Evaluation of a New Rapid Method for the Enumeration of Yeast and Mold Food Samples: Collaborative Study

Introduction: Due to their heterotrophic nature, yeasts and molds are able to adapt and survive in a wide range of environmental conditions. The presence of these microorganisms in food commodities can indicate contamination, inadequately cleaning or inadequate storage conditions. The 3M^™ Petrifilm^™ Rapid Yeast and Mold Count (RYM) Plate uses a cold-water-soluble gelling agent and an indicator system to facilitate the enumeration of yeast and molds after 48 h of incubation at 25^°C or 28°C, compared to 5 -7 days at 25°C  for the reference methods.

Purpose: The purpose of this AOAC^®OMA℠ collaborative study was to compare the new method to the ISO 21527:2008 parts 1 and 2 and the FDA BAM Chapter 18 method in raw ground beef and raw almonds.

Methods: A total of 16 laboratories participated.  Eight artificially contaminated samples, covering three levels of contamination (low level 10 -100 CFU/g, medium level 100-1,000 CFU/g, and a high level 1,000-10,000 CFU/g) and an uninoculated control level (0 CFU/g) were analyzed.  For the new method, replicates were evaluated at both 48 and 60 h at both 25°C and 28°C.  Unpaired replicates were analyzed following a harmonized FDA BAM and ISO reference method (enrichment in 0.1% peptone water).   

Results: A paired t-test was conducted at P = 0.95 and no significant difference was observed between the new method and the harmonized reference methods. No statistically significant differences were observed between the new method when enumerated at 48 h vs. 60 h or when plates were incubated at 25°C or 28°C when compared to the reference methods. The repeatability values obtained were generally similar between the two methods.

Significance: The collaborative study demonstrated the robustness and high level of reproducibility of the new method when enumerating yeast and molds in raw ground beef and raw almonds.