D- Value & Z-Values

D VALUE

Many kinds of microbes are pathogens, meaning they are capable of causing disease to other organisms. Sterilization processes, such as exposure to heat, destroy these pathogens. D-value refers to decimal reduction time - The time required at a certain temperature to kill 90% of the organisms being studied. Thus after an organism is reduced by 1 D, only 10% of the original organisms remain. The population number has been reduced by one decimal place in the counting scheme. Generally, each lot of a sterilization-resistant organism is given a unique D-value. When referring to D values it is proper to give the temperature as a subscript to the D. For example, a hypothetical organism is reduced by 90% after exposure to temperatures of 300 degrees Fahrenheit for 2 minutes, thus the D-value would be written as D_300F = 2 minutes. D-value determination [1] is often carried out to measure a disinfectant's efficiency to reduce the number of microbes, present in a given environment.

Z Value

Z-value of an organism is the temperature, in degrees Fahrenheit or Celsius, that is required for the thermal destruction curve to move one log cycle.It is the reciprocal of the slope resulting from the plot of the logarithm of the D-value versus the temperature at which the D-value was obtained.It may be simplified as the temperature required for one log reduction in the D-value.While the D-value gives us the time needed at a certain temperature to kill an organism, the z-value relates the resistance of an organism to differing temperatures. So, the z-value allows us to calculate a thermal process of equivalency, if we have one D-value and the z-value. So, if it takes an increase of 10°F to move the curve one log, then our z-value is 10. So then, if we have a D-value of 4.5 minutes at 150°F, we can calculate D-values for 160°F by reducing the time by 1 log. So, our new D-value for 160°F is 0.45 minutes. This means that each 10°F increase in temperature will reduce our D-value by 1 log. Conversely, a 10°F decrease in temperature will increase our D-value by 1 log. So, the D-value for a temperature of 140°F would be 45 minutes.

Courtesy : http://en.wikipedia.org/

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