Milk Microbiology

Milk drawn from a healthy milk animal already contains some bacteria.  Most of the changes which take place in the flavour and appearance of milk, after it is drawn from udder are the results of the activities of microbes.  These microbes are of two types i.e. favourable – which brings favourable changes in flavour & appearance while pathogenic – which may cause diseases.  The favourable are carefully propagated while pathogenic (unfavourable) are destroyed to make the milk & its products safe for human consumption.

Following are the important microbes found in milk. 

a.        Bacteria:- Are microscopic, unicellular, occurs in the form of spherical, cylindrical or spiral cells.  Size  1-5m.  Sore forming bacteria produce trouble in dairy industry because of their resistance to pasteurization & sanitization produces.Greater the bacteriological count in milk, the lower is its bacteriological quality. The following bacteriological standards of raw milk are suggested as a guide for grading raw milk in India.

               SPC/ml  (org)                        Grade       

Not exceeding 2,00,000

Very good

Between 2,00,000 and 10,00,000


Between 10,00,000 and 50,00,000


Over 50,00,000


*Pasteurized milk should have a SPC/ml (org) not exceeding 30,000.

 b.       Moulds:-  Multi-cellular, at maturity are as Mycelium.  Useful in cheese making  which is responsible for defects in butter and other milk products.  Most spores of moulds are destroyed by pasteurization.

c.        Yeast:- Unicellular, larger than bacteria.  Destroyed during pasteurization.

d.       Viruses:- Are ultra-microscopic forms of like can be destroyed by pasteurization or higher heat treatment.


Bacteria multiply during production and holding of milk, depending on storage time and conditions.  The changes take place in the physico-chemical properties of milk are result of the activities of the individual microbial cells during their period of growth and reproduction or of substances produced during such activity.

a.        Stages of growth:-

i.                     Initial stationary phase

ii.                    Lag phase (Phase of adjustment)

iii.                  Accelerated growth phase (log phase)

iv.                  Maximum stationary phase

v.                   Phase of accelerated death.

b.        Factors Influencing Growth:-

i.                     Food supply – Milk and its products are good food source, provides all food requirements.

ii.                    Moisture – Milk contains adequate moisture to development.

iii.                  Air – Supplies O2  to aerobic bacteria and moulds.

iv.                  Acidity or pH – Preferably range 5.6 to 7.5.

v.                   Preservatives – Check growth depending upon concentration.

vi.                  Light – More or less harmful.

vii.                Concentration – High sucrose or salt content check growth.

viii.               Temperature – Important means for controlling growth.  According to their optimum growth temperature, bacteria can be classified into :

ix         Psychotropic – can grow at refrigeration temp. 5-70C.

x                Mesophilic – can grow at temp. 20-400C.

xi                Thermophilic – can grow at temp. above 500C.

c.        Products of Microbial Growth:-

i.                     Enzymes

ii.                    Decomposition products (fats, proteins, sugars).

iii.                  Pigments

iv.                  Toxins

v.                   Miscellaneous changes.

d.        Results of Microbial Growth in Milk:-

i.                     Souring:- Most common, due to transformation of lactose into lactic acid & other volatile acids & compounds, principally by lactic acid bacteria.

ii.                    Souring & gassiness:- Caused by coil group, indicates contamination of milk and its products.

iii.                  Aroma production:-  Due to production of desirable flavour compounds s.a. diacetly.

iv.                  Proteoloysis:- Protein decomposition leading to unpleasent odour.

v.                   Ropiness:-  Long threads of milk are formed while pouring. Mainly Alkaligenous viscus.

vi.                  Sweet curdling:- Due to production of a remain like enzyme curdles milk without souring.

e.        Destruction of Micro-organisms:- May be done by following means.

i.                     Heat – Most widely used. Pasteurization & sterlization.

ii.                    Ionizing radiation – Such as ultraviolet rays etc.

iii.                  High frequency sound waves – Supersonic and ultrasonic.

iv.                  Electricity – Microbes are destroyed actually by heat generated.

v.                   Pressure – Should be about 600 times greater than atmospheric pressure.

vi.                  Chemicals – Includes acids, alkalis, hydrogen peroxide, halogens etc.

Action of Microbes on Milk.

        Microbes                  Action                                 Result

1.    Streptococcus lactis


Lactose-lactic acid caseinprecipitation

2. St. bulgaricus



3.       Lactobacillus casei

Cheese ripening

Controls intestinaltermentation.

4. E coil

Souring & gasiness

Lactic acid & gasesAffects cheese ripening.

5. Bacillus substallis


off flavours.

6.       Alkaligenes viscus


Ropi milk

7. St. liquifiecence

Bitter Flavour

Bitter flavour to cream &butter.

8. B. substallis

Sweet curdling

Curd formation

9. St. paracitrovorus

Attacks citric acid

Flavours curd.