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3M Petrifilm RYM Collaborative Study June 2014
OMA-2014-June-XXX
1.2.7 3M Petrifilm RYM plates can be counted using a standard colony counter or other
illuminated magnifier. Gridlines are visible with the use of a backlight to assist with
estimated enumeration.
1.2.8 To differentiate yeast and mold colonies on the Petrifilm RYM, look for one or
more of the following characteristics:
YEAST MOLD
Small colonies Large colonies
Colonies have defined edges Colonies have diffuse edges
Pink-tan to blue-green in color Blue/green to variable upon
prolonged incubation
Colonies appear raised (3 dimensional) Colonies appear flat
Colonies have a uniform color Colonies have a dark center with
diffused edge
1.2.9 The circular growth area is approximately 30 cm2. Plates containing greater than
150 colonies can either be estimated or recorded as TNTC. Estimation can be done
by counting the number of colonies in one or more representative squares and
determining the average number per square. The average number can be multiplied
by 30 to determine the estimated count per plate. If a more accurate count is
required, the sample will need to be retested at higher dilutions. When the sample
contains substantial amounts of mold, depending on the type of mold, the upper
countable limit may be lowered at user discretion.
1.2.10 Food samples may occasionally show interference on the 3M Petrifilm RYM
Plates, for example:
1.2.10.1 uniform blue background color (often seen from the organisms used in cultured
products) these should not be counted as TNTC.
1.2.10.2 intense, pinpoint blue specs (often seen with spices or granulated products).
1.3. ISO 21527:2008. Parts 1 and 2 and FDA-BAM Ch. 18 (April 2001)
1.3.1Using the diluted sample (prepared in section 5) distribute 0.1 ml inoculum of each
dilution onto specific agar plate listed below in triplicate and spread.
1.3.1.1 Low moisture foods (raw almonds): Dichloran 18% glycerol (DG18) agar
1.3.1.2 High moisture foods (raw frozen ground beef (80% lean/20% fat): Dichloran-
rose bengal chloramphenicol agar (DRBC).
1.3.2 Incubate at 25°C for 5-7 days. Do not incubate plates in stacks higher than three
and do not invert.
1.3.3 After 5 days count and record typical colonies. If there is no growth, re-incubate
for another 48 hours.
1.3.4 Record results from the three plates with 10-150 colonies and report average results
in colony forming units (CFU) per gram.
Final Results
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DRAFT DOCUMENT
AOAC Research Institute
Expert Review Panel Use Only
OMAMAN-16A/ Collaborative Study Protocol
ERP Use Only - December 2014