 =1.6-6.0/ 4=1.4

Escherichia coli

At 30ocD = log 10 9.6*101- log 10 4.2*106/5-1 =1.9-6.6 / 4 =1.175

      (iv) The D values and comment briefly

Staphylococcus aureus

At 5oc =   log 10 9.0*102 - log 10 5.0*106/ 5-1 =2.95-6.70/4= 0.937

At 20oc =log 10 1.5*102- log10 3.5*106 / 5-1=2.2-6.5/4=1.075

At 35oc=-log10 5.8*101 - log10 4.0*106/ 5-1=1.8-6.6/4=1.2

Streptococcus faecalis

At 5oc D =log 10 8.2*102-log 10 3.2*106 / 5-1=2.9-6.5/4= 0.9

At 20oc D= log 10 3.0*101-log 10 2.3*106 /5-1=1.4-6.4/4 =1.25

At 30oc D= log10 4.6*101-log 10 1.2*106   /5-1 =1.6-6.0/ 4=1.4

Escherichia coli

At 5oc D = log 10 4.4*102-log 10 5.6*106 /5-1 = 2.6-6.7/ 4 =1.025

At 20oc D = log10 1.2*102-log 10 4.6*106 /5-1 = 2-6 /4 =1.0

At 30ocD = log 10 9.6*101- log 10 4.2*106/5-1 =1.9-6.6 / 4 =1.175

     D= Log 10 N1 – Log 10 N0/ T2 – T1 = log10 1.2*102-log 10 4.6*106/5-1= 2-6/ 4 = 1.0

The D – value obtained ranges from a value of one to two at maximum. The D - value is the decimal reduction time, this is the time required at a certain temperature to kill 90% of the organisms being studied. From the values above, the inactivation was achieved very fast because the D-value shows an average of one. The D - value is used to measure how susceptible a bacterial population is to change in temperature (Doors 2011, p. 111-119). The spore log reduction (SLR) is the value required to reduce the bacterial population to a value of zero or one (Hao, Barbosa & Weiss 2010, p. 665). The D - value is normally calculated using the limited Holcomb- Spearman- Karber method (Gazso & Ponta 2005, p.169).