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Dairy Microbiology | Microorganisms in milk

Dairy Microbiology | Microorganisms in milk

Presence of microorganisms in milk

Milk is a neutral liquid rich in necessary nutrients for the growth of all kinds of living organisms as it contains carbohydrates, proteins, lipids, mineral salts and vitamins, and thus milk is an ideal medium for the growth of microorganisms, especially those capable of breaking down lactose and proteins, but these beliefs were denied due to the following:


1- The lactose sugar in milk is a complex sugar that is difficult to metabolize by a large number of microorganisms and yeast

2- Milk casein protein is difficult to metabolize by microorganisms. A small number of microorganisms have a suitable enzyme system for breaking down casein, and the same is said for lipid

3- Fresh milk contains anti-bacterial material, which prevents its growth

But due to the multiplicity of sources of contamination of raw milk, especially in manual milking, pollution usually occurs in varied kinds and large numbers of bacteria that are able to break down the milk protein, converting it into free amino acids, and these compounds allow many other bacteria that were in the dormant phase to benefit from these materials and this increase the decomposition the milk

Implications and importance of the presence of microorganisms in milk: 

  • The presence of large numbers of bacteria in milk indicates a lack of care for hygiene and production process steps . 
  • Some microorganisms produce enzymes that cause unwanted chemical changes and thus spoil the milk . 
  • Milk is contaminated with many types of pathological bacteria, so necessary precautions must be taken and destroy these kinds of bacteria.
  • When the bacteria grow in milk, they cause changes in its physical properties and chemical nature, causing the decomposition of fats, proteins and lactose, so that the milk becomes unfit for consumption and its nutritional value decreases.


Milk contaminated microorganisms

The microorganism count in milk is less than 1000 cells / mlWhen contamination is minimal during production, the number of microorganisms in the milk reaches more than 1 million cells / ml

 When the number of microorganisms above 100,000 cells / ml  , indicates poor production conditions,

 and a number lower than 10,000 cells / ml indicates good cleaning and sterilization practice during the production stages.


The most important types of microorganisms contaminating raw milk:




First- the Micrococcaceas family

It follows the subspecies: Micrococcaus and Staphylococcus

They are Gram-positive cocci


Second- the Streptococcaceas family

The subspecies related to dairy are Leuconostoc, Streptococcus, which are Gram-positive cocci.


Third- Gram-positive and non-sporogenic bacilli

 It includes the following :

Microbacterium, Corynebacterium, Arthrobacter, Kurthia, Lactobacillus


Fourth- Gram-positive and sporogenic bacilli

Includes the following races:

Bacillus, Clostridium


Fifthly- Gram-negative bacilli

Includes the following races:

Pseudomonas, Acintobacter, Flavobacterium, Enterobacter, Klebsiella, Serratia



They are microorganisms consisting of a single, oval-shaped cell, relatively large, with a diameter of about 4 to 10 microns, and the most important subspecies of it that have a relationship with milk are:

Torulopsis, Candida, Debaryomyces.



fungus has no great importance in contaminating of liquid milk because it needs a long reproduction time compared to bacteria, as it cannot compete with bacteria in such circumstances, but rather it has great importance in contamination of some milk products, especially during the ripening of cheeses.

the most important types of fungus that are found in milk and its derivatives:

Rhizopus, Penicillium, Aspergillus, Geotrichum, Cladosporium.

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