Comparison of New and Traditional Culture-dependent Media for Enumerating Foodborne Yeasts and Molds

Introduction: The 3M Petrifilm Rapid Yeast and Mold (RYM) Count Plate has been reported to compare favorably with traditional culture-dependent media for obtaining actionable yeast and mold counts in foods within incubation times of 48 to 60 h.  Many of these foods were artificially inoculated.

Purpose: The purpose of this study was to compare RYM Petrifilm with other culture-dependent media for enumerating yeasts and molds in naturally contaminated foods.

Methods: Homogenates of 56 foods (6 dairy products, 5 meat and meat products, 15 fruits and fruit products, 5 vegetables, 9 cereal products, 8 seeds and nuts, and 8 dry seasonings and tea) were plated on RYM Petrifilm, YM Petrifilm, Dichloran Rose Bengal Chloramphenicol (DRBC) agar, Acidified Potato Dextrose Agar (APDA), and Dichloran 18% Glycerol (DG18) agar.  Colonies were counted after incubating plates for 48, 72, and 120 h at 25°C.

Results: Out of 56 foods in which yeasts and/or molds were detected on one or more media (RYM, YM, DRBC, APDA, DG18) incubated for 120 h, neither yeasts nor molds were detected in 55.4, 73.2, 21.4, 19.6, and 71.4% and 3.6, 1.8, 0, 0, and 0% of foods plated on the five respective media and incubated for 48 and 120 h, respectively.  The general orders of performance were DRBC = APDA > RYM Petrifilm > YM Petrifilm ≥ DG18 when plates were incubated for 48 h, DRBC > APDA > RYM Petrifilm > YM Petrifilm ≥ DG18 at 72 h, and DRBC > APDA > RYM Petrifilm = YM Petrifilm > DG18 at 120 h.

Significance: The incubation time at which actionable levels of yeasts and molds naturally present in a foods can be determined will depend on the origin, composition, pH, and a_w of the food being examined, the formulation of the enumeration medium, and standards set by the manufacturer and regulatory agencies.