Article: Z-Value Calculation

What is a Z-value?
A Z-value is defined as the number of degrees (Celsius or Fahrenheit) required to change a D-value by one factor of ten. In the practical sense, it is a measure of how susceptible a spore population is to changes in temperature. For example, if the Z-value of a population is 10 degrees, then increasing the sterilization temperature 10 degrees will result in a log reduction of the D-value.

The Z-value can be found by plotting D-values against temperatures on a semilogarithmic scale and adding a line that best fits the data. The absolute value of the reciprocal of the slope of this line will be the Z-value.