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Chapter 23

Culture Media for Detecting and Enumerating Yeasts and Moulds

Dilution plating techniques are designed to determine populations of viable fungal, i.e. yeast and mould, propagules per unit weight or volume of food. Direct plating techniques, on the other hand, are designed to assess the internal mycoflora of individual pieces of foods, e.g. seeds, nuts or dried fruits, and results are expressed as a percentage of infected pieces. Both techniques are used by industry and regulatory agencies to monitor fungal contamination at various stages of food handling, storing, processing and marketing. Peptone (0.1%) water is commonly used as a diluent for samples to be homogenized, pummelled or blended. Buffered diluents containing up to 30% glycerol, 40% glucose or 60% sucrose are recommended for enumerating xerophiles. No one medium is satisfactory for detection or enumeration of all yeasts and moulds in all foods. Antibiotic-supplemented media are superior to acidified media for general enumeration of yeasts and moulds. Dichloran rose bengal chloramphenicol agar is most suitable for this purpose. Dichloran 18% glycerol agar performs well for enumerating moderately xerophilic yeasts and moulds. Fastidious xerophiles require media containing high concentrations of sugars and/or sodium chloride. Media have been formulated to detect potentially aflatoxigenic aspergilli and mycotoxigenic strains of penicillia, fusaria and other moulds, but media are needed with increased selectivity and specificity for detecting mycotoxigenic moulds. Ascospores of heat-resistant moulds often require heat treatment prior to plating in order to activate the germination process. The spread-plate technique is strongly preferred over the pour-plate technique for enumerating yeasts and moulds. The recommended incubation temperature is 25°C, but incubation time between plating and counting colonies ranges from 5 days for determination of general populations of mycoflora to 4 weeks or more for fastidious xerophiles. There is a need for new and improved media for selectively isolating various groups, genera, species and/or strains of fungi capable of growing only under specific environmental conditions, e.g. low aw, low pH, low oxygen tension or, in the case of sublethally injured cells, under conditions which facilitate resuscitation. Improved media are needed which accurately detect moulds capable of producing specific mycotoxins in a wide range of food types.

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06 Dec 2011
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