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Colonies have defined edges
Colonies have diffuse edges
1
Pink-tan to blue-green in color
Blue/green to variable upon prolonged
2
incubation
3
Colonies appear raised (3 dimensional)
Colonies appear flat
4
Colonies have a uniform color
Colonies have a dark center with
5
diffused edge
6
11. The circular growth area is approximately 30 cm
2
. Plates containing greater than 150 colonies
7
can be either estimated or recorded as TNTC. Estimation can only be done by counting the
8
number of colonies in one or more representative squares and determining the average number
9
per square. The average number can be multiplies by 30 to determine the estimated count per
10
plate. If a more accurate count is required the sample, will need to be retested at higher dilutions.
11
When the sample contains substantial amounts of mold, depending on the type of mold, the upper
12
countable limit may be at user discretion.
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12. Food samples may occasionally show interference on the 3M Petrifilm RYM Count Plates for
14
example:
15
a. Uniform blue background color (Often seen from the organisms used in cultured
16
products) these should not be counted as TNTC.
17
b. Intense pinpoint blue specs (often seen with spices or granulated products).
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c. Report final results as colony forming units/gram (cfu/g)
19
13. If required, colonies may be isolated for further identification by direct microscopy or
20
biochemical analysis. Lift the top film and pick the colony from the gel.
21
22
Results of the Collaborative Study
23
In this collaborative study, the 3M Petrifilm RYM Count Plate was compared to two reference
24
methods for enumerating total yeast and mold: FDA BAM Chapter 18 and ISO 21527 Part 1 or
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Part 2. A total of 15 laboratories throughout the United States participated, with 11 laboratories
26
submitting valid data for the frozen raw ground beef patties and 14 laboratories submitting valid
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data for the raw almonds as presented in Table 1. For the frozen raw ground beef patties,
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laboratories 4 (failed to enumerate 3M RYM Petrifilm plates at 60 hours), 6 and 8 (plated the
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reference method samples in duplicate and not the required triplicate plating) and 12 (plated the
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3M Petrifilm RYM plates and reference method plates on two different days (enrichments were
31
held at 2-8
o
C overnight and the Petrifilm RYM plates were plated 24 hours after the reference
32
method plates)) reported deviations from the protocol; ; and were therefore excluded from
33
statistical analysis. For the raw almonds, laboratory 4 reported a deviation from the protocol
34
(failure to enumerate 3M RYM Petrifilm Plates at 60 hours) and was therefore excluded from the
35
statistical analysis.
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The 3M Petrifilm RYM Count Plate results along with FDA BAM and ISO results reported by
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each lab were converted to logarithmic values for statistical analysis and were plotted using a
38
Youden’s plot (see Figures 1-4). The log
10
individual lab results are presented in Tables 2-9.
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Using the Youden’s plots, an initial review of the data to determine outliers was conducted by
40
observing the mean replicate results for each laboratory at each contamination level for each
41
matrix. The transformed data was than statistically analyzed for outliers by the Cochran and
42
Grubb’s tests. No evidence of physical cause or suspicion of cause was noted, so all outliers
43
identified were included in the statistical analysis. A paired t-test, the difference of means, and
44
Candidates for 2016 Method of the Year
81