#1.2 STERILIZATION

Continued from #1.1

The semi logarithm relationship does not accurately fit all experimental microbial thermal destruction data, but to the date no other model is known that fits all experimental data.

In order to apply the semi logarithm model for survivor curve the challenge must consists homogeneous culture and a constant lethal stress must be applied to the challenge.

Considering sterilization by Thermal stress the survivor curve can be described as:

Log N_F = -F_(T,z)/D_T+Log N₀  (First order reaction) 

N_F: Number of microorganism after exposure of F equivalent minutes

F_(T,z): Equivalent lethality of a cycle calculated as minutes at a reference temperature(T) using a defined temperature coefficient (z)

D_T: Thermal resistance value_, in minutes of the microorganism at a specific temperature (T)

N₀: Number of microorganism prior to exposure

Graphical Representation for the above equation will be:

Resistance Value (D_T Value):  D_T value is the time in minute for a one log or 90% reduction of a microbial population under specific lethal condition i.e. temperature

One logarithm cycle on y axis represents a tenfold change in number of survivors or it can be said that 1 log reduction means a reduction of tenfold in microbial population.

D_T value is the time or equivalent time on x axis for the survivor curve to transverse 1 log cycle.

Temperature Coefficient (Z Value): Z value is the number of degrees of the temperature change necessary to change the D_T value by a factor of 10.

For example: D_T value of a BI challenge system with a Z value of 8^o will change by a factor of 10 for each 8^o change in temperature.

If the D₁₂₁ of the BI is 1.6 min then the D₁₂₉ will be 0.16 min and D₁₁₃ will be 16 min.

Moist heat sterilization processes are usually carried out within a small temperature change, 110 ^o to 135 ^o therefore experimentally determined z value is usually considered constant for practical purpose. Z value of 10^o is generally used.

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Article Reference:  PDA Technical Report No.1 “Validation of moist heat sterilization process. Cycle design, development, qualification and ongoing control”