D-value

The time or dose needed under stated conditions to reduce a microbial population by 90%, which is one log. For example, a D-value of 2 minutes at 121 °C means that every 2 minutes at that temperature reduces the surviving population tenfold. D-values depend on the organism, the temperature and the material the organism is in. They are used to choose biological indicators and to calculate the lethality needed for a target sterility assurance level.