What is D value in Microbiology

 D value (Definition)

It is time required for a 90% reduction in microbial population. Quantitative expression of rate of killing of micro organism. 

Importance of D value:

1.    It is specific kinetic expression

2.  Important for calculation of Z value especially for heat sterilization at different temperature.

3.    For calculation of F value

4.    Extrapolation of D value from large microbial population value to fractional value predicts number of log reduction at given exposure period.  

Two methods are available to find the D value:

1. Survivor curve method (log number of surviving organism versus time/gas concentration/radiation dose)

Determining the D value of a culture A culture is incubated at a specific temperature and portions removed at various time points. The population in each sample is determined. The data is plotted with the population in the y-axis vs. time in the x-axis as shown in the upper right of the figure. The D value is the negative inverse of the slope.

Log N =A + Bt

Where: -        N is number of surviving organism.

A is the y intercept.

B is the slope of the linear line in the graph

D value is the of the linear slope: D=1/B

2.  Fraction negative method

Here replicate sample containing identical spores treated in identical manner and determining fraction of spore remaining after treatment and incubation.

Disadvantage is it is not applicable to ethylene oxide sterilization, because heat an humidity are two variables which will interfere in the result