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Calculate D value

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Joined: 01 Sep 2009
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    Posted: 11 Feb 2012 at 11:37pm

Welcome back.

I am going to explain how to do this by calculating the slope of the thermal death curve for the organism you have studied using an Excel function called "slope". The D-value is then obtained by obtaining the negative reciprocal of the slope.

1. I presume that you have these values in an Excel spreadsheet? If not create a workbook with 3 columns, Time (minutes), Colony forming units/ml, and Log10 colony forming units/ml.

2. Convert your colony forming units (CFU)/ml values to their log10 equivalents.

3. Excel 2010 has a useful slope function to calculate the slope of the linear regression line for x and y values and I will explain how to use this function.

4. Lets assume that your time values (x values) are in column A and range from A2 to say A15 and the corresponding log10 colony forming units/ml are in column C and range from C2 to C15. The slope can then be calculated using =slope(C2:C15, A2:A15).

5. The D-value is obtained by obtaining the reciprocal of the slope and multpling this value by -1.

6. I suggest that you produce a figure showing the relationship between time and the reduction in Log10 CFU/ml. Show the experimental values and add a trendline. Use Excel to calculate the linear regression equation and show the r2 value. If this is less than 0.9 consider your error sources and look for "tailing" of your data points.


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Joined: 30 Nov 2010
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mstrange Quote  Post ReplyReply Direct Link To This Post Posted: 11 Feb 2012 at 5:09pm
Hi again!
Cry Got table of time (mins) and nos of survivors at 200F. Pl can you tell how to calculate D at 200F?
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