4. AOACRIMicroMethods-2018Awards

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666 B ird et al .: J ournal of AOAC I nternational V ol . 99, N o . 3, 2016

rinsate, fresh swai, fresh tuna, fresh tiger shrimp, raw easy-peel shrimp, cherry tomato wash, frozen blueberries, Mediterranean apricots, creamy salad dressing, fresh pasta, vanilla ice cream, instant NFDM, and pasteurized skim milk.) See Tables 2015.13A and 2015.13B for a summary of results of the collaborative study. See Tables 2015.13C–G for detailed results of the collaborative study. The 3M Petrifilm RAC Plate is a sample-ready culture medium system that contains nutrients, a cold-water-soluble gelling agent, and an indicator system that facilitates aerobic bacterial enumeration. 3M Petrifilm RAC Plates are used for the enumeration of aerobic bacteria in as little as 24 h for most food matrixes. 3M Food Safety is certified to ISO 9001 for design and manufacturing. A. Principle (a)  3M Petrifilm RAC Plate .—25 plates per pouch, two pouches per box. Available from 3M Food Safety (St. Paul, MN; Cat. No. 6478). (b)  Sterile diluent .—Butterfield’s phosphate-buffered diluent. (c)  Pipets .—Capable of pipetting 1000 μL or a serological pipet. (d)  Sterile pipet tips .—Capable of 1000 μL. (e)  Stomacher .—Seward or equivalent. (h)  Incubators .—Capable of maintaining 32 ± 1°C and 35 ± 1°C and having a solid front to maintain a dark interior. (i)  Refrigerator or freezer .—Capable of maintaining temperature between –20 to 8°C for storing unopened 3M Petrifilm RAC Plates. (j)  Freezer .—Capable of maintaining temperature at less than −15°C for storing 3M Petrifilm RAC pouches after incubation. (k)  Standard colony counter or illuminated magnifier. (f)  Filter stomacher bags .—Seward or equivalent. (g)  3M Petrifilm Flat Spreader (Cat. No. 6425). B. Apparatus and Reagents

standard colony counter. Plates containing >300 colonies were either estimated or recorded as too numerous to count (TNTC). Estimations were conducted by counting the number of colonies in two or more representative squares and determining the average number per square. The average was multiplied by 30 to determine the estimated count per plate. All matrixes analyzed by the 3M Petrifilm RAC Plate were also analyzed using BAM (shrimp) or SMEDP (milk and NFDM) reference methods in a paired study design. Serial dilutions for each sample were plated in duplicate onto plate count agar (BAM) or standard methods agar (SMEDP). For raw easy-peel shrimp and pasteurized skim milk, agar plates were incubated for 48 ± 4 h at 35 ± 1°C or 32 ± 1°C, respectively. Instant NFDM agar plates were incubated for 72 ± 3 h at 32 ± 1°C. Typical colonies in the countable range (25–250) were enumerated using a standard colony counter. Each collaborating laboratory recorded the CFU/g results for the reference methods and the 3M Petrifilm RAC Plate on the electronic spreadsheet provided in the collaborator study outline. The data sheets were submitted to the study director at the end of each week of testing for analysis. The data from each duplicate set of plates were averaged. A logarithmic transformation of the averaged counts was conducted for data analysis. Outliers were identified using Cochran and Grubbs’ tests. The differences of means, including 95% upper and lower confidence limits, were determined for each contamination level for each matrix (6). If the confidence interval (CI) for the difference of means passed through the point 0, there was no statistical difference between the two methods (7). The reversed transformed difference of means, with a 95% CI, and the repeatability (s r ) and reproducibility (s R ) of the 3M Petrifilm RAC Plate and reference methods were also determined (7). AOAC Official Method 2015.13 Enumeration of Aerobic Bacteria in Food 3M Petrifilm Rapid Aerobic Count Plate First Action 2015 (Applicable to the enumeration of aerobic bacteria from raw groundbeef, rawgroundpork, rawground turkey, chickencarcass Statistical Analysis

Table 2015.13A. Interlaboratory study results of 3M Petrifilm RAC Plate vs FDA BAM Chapter 3 method for raw easy-peel shrimp

Difference of means d

Reverse- transformed difference of the mean, CFU/g

Reverse- transformed difference of means LCL, UCL

3M Petrifilm RAC Plate

FDA BAM Chapter 3

Matrix raw easy-peel shrimp

Mean log 10 CFU/g s r

95% LCL, UCL

Difference of means

N a

b

c

s r

s R

Lot

N

s R

Lot

32°C Low 16 2.96 0.132 0.280 Low 16 3.02 0.218 0.356 0.06 0.77, 1.72 Medium 16 4.29 0.202 0.215 Medium 16 4.23 0.095 0.298 –0.06 –0.18, 0.06 –2424.10 0.67, 1.15 High 16 5.56 0.110 0.248 High 16 5.76 0.097 0.214 0.20 –0.01, 0.42 214352.79 0.97, 2.61 35°C Low 16 2.80 0.121 0.335 Low 16 3.02 0.218 0.356 0.22 –0.03, 0.48 422.68 0.92, 3.03 Medium 16 4.22 0.172 0.273 Medium 16 4.23 0.095 0.298 0.01 –0.08, 0.11 539.37 0.83, 1.28 High 16 5.67 0.141 0.174 High 16 5.76 0.097 0.214 0.09 –0.09, 0.26 105217.30 0.82, 1.83 a  Number of laboratories that reported complete results. b  s r = Repeatability. c  s R = Reproducibility. d  95% lower and upper confidence limits. A 95% CI that contains the point 0, indicates no statistical significant difference between methods. –0.11, 0.24 139.47

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