In microbiology, in the context of a sterilization procedure, the D-value or decimal reduction time (or decimal reduction dose) is the time (or dose of an antimicrobial drug) required, at a given condition (e.g.

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

) or set of conditions, to achieve a

log reduction Log reduction is a measure of how thoroughly a decontamination process reduces the concentration of a contaminant. It is defined as the common logarithm of the ratio of the levels of contamination before and after the process, so an increment of ...

, that is, to kill 90%(or 1 log) of relevant microorganisms. A D-value is denoted with the capital letter "D". Thus, after an exposure time of 1 D, only 10% of the organisms originally present in a microbial colony would remain. The term originated in assessments of microbes' thermal resistance and in

thermal death time Thermal death time is how long it takes to kill a specific bacterium at a specific temperature. It was originally developed for food canning and has found applications in cosmetics, producing salmonella-free feeds for animals (e.g. poultry) and phar ...

analysis; however, it now has analogous uses in other microbial resistance and death rate applications, such as for

ethylene oxide Ethylene oxide is an organic compound with the formula . It is a cyclic ether and the simplest epoxide: a three-membered ring consisting of one oxygen atom and two carbon atoms. Ethylene oxide is a colorless and flammable gas with a faintly sw ...

and radiation processing.

Details

Use of D-values is based on the assumption that the procedure in question causes the number of living microorganisms to decay exponentially. From this perspective, D-values can be understood as roughly analogous to

half lives Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of radioactive substances, however a half life involves a reduction of 50% rather than 90%. The half life is actually roughly 30% () of the D-value, so if D = 10 minutes, the number of living microorganisms will be halved in about 3 minutes. Generally, each lot of a sterilization-resistant organism will have its own specific D-value. Determining a D-value requires an experiment, but only gives the D-value under the specific conditions of that experiment. D-values are unique to the conditions of the environment that the bacteria currently exists in. In the context of thermal analysis it is typical practice to subscript the "D" with an indication of temperature. For example, given a hypothetical organism which is reduced by 90% after exposure to temperatures of 150° C for 20 minutes, the D-value would be written as D_150C = 20 minutes. In the US, the temperature is usually indicated in

degrees Fahrenheit The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined his ...

; a notation like D₂₃₀ should be understood to mean D_230F (D_110C). When describing D-value generally for any temperature, like in the heading of a table, a common abbreviation is D_''T'' (where ''T'' stands for the temperature), where specific values for ''T'' may be given elsewhere. A numeric subscript may also be used to indicate some other level of reduction than 90%; for example, D₁₀ denotes the time required for a 10% reduction. D-values are sometimes used to express a disinfectant's efficiency in reducing the number of microbes present in a given environment.

References

{{reflist Microbiology

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See also

References