Milk and Milk Products Technology

YEAST AND MOULD COUNT

Aim

• To enumerate yeast and mould in the given sample of butter.

Apparatus required

• Sterile petridishes, 1.1 ml pipettes, BOD incubator (Biological Oxygen Demand), Pipette can, petridish can refrigerator, conical flask, test tubes.

Medium required

• Potato dextrose agar medium, 10% Lactic acid or tartaric acid.

Composition of the medium

• Potato infusion from white potato - 200 gm
• Dextrose - 20 gm
• Agar - 15 gm
• Distilled water - 1000 ml
• pH = 5.6 ± 0.2

Procedure

• The medium potato dextrose is sterilized by autoclaving at 121⁰ C for 15 minutes at 15 pounds pressure. The pH of the medium is adjusted to 3.0 – 3.5 using 10 % lactic or tartaric acid. Yeast and mould count is normally done in products like butter, cream and fermented milk products. About 5g of butter is melted in water bath at 45⁰ C to liquefy and there after 1 ml of this liquefied butter is transferred to 9ml of the sterile distilled water and mixed gently till a uniform emulsion is formed. Serial dilution is carried out up to 1:1000. The required dilution of serially diluted sample is transferred to a sterile petridish followed by the addition of potato dextrose agar medium.
• Mix the contents of the petridish gently and allow it to remain undisturbed for 15-20 minutes for solidification. Then the petridish is transferred to BOD incubator and incubated for 5 days at 21⁰C. After 5^th day take the plate and count the colonies.
• Yeast appears as convex glistening surface colonies on the agar. The Mould colonies appear in different colours viz white, black, green and are cottony, wooly and velvety in texture. The colour of mould colonies is due to the colour of the spores and due to the aerobic nature yeast and mould form surface colonies.

BIS standard for butter

Yeast and mould count	Grade
Less than 20	Good
20-50	Fair
50-100	Poor
Above 100	Very poor

Result

• Yeast and mould count per gram or ml = No.of colonies x dilution factor.

Comment

Questions

1. How does butter favour the growth of yeast and mould?
2. What is the significance of high yeast and mould count in butter?