#1.0 STERILIZATION

#1.0

1. Sterilization

"Sterile" and "sterilization", in a strictly biological sense, describes the absence, and respectively, the destruction of all viable microorganisms.

Microbial Growth and Death:

In order to increase in number (or replicate), aerobic and facultative microorganisms require: food, water & air. Hence death of microorganism can be accomplished by removing what they need to survive (i.e. food, water & air).

Death rate of microorganisms can be accelerated by subjecting the microbes to some degree of stress. The stress can be thermal, chemical, or ionic in nature (filtration is not a lethal process) in which the death rate is accelerated.

The mechanism of death may be fundamentally differ for all stress methods, but each death process follows a chemical reaction that follows first order kinetics.¹

A first-order reaction is a reaction that proceeds at a rate that depends linearly on only one reactant concentration.²

Example of Death model (in terms of first order kinetics)

Suppose a vial containing certain microorganism is subjected to stress condition via saturated steam. Under such condition thermal degradation of the microorganism supposed to obeys the laws of chemical reactions.

The variation in the number of microorganisms as the function of a chosen time “t” of exposure to the selected sterilization temperature can be written as:

 -K N=  dN / dt

Where:   N is number of microorganisms present in the system,

              K is a constant which is typical of the species and condition of the  chosen
               microorganism.

Or equation may be written as    -Kdt  =  dN / N

By converting from base e to base 10 logarithms, the following is obtained

log N = -k t + constant    .....Equation 1

Where, K = k/ 2.303 due to the shift from base e logarithms to base 10. 

At time zero or at initial,   t=0 & N=N₀, the equation shall become

log N₀  = Constant

Or ....Equation 1 can be expressed as:  log N = -k t + log  N₀ 

Or    log N / N₀ = -k t 

Or    N / N₀ = 10^-kt  .....Equation 2   

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      

.....Equation 2 shows that the number of microorganism decreases exponentially depending on the sterilization time.³

^{______________________________________________}

¹Remington-Essentials of Pharmaceutics, Chapter 25 “Sterilization Processes and Sterility Assurance”

²https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Module       s_(Physical_and_Theoretical_Chemistry)/Kinetics/Reaction_Rates/First-Order_Reactions

³F0 - A technical note –  Doc. 352178v2 - Fedegari Group