Numbers in brackets refer to illustrations in the Extension materials.

Introduction

Milk in the udder of healthy animals has very little bacteria.  It is only after leaving the udder that the milk can be contaminated with bacteria if proper hygenic handling is not undertaken.  Sources of contamination include: 

 
Milking Utensils 
The milker 
The air 
The skin of the animal 
Inflammed udders

They visit your farm and test your milk.

They test for: 
- hygiene and physical properties
- animal health and bacteria
- composition and suitability for processing. 

They have special equipment for each test.

Testing for Hygiene

During the milking and handling of the nilk all kinds of dirt can get into the milk, such as straw, dust, hairs, flies etc.  By filtering a representative sample of the milk through a white disc (The Sediment Test) these dirt particles become visible. 

This test gives a very good indication of the cleanliness of the milk with respect to visible dirt.  The test is also useful when training farmers in clean milk production and can also be used for the classification of  milk (milk payment).  It is obvious that a high amount of visible dirt also indicates high bacteria content.

Testing for Physical Properties 

Flavours which are usually absorbed by milk are: feed flavours, medicine flavours, chemical flavours and unclean flavours. Silage flavour is easily absorbed by the milk either directly or indirectly via the cow. However, if the silage is well-preserved and fed in reasonable quantities there will be no problem with silage flavour. Flavours from onions and certain weeds may be absorbed via the cow. 

Unclean flavours are often due to unclean sheds and animals. Keeping the milk too long in an open container in the milk shed may also cause unclean flavour. Medicine and chemical flavours usually come from medicine used to treat the cows and from detergents and disinfectants which have not been properly rinsed off the milking utensils. (11)

Testing for Healthy Animals 

In the case of diseased cows, blood clots may get into the milk.  (14) 
 

Milk containing antibiotic residues cannot be used for dairy products. This is particularly true for products which have to be fermented by lactic acid producing bacteria, as this will be inhibited by the antibiotics. (15)

Testing for Bacteria 

The two most common dye reduction tests are the Methylene Blue Test and the Resazurin Test. These dyes will, when added to milk which is incubated at 37 C, be chemically reduced if there is microbial activity in the milk. Generally, the time required to change the colour of the dye is shorter if there is high bacterial activity in the milk (many bacteria) and longer if there is low bacterial activity (few bacteria). 

Dye reduction tests do not indicate anything about the kind of bacteria in the milk, they only indicate the number of bacteria. 

Plate Count Test (18) 

The plate count method is used for determining total number of bacterial colonies per ml sample. A measured amount of milk is placed on agar plates and the number of colonies are counted after incubation. 

Microscope Count Test (18) 

To obtain the amount of single bacteria present in milk, a direct count must be carried out, using a microscope.

17 Then they check the time for the colour to disappear.  The shorter time, the more bacteria.
 
 

Testing for Composition 

The normal freezing point of milk is a little lower than the freezing point of water owing to dissolved components (mainly lactose and salts). The freezing point, being one of the most constant physical characteristics of milk, is used to detect adulteration of milk with water. 

The freezing point of cows milk varies between -0.53 C and -0.59 C with -0.54 C being the average value. Values below -0.53 C usually indicate extraneous water. 

A cryoscope is used for determination of the freezing point. 

Measuring density (20) 

Another method to detect adulteration with water is by measuring the relative density of the milk. The method is not as reliable as detection by determination of freezing point but is easier to carry out and requires less sophisticated equipment. The density of milk normally varies between 1.028 and 1.034 g per ml at 15^oC depending on the composition. The density of water is 1.0 g per ml. A lactodensimeter and glass cylinder are required for the test. 

Measuring fat content (21) 

For small to medium scale operations the Gerber test is used for determination of fat content in the milk. The test is a volumetric method in which fat is separated from skimmilk by centrifugal force. Sulphuric acid is used to dissolve the protein that forms the membrane around the fat (fat globules) and amyl alcohol is added to improve the separation of fat from protein.
 

Testing Suitability for Processing 

 

How to control milk quality

1.   Farm visits	(4)
2.   Milk tests	(5)

How to test for hygiene

1.   Farm inspection	(7)
2.   Visual checks and filtrations	(8)
3.   Samples for laboratory testing	(9)

How to test physical properties

1.   Visual checks - colour and textures	(10)
2.   Organic checks - smell and taste	(11)
3.   Temperature	(12)

How to test for animal health

1.   Cell counts for mastitis and other diseases	(13-14)
2.   Incubate with bacteria for antibodies	(15)

How to test for bacteria

1.   Dye test	(16-17)
2.   Bacteria count	(18)

How to test for composition

1.   Freezing point for adultration	(19)
2.   Density for adultration	(20)
3.   Fat content	(21)
4.   Toral solid content	(22)

How to test suitability for processing

1.   Acidity for quality	(23)
2.   Alcohol test for curdling
- suitability for pasturization	(24)