Effect of Temperature on Growth of Bacteria in Raw Milk

1
Effect of Temperature on Growth of Bacteria in
Raw Milk

• Bhushan Jayarao
• Extension Veterinarian
• Department Veterinary Science
• Pennsylvania State University

2
How do bacteria get into raw milk ?

• Udder
• Exterior of the teats and udder
• Milker
• Aerial contamination
• Milking equipment
  bulk tank

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Udder-1

• Healthy udder - 0 to lt 100 bact./ ml
• Mastitis
• Subclinical mastitis
• - lt 10,000 bacteria / ml
• Clinical mastitis
• - gt 10,000,000 bacteria / ml
• Streptococcus agalactiae
• Staphylococcus aureus

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Udder-2

• Mastitis organisms can increase the bulk tank
  count by 100,000 organisms/ ml.
  
  
  
  
  
• Usually observed with Streptococcal and Coliform
  mastitis.

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EXTERIOR OF UDDERS AND TEATS

• Feces/ Manure
• Soil
• Bedding
• Feed

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EXTERIOR OF UDDER AND TEATS-2

• Between milkings cows teats may become soiled
  with dung, mud, bedding materials.
  
  
  
  
  
• If not removed, microorganisms present in the
  dirt, are washed into the milk.

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EXTERIOR OF UDDER AND TEATS-3

• Milk from cows with unwashed teats and heavily
  soiled dung may have a bacterial counts
  gt 100,000 bacteria /ml

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EXTERIOR OF UDDER AND TEATS-4

• Clipping cows around the flanks and udders
  reduces amount of
  contamination.
  
  
  
  
  
• Average bacteria/ml of raw milk
• ________________________________________________
• Farm of cows not clipped
  clipped
• ________________________________________________
  
• A 80 20,000 5,000
• B 45 48,000 10,000
• C 35 200,000 80,000
• D 20 350,000 200,000
• ________________________________________________

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EXTERIOR OF UDDER AND TEATS-5

• Average bacteria/ml raw milk
• _______________________________________________
• Farm of cows Not washed
  Washed
• _______________________________________________
  
• A 80 20,000 6,800
• B 45 48,000 20,000
• C 35 200,000 100,000
• D 20 350,000 230,000
• _______________________________________________
• Wash the udder with warm sanitizer solution
• Iodophors commonly used
• Udder is dried with single use paper towel

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Aerial contamination

• Least important source of bacteria in milk
• Few bacteria carried into milk by air entering
  the milking machine
• Cowsheds/ parlors lt 200 bacteria / L
• Few numbers of Streptococci and Gram-negative
  bacteria

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Milker

• Risk of contamination from the milker are much
  less with machine milking.
• 
  
  
  
  
• Milkers with infectious diseases contaminating
  the milk is not ruled out.

12
Water supply

• Many farms rely on untreated water supplies
• The number and types of bacteria in untreated
  waters vary considerably
• Organisms could originate from
• feces
• soil or vegetation

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Water supply -2

• Fecal origin
• Coliforms
• Fecal Streptococci
• Clostridia
• Soil or vegetation
• Pseudomonas
• Coliforms
• Bacillus spores

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Water supply -3

• Washing equipment with untreated water may not
  significantly increase bacterial numbers in raw
  milk.
  
  
  
  HOWEVER
• Multiplication of bacteria in residual water in
  the equipment will result in more serious
  contamination and problems.
  
  
  
  
  ALSO
• Lead to establishment and development of some
  undesirable types such as gram-negative
  psychrotrophic bacteria.

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Milking equipment -1

• Growth of contaminating bacteria possible if
  temperature control not maintained.
• 
  
  
  
  
• Milk should be cooled to lt40 F within 2 hours and
  not warmed above 50 F when next milking's milk
  is added.

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Milking equipment-4
Increase in bacteria/ml in "clean" milk over a
12-h period. Storage Temp. Increase 40 F
None 50 F 5X 60 F 15X 70 F
700X 80 F 3,000X
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Milking equipment -5

• The microflora is relatively restricted with one
  particular group or two
  
  
  
  
  
• The groups are
• Micrococci, Streptococci, Gram-negative rods,
  and asporogenous gram positive rods
• 
  
  
  
  
• In any one milking machine, the microflora can be
  consistent or vary from time to time

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Milking equipment -6

• Organisms that grow at refrigeration temperatures
  are of concern in the bulk tank
  
  
  
  
  
• These organisms are mainly
• gram-negative rods e.g. Pseudomonas
• gram positive rods e.g. Bacillus

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Putting it all together
__________________________________________________
_____ Milk Production
Storage SPC/ml
Conditions Temp. Fresh
24 hours 48 hours __________________________
_____________________________ 1. Clean cows
40 F 4,300 4,100
4,600 environment, 50 F 4,300
14,000 130,000 and utensils 60
F 4,300 1,600,000 33,000,000 2.
Clean cows, 40 F 39,000
88,000 120,000 dirty environment, 50 F
39,000 180,000 830,000 and
utensils 60 F 39,000
4,500,000 99,000,000 3. Dirty cows, 40 F
140,000 280,000 540,000
environment, 50 F 140,000
1,200,000 14,000,000 and utensils 60
F 140,000 25,000,000 640,000,000 _____________
___________________________________________
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Why low bacterial numbers in raw milk ?

• Low bacterial numbers
• good quality cheese, yogurt, buttermilk
• longer storage life of the product
• lower risk of pathogenic organisms in milk that
  grow at refrigeration temperatures such as
• Yersinia entercolitica
• Listeria monocytogenes