Evaluation of a culture film (Petrifilm™ YM) method for enumerating yeasts and molds in selected dairy and high-acid foods

Abstract

The Petrifilm™ Yeast and Mold (YM) plate was compared to acidified potato dextrose agar (APDA) and chloramphenicol-supplemented plate count agar (CPCA) using pour- and surface-plating techniques for its ability to recover yeasts and molds from hard and soft cheeses, cottage cheese, yogurt, sour cream, fruit juice, salad dressing, relishes, and tomato-based sauces. Correlation coefficients of Petrifilm™ YM plates versus pour-APDA, surface-APDA, pour-CPCA, and surface-CPCA for recovering total yeasts and molds from a composite of the eight test foods were, respectively, 0.993, 0.993, 0.994, and 0.995. Slope and intercept values for populations detected using Petrifilm™ YM plates versus traditional systems ranged, respectively, from 0.984 to 1.008 and -0.051 to 0.149. The coefficient of variation for total yeast and mold populations recovered on Petrifilm™ YM plates was 1.0% compared to 1.2 to 1.7% for traditional enumeration systems. Regardless of the enumeration system employed or the type of fungal cell, i.e., yeast or mold, being enumerated, significantly (P < 0.05) higher populations were generally detected after 5 d compared to 3 d of incubation. After 5 d of incubation, in no case were yeast or total yeast and mold populations detected in the eight food products using Petrifilm™ YM plates significantly lower than respective populations detected using traditional pour- and surface-plating techniques and media. When Petrifilm™ YM plates were used, significantly higher total yeast and mold populations were detected in 3, 1, and 1 out of eight food products compared to using, respectively, pour-APDA, surface-APDA, and surface-CPCA enumeration systems. The Petrifilm™ YM plate offers an acceptable alternative to traditional methods for enumerating yeasts and molds in the dairy and high-acid products evaluated in this study.