Yeasts and Moulds

Selective media for yeasts and moulds include acidified media and antibiotic media. The method described below uses acidified potato dextrose agar.

Equipment and Material

Potato dextrose agar

Equipment for plating

Tartaric acid solution (10 aqueous)

Incubator set at 22-25C

Procedure

  1. Prepare cheese homogenates and serial dilutions as described in Section A.
  2. Predetermine the quantity of sterile 10% tartaric acid solution necessary to obtain a pH of 3.5 + 0.1. Put a portion of the medium in a small beaker and titrate to pH 3.5 at 45C. Check the accuracy of the titration by allowing the agar to cool to incubation temperature, place electrodes directly into the solidified medium, and read the pH. It should be 3.5 + 0.1. Calculate the amount of sterile 10% tartaric acid solution necessary for the volume of tempered agar to be used for pouring plates.
  3. Place 5 ml of the 0.1 dilution and 1 ml of additional dilutions as required into each of duplicate petri dishes.
  4. Add the tartaric acid solution to the tempered agar immediately before pouring 15 - 20 ml into each of the plates containing the sample dilutions.
  5. Mix well and let solidify before inverting the plates. Incubate at 22 - 25C.
  6. Count the plates at 3 and 5 days of incubation. Yeast cells will appear as cream coloured shiny colonies.