refers to decimal reduction time and is the time required at a certain temperature to kill 90% of the organisms being studied.Thus after a colony is reduced by 1 D, only 10% of the original organisms remain, For example, given a hypothetical organism which is reduced by 90% after exposure to temperatures of 300° F for 20 minutes, the D-value would be written as D300F = 20 minutes. D-value determination is often carried out to measure a disinfectant's efficiency to reduce the number of microbes present in a given environment.