Yeast and Mold Count

Historically, yeast and mold enumeration, with its requirement for a five-day incubation period, has been one of the slowest of the routine microbiology analyses to perform. Several attempts have been made over the years to shorten the incubation period by taking advantage of the ability of the HGMF to separate microorganisms from the food matrix and to confine the surface spread of colonies.

The first effort, reported in 1982, consisted of incubating the HGMF on Potato Dextrose Agar and staining the filter with safranin solution at the end of the incubation period to provide contrast between the colonies and the surface of the membrane filter (27). The method was refined by Lin et al. (44), who incorporated trypan blue dye into the Potato Dextrose Agar. The trypan blue was taken up by both yeasts and molds, producing blue colonies against the white background of the membrane filter. After determining that some molds would not develop visible colonies reliably within 48 h on Potato Dextrose Trypan Blue Agar, Entis and Lerner developed a new culture medium, YM-11 Agar, to optimize recovery of both yeasts and molds on the HGMF (45). YM-11 Agar enabled the enumeration of even slower growing yeasts and molds within a 50 ± 2 h incubation period at 25 ± 1°C. The performance ofYM-11 Agar was evaluated in a precollaborative study and then further validated in an AOAC-sponsored collaborative study. The two-day HGMF method using YM-11 Agar performed equivalently to the five-day Potato Dextrose Agar pour plate method in both studies. As a result, AOAC recognized this HGMF method in 1995 (46).

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