314 / 350
colonies and results were reported by averaging counts between triplicate plates. Final results were
1
presented as colony forming units per gram (CFU/g).
2
3M Petrifilm Rapid Yeast Mold Plate Method
3
Paired test portions for each matrix were prepared as described in the ISO and FDA BAM reference
4
methods. A 1 mL aliquot of each sample dilution was placed on the RYM Petrifilm. The top film was
5
gently lowered and the aliquot was spread with the 3M Petrifilm Flat Spreader. The plates were left
6
undisturbed for one minute, and incubated for 48 ±2 hours at 25±1°C. Typical colonies were
7
enumerated and reported as CFU/g. The RYM plates were reincubated for an additional 12 hours (60
8
total hours) and then enumerated using a Quebec colony counter. Typical yeast colonies were small
9
pink tan or blue green in color with defined edges. Molds s typically formed large blue-green colonies
10
with diffuse edges.
11
12
RESULTS
13
For each matrix, the colony forming units/g were converted to log
10
and analyzed. A t-test was
14
performed using the appropriate assumption for equal or unequal variance to evaluate the mean of the
15
log
10
values for statistical difference between two methods at the 95% confidence level.
16
Repeatability of the Petrifilm RYM method and the reference methods was calculated as the standard
17
deviation (S
r
) for each contamination level for each matrix.
18
Yogurt 48 hours
19
No statistically significant difference was observed between all 3 levels analyzed between the two
20
methods. The p-values calculated at a 95% confidence level for the low, medium, and high levels were
21
0.6343, 0.4633, and 0.6776, respectively. The t-test indicated no significant differences between the 3M
22
RYM method and the ISO and FDA BAM methods. The 3M RYM method had a higher repeatability value
23
than the ISO and FDA BAM methods for the medium and high levels, with S
r
values, respectively, of
24
0.0279 and 0.0850 for the medium level and 0.0787 and 0.0895 for the high level. For the low level the
25
ISO and FDA BAM had higher repeatability value compared to the RYM method, with S
r
values,
26
respectively, of 0.1026 and 0.0445. Detailed results are presented in Table 12.
27
28
29
30
31
18
AOAC Research Institute
Expert R view Panel Use Only
OMAMAN-16D/ PTM Report
ERP Use Only - December 2014