Yeast and Molds

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Yeast and Molds

• Food Microbiology Laboratory

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Use of Acidified Media

• Historically
• Acidification of Media Used to Prevent growth of
  Bacterial Cells
• Inferior to Use of Antibiotic Added Media
• Use Only if pH of Food is gt8
• Corn
• Eggs
• Antibiotics Not Effective at this pH
• Acidify to pH 5.0 with 10 tartaric acid
• Acidify AFTER autoclaving

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Antibiotic Added Media

• Supresses Growth of Bacterial Cells
• Allows Recovery of Injured Cells
• Minimizes precipitation of Food Particles

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Media

• Antibiotic Added
• Plate Count Agar
• Dichloran Rose Bengal Agar
• Yeast and Mold Enumeration using dilution Methods
• Use SURFACE SPREAD PLATE TECHNIQUE
• Hot media will kill yeast and molds

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Antibiotics

• Tetracycline
• Chloramphenicol
• Oxytetracycline
• Gentamicin
• Streptomycin
• Kanamycin
• Recommended because of broad spectrum and heat
  tolerance (can be autoclaved, other must be added
  after autoclaving)

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Rose Bengal

• Restricts Excessive Mycelial Growth
• Do Not Expose to Light
• Photodegradation of rose bengal results in
  formation of Dichloran which can restrict mold
  growth

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Enumeration

• Plates should be DRIED overnight prior to plating
  Prevents Spreaders
• Incubate UPRIGHT
• 22 to 25 C (room temp) for 5 days
• Countable Plates
• 15-150 colonies
• Do not remove plate lids
• Allergic Reactions
• DO NOT SMELL

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Laboratory Assignment

• Enumerate Yeast and Molds in Assigned Food
• Pre-poured Plates
• Yeast and Mold Petrifilm