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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 1 corresponds to a reduction in concentration by a factor of 10.
What is often called a "log reduction" (technically a reduction by one order of magnitude) represents a 90% reduction in microbial population. Thus a process that achieves a "6-log reduction" (10 −6 ) will theoretically reduce an initial population of one million organisms to very close to zero.
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) or set of conditions, to achieve a one-log reduction, that is, to kill 90% of relevant microorganisms. [1]
A serial dilution is the step-wise dilution of a substance in solution, either by using a constant dilution factor, or by using a variable factor between dilutions. If the dilution factor at each step is constant, this results in a geometric progression of the concentration in a logarithmic fashion.
The graph of the logarithm base 2 crosses the x-axis at x = 1 and passes through the points (2, 1), (4, 2), and (8, 3), depicting, e.g., log 2 (8) = 3 and 2 3 = 8. The graph gets arbitrarily close to the y-axis, but does not meet it. Addition, multiplication, and exponentiation are three of the most fundamental arithmetic operations.
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For example, on a plot axis showing log 2 fold changes, an 8-fold increase will be displayed at an axis value of 3 (since 2 3 = 8). However, there is no mathematical reason to only use logarithm to base 2, and due to many discrepancies in describing the log 2 fold changes in gene/protein expression, a new term "loget" has been proposed. [10]
The survival ratio is commonly expressed as an inactivation ratio (in %) or as the number of reductions in the order of magnitude of the microorganism concentration. For example, a situation where N 0 =10 7 CFU/L and N=10 4 CFU/L would be reported as a 99.9% inactivation or "3-log 10" removal.