Plate count agar (PCA), standard methods ( M124) add 100 mg chloramphenicol/liter when this medium is used for yeast and mold enumeration.Dichloran 18% glycerol (DG18) agar ( M184).Dichloran rose bengal chloramphenicol (DRBC) agar ( M183).Equipment for plating samples, see Chapter 3.Basic equipment (and appropriate techniques) for preparation of sample homogenate, see Chapter 1.Methodology for testing the ability of isolates of toxigenic mold species to produce mycotoxins on sterile rice water substrate is included here.Įnumeration of Yeasts and Molds in Food-Dilution Plating Technique It is also used to determine whether the presence of mold is due to external contamination or internal invasion.
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The direct plating method is more efficient than the dilution plating method for detecting individual mold species, including most of the toxin producers, but it is less effective in detecting yeasts. The dilution plating and the direct plating methods may be used to detect fungi in foods. Although most foodborne fungi are not infectious, some species can cause infection, especially in immunocompromised populations, such as the aged and debilitated, HIV-infected individuals, and persons receiving chemotherapy or antibiotic treatment. Certain foodborne molds and yeasts may also elicit allergic reactions or may cause infections. Even though the generating organisms may not survive food preparation, the preformed toxin may still be present. Most mycotoxins are stable compounds that are not destroyed during food processing or home cooking. Several foodborne molds, and possibly yeasts, may also be hazardous to human or animal health because of their ability to produce toxic metabolites known as mycotoxins. Contamination of foods by yeasts and molds can result in substantial economic losses to producer, processor, and consumer. Occasionally, a food appears mold-free but is found upon mycological examination to be contaminated. Abnormal flavors and odors may also be produced. Their detectability in or on foods depends on food type, organisms involved, and degree of invasion the contaminated food may be slightly blemished, severely blemished, or completely decomposed, with the actual growth manifested by rot spots of various sizes and colors, unsightly scabs, slime, white cottony mycelium, or highly colored sporulating mold. They also grow on processed foods and food mixtures. They can invade and grow on virtually any type of food at any time they invade crops such as grains, nuts, beans, and fruits in fields before harvesting and during storage. Moisture requirements of foodborne molds are relatively low most species can grow at a water activity (a w) of 0.85 or less, although yeasts generally require a higher water activity.īoth yeasts and molds cause various degrees of deterioration and decomposition of foods.
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Their temperature range (10-35☌) is also broad, with a few species capable of growth below or above this range.
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Although the majority of yeasts and molds are obligate aerobes (require free oxygen for growth), their acid/alkaline requirement for growth is quite broad, ranging from pH 2 to above pH 9. The ability of these organisms to attack many foods is due in large part to their relatively versatile environmental requirements. The large and diverse group of microscopic foodborne yeasts and molds (fungi) includes several hundred species.
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Bacteriological Analytical Manual (BAM) Main PageĪuthors: Valerie Tournas, Michael E.