Appendix C

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Volumes 1-3
January 1993

Calculating the Most Probable Number of Growth Units

1. Use the Conversion Table of Most Probable Number of Growth Units (MPNGU) provided or calculate using the equation in Table 1.
2. For each of the duplicate HGMF scores, calculate or look up the corresponding MPNGU value.
3. Calculate the MPNGU and then determine the average. DO NOT average the scores of the duplicate filters to use in calculating the average MPNGU.
4. If 1.0 ml of the 1:10 suspension was used to inoculate the HGMF, multiply the average MPNGU by 10 to calculate the number of aerobic organisms per g (ml) of product.
5. If V ml of the 1:10 suspension was used to inoculate the HGMF, multiply the average MPNGU by 10/V to calculate the number of aerobic organisms per g (ml) of product.
6. If V ml of a 1:C dilution was used to inoculate the HGMF, multiply the average MPNGU, by C/V to calculate the number of aerobic organisms per g (ml) of product.

Table 1. Calculation ff Most Probable Numbers Of Growth Units

• 1.1 Colony overlapping at high inoculum levels occurs just as much on HGMF as on plates or membrane filters. In contrast to the latter, however, the error on HGMF is not only calculable, but is turned to advantage by allowing unusually high inoculum levels to be accommodated. If x grid-cells on the HGMF show green growth, the most probable number of growth units (i.e., aerobic organisms) in the inoculum is given by: -
• 1.2 MPNGU = N log_e (N/(N-x))
  
  Where N = the total number of grid-cells on the HGMF. Usually, this will be 1600. Equation 1.2, therefore, will normally be: -
• 1.3 MPNGU = 1600 log_e (1600/(1600-x))
• 1.4 The following table allows MPNGU to be read directly from the HGMF grid-cell score x. DO NOT use the table for HGMF containing other than 1600 grid-cells.

Examples of Calculations

Counts of green grid-cells from the duplicate HGMF are 157 and 164.

1. MPNGU of filter 1 = 165
   MPNGU of filter 2 = 173
2. Average MPNGU = 169
3. No. of organisms/ml = 1700