N / N0 = 10-kt
Where
N0 - initial number of microorganism, t - elapsed exposure (= sterilization)
time, N - number
of microorganism after the exposure time t, K - reaction rate constant which depends on the
species and conditions of the microorganism
As Equation 2
describes the survivor curve for microorganism and shows that number of
microorganism decrease exponentially depending on sterilization time.
The graphical
representation of survivor curve (semi-logarithmic plot) shall be as follows:
The semi-logarithmic
plot of the Survivor Curve has the difficulty of being undefined mathematically
at zero, which is the definition of “sterile”1.
The sterilization reaction is therefore neither an
"all-or-nothing" process nor a "potential barrier" process
as its definition says2"Sterile" means absence of all viable microorganisms.
In
the pharmaceutical industry the concept of “Sterility” expresses sterility as a
probability in the region of the survivor curve below one surviving organism.
This region has negative values for the log of concentration and is interpreted
as non-sterile containers in a group being sterilized (e.g., one non-sterile
unit in ten units for a survivor concentration of 10-1). This approach is
called Probability of a Non-Sterile Unit (PNSU) or, as applied to all processes,
Sterility Assurance Level (SAL) 1.
______________________________________________
1Remington-Essentials
of Pharmaceutics, Chapter 25 “Sterilization Processes and Sterility Assurance”
2F0 - A technical note – Doc.
352178v2 - Fedegari Group
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