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    Dr. Ibrahim MM

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    Ahmed Eiqan Baig

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    © All Rights Reserved
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    Dr. Ibrahim MM

    Uploaded by

    Ahmed Eiqan Baig
    7 views14 pages

    Copyright

    © © All Rights Reserved

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    PPTX, PDF, TXT or read online from Scribd

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    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    7 views14 pages

    Dr. Ibrahim MM

    Uploaded by

    Ahmed Eiqan Baig

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    of 14

    Milk Microbiology and

    Spoilage
    Dr. Muhammad Ibrahim
    Associate Professor
    Department of Biosciences
    CUI Sahiwal
    Milk industry
     Milk is a highly nutritious media for bacteria
     Can spoil easily
     Initially milk may have few organisms , but bacteria can enter milk during
    milking process and handling
     Refrigeration improved milk industry, but still other improvements needed
    to be done to reduce No’s of non-pathogenic bacteria.
    Non pathogens in milk
     Air borne contaminants : Pseudomonas, Flavobacterium,
    Alcaligenes ,Some coliforms, micrococcus, fungi
     Psychrotrophs :more detrimental to milk
    They are able to grow at 5°c and less and make up a large
    number of the microbes in milk, they are natural soil
    inhabitants and wide spread in environment esp. water.

    • Spoilage of refrigerated milk consists usually of bitter,


    rancid, fruity flavours
    • Spoilage of milk at room temp. consists usually of souring
    Sources of Pathogens in milk
     Animal itself : Mycobacterium bovis, Brucella species,
    Coxiella burneti ,Campylobacter sp.

     Humans : Salmonella, Shigella

     Environment : Bacillus anthracis, Clostridium


    perfringens, Clostridium botulinum
    Pasteurization of milk
     LTLT : Low temp. Long time 63°C 30 minutes
     HTST: High temp. Short time72°C 15 seconds
     UHT : Ultra high temp.,130-135 °C 1 second
    Pasteurization:
     Kills and removes all pathogens(100%)
     Kills most non pathogens (99%)
     Surviving organisms are spore formers, thermoduric and thermophilic
    organisms
    Pasteurization of milk
    • Common thermoduric organisms:

    Bacillus, Micrococci, Lactococci, Arthrobacter


    Microbacteria, Corynebacteria
     Pasteurization provide a product with average shelf life 7-10 days( LTLT-
    HTST)
     Types and numbers of bacteria in milk depend on the microbial load before
    pasteurization
    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.
    Viable count method
    Advantages Disadvantages
     Can see cell morphology  Take long time
     Count viable cell only  Lots of tubes , tedious.
     Can be used to different
    types of samples( milk,
    milk products
    Dye reduction test
     A quick test for determination of microbial load of
    milk
     Use Methylene blue or resazurin dyes
     Add 1ml of dye to 10ml of milk, incubate 1 hr at
    36c
     If there is change in color , it means high bact.
    Load .It is not good quality milk, shorten expiration
    date.
     If no change in color (still blue), low bact. Load ,
    good quality milk , expand expiration period
     A long history of use in the dairy industry, especially
    to measure microbial quality from raw milk
    Dye reduction test
     Advantages  Disadvantages

    1. Simple, rapid, 1. Not all organisms


    and reduce the dyes
    inexpensive equally
    2. Not applicable to
    2. Only viable
    food specimens
    cells actively that contain
    reduce the reductive
    dyes enzymes
    Direct microscopic count
     Method
     Deliver 0.01 ml of milk on slide ,air dry
     Stain with LW (Levowitz- Weber )stain for 10 min.
     Count bacterial clumps with the microscope
     Reading results:
     Bacterial clump: dark blue
     Protein, Leucocytes and somatic cells :light blue
    background
     Fat globules : colorless
     Dirt: brown
    * clump : is one cell or group of cells of same type separated by a
    distance from other clumps or cells
    Direct Microscopic Count
    Advantages Disadvantages

    1. It is rapid and simple 1. Results depends on


    each analyst
    2. Cell morphology can be
    determined 2. Both viable and
    nonviable are counted
    3. Can se whole picture
    of milk 3. Some bact. may not
    take the stain well
    Diary products safety issues:

    • Outbreaks have been associated with cheeses


     Salmonella serovar Zanzibar in goat milk cheese

     Listeria monocytogenes in cheese

     Salmonella and S. aureus die off during cheese aging due to antagonistic
    activity by starters, salt and acid

     No outbreaks associated with yogurt

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