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Media for detecting and enumerating yeasts and moulds
1Department of Food Science and Technology, University of Georgia, Griffin 30223-1797.
Abstract:
Dilution plating techniques are designed to determine populations of viable fungal 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 or dried fruits, and results are expressed as a percentage of infected pieces. Both techniques are used by industry and regulatory agencies to monitor levels of 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 or blended. Buffered diluents containing up to 30% glucose or 60% sucrose are recommended for enumerating xerophiles. No one medium is satisfactory for detection or enumeration of yeasts and moulds in all foods. Dichloran rose bengal chloramphenicol agar, oxytetracycline glucose yeast extract agar and rose bengal chloramphenicol agar are superior to acidified potato dextrose agar for enumeration of yeasts and moulds. 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 and fusaria, but increased selectivity and specificity of media for detecting mycotoxigenic moulds are needed. Heat-resistant mould ascospores 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 degrees 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 or, in the case of sublethally injured cells, under conditions which facilitate resuscitation. Improved media are needed which accurately detect moulds producing specific mycotoxins in a wide range of food types.
Insights
Accurate fungal contamination monitoring in food relies on specific plating techniques and media. Research highlights the need for improved methods to detect various fungi and mycotoxins in diverse food products.
Area of Science:
- Food microbiology
- Mycology
- Food safety analysis
Background:
- Dilution and direct plating are key techniques for quantifying fungal propagules and assessing internal mycoflora in food.
- Industry and regulatory bodies use these methods to monitor fungal contamination throughout the food supply chain.
- Current methods face limitations in detecting all yeast and mould species, especially xerophiles and mycotoxin producers.
Purpose of the Study:
- To review and compare existing dilution and direct plating techniques for fungal enumeration in food.
- To evaluate the efficacy of various culture media for detecting and quantifying yeasts and moulds.
- To identify areas for improvement in fungal detection methods, particularly for mycotoxigenic fungi and xerophiles.
Main Methods:
- Comparison of dilution plating and direct plating techniques for fungal enumeration.
- Evaluation of different selective and differential culture media, including Dichloran Rose Bengal Chloramphenicol Agar and Dichloran 18% Glycerol Agar.
- Discussion of optimal diluents, incubation temperatures, and plating methods (spread-plate vs. pour-plate).
Main Results:
- Specific media like Dichloran Rose Bengal Chloramphenicol Agar and Oxytetracycline Glucose Yeast Extract Agar are superior to acidified potato dextrose agar for yeast and mould enumeration.
- Buffered diluents with high sugar concentrations are effective for xerophiles.
- The spread-plate technique is preferred over pour-plate for yeast and mould enumeration.
Conclusions:
- No single medium is universally suitable for detecting all fungi in all food types.
- Further development of selective and specific media is crucial for accurately detecting mycotoxigenic fungi and fungi under specific environmental conditions.
- Improved methods are needed for detecting sublethally injured fungal cells and mycotoxin-producing moulds in a wide range of foods.