1. Sterilization
on a practical basis means the absence of any detectable viable organism.
: only the desired organism is detectably present.
Sterilization methods:
Medium & equipment:
Heat-sensitive materials in medium: (microfiltrator pore sizes<0.2μm).
Heat-sensitive equipment: UV radiation (energy intensive) or chemical agents (toxic to the desired culture. e.g.70% ethanol-water, and ethylene oxide (gas).
Gas (air): sterilized through adiabatic compression process for supplying oxygen (~2200C) followed by (e.g. glass wool filter).

2. Sterilization Thermal deactivation
The probability of an unsuccessful sterilization:
1- P0(t)
The probability of extinction (successful sterilization):
P0(t)=[1-p(t)]N0
where p(t) is the probability that an individual will still be viable at time t. N0 is the number of individuals (cells or spores) initially present.
Assume first order death rate:
p(t)=e-kdt
1- P0(t)=1-[1-p(t)]N0 = 1-[1-e-kdt]N0
where the specific death rate kd = αe-E0d/RT(1/time), constant at specified T. Eod is the activation energy for the death of the organism.
When dealing with sterilization by inactivating living cells, spores or viruses, the probabilistic nature of cell death should be considered.

3. Sterilization
(D): time for the number of viable cells to decrease tenfold.
E[N(t)] = N0 p(t)
E[N(t)] = N0 e-kdt
0.1= e-kdD
D=2.303/kd
E[N(t)] is the expected value of the number of individuals present at time t.

4. Sterilization 1- P0(t)= 1-[1-e-kdt]N0 From the above equation:
kd = αe-E0d/RT
From the above equation:
Known N0, T, t, determine Kd, the probability of an unsuccessful sterilization is determined.
Given N0, T, acceptable probability of failure e.g. 10-3, required time can be determined
Higher Kd tends to achieve low probability of sterilization failure. Normally at 121oC.
Kd of vegetative cells > 1010 min-1, spores min-1. The major concern is spores.

5. Sterilization Chart e.g. N0=108, Kd=1min-1 (1210C), t=26min Kdt=26
Probability of failure:
1-P=0.001
e.g. N0=108,
Kd=1min-1 (1210C), 1-P=0.001
Kdt=26
Required sterilization time:
t=kdt/kd=26 min
(M. Shular, Textbook, p.319)

6. Batch Sterilization T (K) t time Kd2 T (K) Kd3 Kd1 t time holding
heating
holding
cooling
Temperature verse time in a batch sterilization process
T (K)
t time
Kd1
Kd2
Kd3
Simplified calculations for deactivation of spores and medium components

7. Sterilization
Degradation of components in the medium in sterilization process.
vitamin and growth factor
The inactivation of viability is much more sensitive to temperature changes than the degradation of important growth factors in the medium
It is important to assure complete killing of foreign organisms without the destruction of important components in the medium.

8. Sterilization Degradation of components in medium
Assume the degradation rate of such components is first order.
dC/dt=-kdC
The degradation rate constant kd can also be related to temperature by Arrhenius equation.
Integrating the above equation,
ln C/C0=-kdt or C=C0e-kdt
where C0 is the initial concentration of the component.
To determine the components remaining active:
the temperature T → determine kd → with known t, determine C.

9. Sterilization Degradation of components in the medium. kd = αe-E0d/RT
T (K)
t time C0
Δt1 C1 C2
Δt3 C3
Kd,3
Kd,1
kd = αe-E0d/RT