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).

SterilizationThermal 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.

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.

Sterilization1- P0(t)= 1-[1-e-kdt]N0

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 0.5-5 min-1. The major concern is spores.

Sterilization Charte.g. N0=108,Kd=1min-1 (1210C), t=26minKdt=26Probability of failure:1-P=0.001

e.g. N0=108,Kd=1min-1 (1210C), 1-P=0.001 Kdt=26Required sterilization time:t=kdt/kd=26 min

(M. Shular, Textbook, p.319)

Batch Sterilization

Temperature verse time in a batch sterilization process

T (K)

t time

heat

ing

holding

cooling

Simplified calculations for deactivation of spores and medium components

T (K)

t time

Kd1

Kd2

Kd3

SterilizationDegradation 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.

SterilizationDegradation 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.

T (K)

t time

Kd,1

Kd,2

Kd,3

SterilizationDegradation of components in the medium.

33,

23tkdeCC

C0 C1

C2C1

C2 C3

kd = αe-E0d/RT

Δt1

Δt2

Δt3

12,

12tkdeCC 11,

01tkdeCC