926-927 / 1195
18
contamination by
Listeria
was not detected for any of the foods tested. A total plate count was also performed on each food to
1
determine the microbial load. Within each sample set there were 5 x 25g uninoculated samples and 20 x25g low-level inoculated
2
samples for each method. The target levels of the organism used for challenging each food matrix was as follows: 0.2-2 colony
3
forming units (cfu) /25 g food for the low level inoculation and 0 cfu/25 g for the uninoculated control samples. The aim was to
4
achieve fractional positive data within the range 5-15 positives for the low inoculation level.
5
6
A 3-tube Most Probable Number MPN analysis of each food was performed.
MPN was calculated using the AOAC MPN calculator
7
(9). On the day of initiation of analysis, 3 samples of 100 g, 20x25 g, 3x5 g from each level were prepared for each food, enriched
8
with the appropriate reference method broth and confirmed according to the specified reference method procedures. A different
9
culture of
Listeria monocytogenes
was used for each inoculated food. Cultures were tested to confirm biochemical reactions prior to
10
use in the study. In this study all cultures were biochemically typical of the species.
11
12
Preparation inoculated food
13
14
A plate count was performed using TSA spread plates on an 18-24h broth culture of the inoculating organism. The dilutions were kept
15
refrigerated overnight and the appropriate inoculum was added drop wise to bulk food, followed by thorough mixing. After
16
inoculation,
Listeria
was allowed to equilibrate with the food environment for 48-72 h at 2-8°C in refrigerated foods, for 2 weeks
17
frozen at -20◦C for vanilla ice cream and frozen spinach and for 2 weeks at room temperature for peanut butter, before testing. All of
18
the foods required different sample preparation procedures for the LMX and Reference methods (Table 1), the inoculated bulk food
19
was mixed thoroughly then the 2 sets of 25g samples and MPN samples were prepared.
20
21
Table 8. Method Comparison Study: PTM Food Matrices Reference Method
22
23
Matrices
Inoculating Organism
Reference Method
Cooked shrimp
L.monocytogenes
Serotype 3b
IMVS 1532
FDA-BAM
Smoked white fish
L.monocytogenes
Serotype 1/2b
IMVS 2551
FDA-BAM
Peanut butter
L.monocytogenes
Serotype 4a
NCTC 11994
FDA-BAM
Deli ham
Independent study
L.monocytogenes
Serotype 4b
ATCC 13932
USDA
PTM Certification Report