Microbiology and Deterioration of Milk and Milk Products

Pasteurization of milk

• No of non pathogens in raw milk is 500,000 to 3,000,000 organisms/ml of milk
• While No of non pathogens in pasteurized milk is 2,000 to 20,000 organisms /ml
• Different methods to count non pathogens in milk such as:
• Viable count ( pour plate method)
• Dye reduction test
• Direct (clump) microscopic count

Viable count method

• Pour plate method
• Deliver 1ml of milk to 9ml of sterile saline tube( 10-1), mix and transfer 1ml diluted milk from tube 1 to tube 2 ( 10-2).
• Deliver 1ml of milk to an empty Petri dish ,1ml of diluted milk 10-1,10-2 to empty Petri dishes .
• Pour 9ml of molten nutrient agar to each Petri dish, mix well
• Leave to solidify, then incubate at 37 c 48 hrs.
• Count No of colonies and multiply by dilution factor to determine No of viable non pathogens on milk
• A plate containing more than 300 colonies should not counted, plate with more dilution is counted instead.