Kinetics of thermal death of microorganisms

• Steam (or moist heat) is used almost universally for the sterilization of fermentation media.

• Except the use of filtration for the sterilization of media for animal-cell culture – because such media are completely soluble and contain heat labile components making filtration is the method of choice for sterilization.

• The destruction of microorganism by heat is considered as loss of viability not destruction.

• The destruction of micro-organisms by steam (moist heat) at specific temperature can be described as a first-order chemical reaction provided if we considerer loss of viability not destruction.

• The thermal death kinetics may be represented by the following equation:

-dN/dt = kd N 1

• Where,– N, is the number of viable organisms present,– T, is the time of the sterilization treatment– kd, is the reaction rate constant of the reaction,

or the specific death rate per time.• On integration of equation (i) from t=0’ to t=t,we have the following expression :

Nt/N0 =e-kdt 2

• where – No is the number of viable organisms present at the start of the sterilization treatment,– Nt is the number of viable organisms present after a treatment period, t.On taking natural logarithms, equation (2) is reduced to:

ln(Nt/N0) = - kd t 3

The graphically equations (1) and (3) are represented as,

Fig: Plots of the proportion of survivors and the natural logarithm of the proportion of survivors in a population of microorganisms subjected to a lethal temperature over a time period.

The relationship observed in the above graph would be found only with the sterilization of a pure culture in one physiological form, under ideal sterilization conditions.

From Principles of Fermentation Technology,- Peter F. Stanbury, Allen Whitaker, Stephen J. Hall, Second Edition,

We have got two predictions from this kinetic description-

– (i) An infinite time is required to achieve sterile conditions (i.e. Nt= 0).

– (ij) After a certain time there will be less than one viable cell present.

• The ratio of No/Nt is the inactivation factor,

• The ratio of Nt/No is the survival factor and

• the In of No/Nt=V is the design criterion, a parameter which encompasses the contamination level of the medium to be sterilized, No, and the desired sterility level, Nt.

• In the above equations,

– k is a constant which expresses the specific death rate.

– It increases sharply with temperature and

– can be experimentally determined far an organism using equation 3.

If the experimentally determined Ink value from this equation is plotted against the reciprocal temperature (1/T) value, a straight line should be obtained from which the k value can be calculated for a desired temperature.

The value of k is not only species dependent, but dependent on the physiological form of the cell (i.e. vegetative and spore form)

From Principles of Fermentation Technology,- Peter F. Stanbury, Allen Whitaker, Stephen J. Hall, Second Edition,

Initial population increase resulting from the heat activationof spores in the early stages of a sterilization process

From Principles of Fermentation Technology,- Peter F. Stanbury, Allen Whitaker, Stephen J. Hall, Second Edition,

An initial stationary period observed during a sterilizationtreatment due to the death of spores being completly compensatedby the heat activation of spores

From Principles of Fermentation Technology,- Peter F. Stanbury, Allen Whitaker, Stephen J. Hall, Second Edition,

Initial population decline at a sub-maximum rate during a sterilization treatment due to the death of spores being compensatedby the heat activation of spores

From Principles of Fermentation Technology,- Peter F. Stanbury, Allen Whitaker, Stephen J. Hall, Second Edition,

The effect of a sterilizaIion treatment on a mixed culture consisting of a high proportion of a very sensitive organism

From Principles of Fermentation Technology,- Peter F. Stanbury, Allen Whitaker, Stephen J. Hall, Second Edition,

The effect of a sterilization treatment on a mixed culture consisting of a high proportion of a relatively resistant organism