MC-Media Pad ACplus™ for Enumeration of Aerobic Counts in a Variety of Foods

Abstract

The MC-Media Pad ACplus™ is a dry, rehydratable film medium for the enumeration of aerobic bacterial colonies. The performance of the method in a variety of foods was compared to that of U.S. reference methods: U.S. Department of Agriculture (USDA) Food Safety and Inspection Service (FSIS) Microbiology Laboratory Guidebook (MLG) Chapter 3.02 "Quantitative Analysis of Bacteria in Foods as Sanitary Indicators" (USDA/FSIS MLG 3.02); Standard Methods for the Examination of Dairy Products (SMEDP) Chapter 6 "Microbiological Count Methods, Standard Plate Count Method" (SMEDP 6); AOAC Official MethodSM 966.23 Microbiological Methods; and ISO 4833-1:2013 "Microbiology of the food chain-Horizontal method for the enumeration of microorganisms-Part 1: Colony count at 30 degrees C by the pour plate technique." The validated matrixes included raw chicken breast and raw ground pork for USDA/FSIS MLG 3.02; cream cheese and yogurt drink for SMEDP 6; parsley, vegetable juice, prawns, tuna pate, sandwiches, and pasta salad for AOAC Method 966.23, and raw chicken breast, raw ground pork, cream cheese, yogurt drink, parsley, vegetable juice, prawns, tuna pate, sandwiches, and pasta salad for ISO 4833-1:2013. In each matrix study, five replicates at each of three contamination levels were tested as paired test portions. All 10 matrixes were compared to the appropriate U.S. reference methods under MC-Media Pad ACplus standard-usage conditions (35 ± 1°C for 48 ± 2 h). Across all matrixes, the difference of mean log10 values ranged from -0.43 to 0.44, within the acceptable range of -0.50 to 0.50. The candidate method repeatability SD (sr) varied from 0.03 to 0.23 log10 CFU/g, comparing favorably to the reference method SD, which ranged from 0.06 to 0.30 log10 CFU/g. Seven matrixes were compared to the appropriate U.S. reference methods under MC-Media Pad ACplus rapid-usage conditions (35 ± 1°C for 24 ± 2 h). Of the 21 matrix/concentration combinations, only three instances of difference of mean >0.5 log were observed. The ranges of sr values of the rapid-usage candidate method (0.023-0.324) and the reference method (0.013-0.236) were similar for the seven matrixes tested. All 10 matrixes were compared to the International Organization for Standardization (ISO) reference method under MC-Media Pad ACplus alternate-method conditions (30 ± 1°C for 72 ± 3 h). All 10 matrixes yielded a mean difference between methods of <0.5 log, and the ranges of sr values were similar between the candidate alternate method (0.037-0.378) and the ISO reference method (0.037-0.437). The product consistency study demonstrated no significant difference between lots of product and supported the 2-year shelf life. Robustness testing yielded no significant differences when small variations were made in sample volume, incubation temperature, and incubation time. Thus, the data show equivalent or better performance of the MC-Media Pad ACplus method compared to the relevant reference methods in support of AOAC Performance Tested MethodSM certification.