302 / 350
sample will need to be retested at higher dilutions. When the sample contains substantial amounts of
1
mold, depending on the type of mold, the upper countable limit may be lowered at user discretion.
2
Food samples may occasionally show interference on the 3M Petrifilm RYM Plates, for example:
3
a) a uniform blue background color (often seen from the organisms used in cultured products) these
4
should not be counted as TNTC. b) intense, pinpoint blue specs (often seen with spices or granulated
5
products).
6
When necessary, colonies may be isolated for further identification. Lift the top film and pick the colony
7
from the gel.
8
9
Internal Validation Studies
10
Ruggedness Testing
11
Minor perturbations were introduced within parameters that might be expected to occur when Petrifilm
12
RYM Plates are used. The results are listed in Tables 2 and 3.
13
14
Methodology
15
The effects of perturbations in three (3) method parameters were investigated:
16
1. Incubation time (44 and 62 hours)
17
2. Incubation temperature (24 ± 1°C and 29 ± 1°C)
18
3. Dilution fluid (0.1% peptone Water, 0.1% peptone water (45°C), Butterfields Buffer, Letheen Broth,
19
saline, peptone salt diluent, water, buffered peptone water).
20
The Petrifilm RYM Plate method was evaluated for ruggedness by examining the performance of the
21
plates when small changes in the operating conditions were made. All ruggedness parameters were
22
evaluated with one yeast (
Candidia tropicalis
) and one mold (
Geotrichum candidum
) at high and low
23
level inoculums, and one non-target strain (
Pseudomonas aeruginosa
ATCC # 27853) at a high level
24
inoculum. For the target organisms, five replicates were inoculated to achieve the target levels of 50-75
25
CFU/mL for high level yeast and 25-50 CFU/mL for high level mold, and 1-20 CFU/ml for the low level
26
yeasts and 1-10 CFU/ml for the low level mold. For the non-target strain, five replicates were inoculated
27
to achieve target levels of 80-100 CFU/mL for the high level
.
28
29
Table 2.
Summary of Ruggedness Test Results for
Candida tropicalis
– Inoculation Level, Incubation
30
Time, Incubation Temperature, and Dilution Fluid.
31
6
AOAC Research Institute
Expe t Revie Panel Use Only
OMAMAN-16D/ PTM Report
ERP Use Only - December 2014