Calculate the decimal reduction time (D) for the two temperatures in the following graph.

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Identify the decimal reduction time (D) definition: it is the time required at a certain temperature to reduce the number of living microbes by 90%, which corresponds to a 1-log reduction on a logarithmic scale.

On the graph, locate the initial number of living microbes for each temperature (45°C and 20°C) at the starting time point.

For each temperature, find the time it takes for the number of living microbes to decrease by one logarithmic unit (i.e., from 10^4 to 10^3, or from 10^3 to 10^2, etc.). This time interval corresponds to the decimal reduction time (D).

Calculate the difference in time between these two points on the x-axis (time in minutes) for each temperature curve. This difference is the decimal reduction time (D) for that temperature.

Repeat the process for both the 45°C and 20°C curves to find their respective D values, noting that the steeper slope (45°C) will have a smaller D value compared to the gentler slope (20°C).

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Decimal Reduction Time (D-value)

The decimal reduction time (D-value) is the time required at a specific temperature to reduce the microbial population by 90%, or one log cycle. It is a key measure of microbial resistance to heat and is used to compare the effectiveness of different temperatures in killing microbes.

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Calculating Generation Times

Logarithmic Scale in Microbial Death Curves

Microbial death over time is often plotted on a logarithmic scale to linearize the exponential decline in the number of living microbes. This allows easier calculation of the D-value by measuring the time interval for a one-log reduction in microbial count.

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Microbial Growth Curves in a Closed System

Effect of Temperature on Microbial Death Rate

Higher temperatures generally increase the rate of microbial death, resulting in a shorter D-value. The graph shows two temperatures (20°C and 45°C), illustrating that microbes die faster at 45°C compared to 20°C, which is reflected in the steeper slope of the 45°C line.

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