In
microbiology
Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, prot ...
, 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
An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ar ...
) required, at a given condition (e.g.
temperature) or set of conditions, to achieve a
log reduction, that is, to kill 90%(or 1 log) of relevant
microorganisms
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
. 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 analysis; however, it now has analogous uses in other microbial resistance and death rate applications, such as for
ethylene oxide and
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
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 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; a notation like D
230 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, D
10 denotes the time required for a 10% reduction.
D-values are sometimes used to express a
disinfectant
A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than st ...
's efficiency in reducing the number of microbes present in a given environment.
See also
*
Food microbiology
*
Pasteurisation
*
Sterilisation
*
z-value
References
{{reflist
Microbiology