SAMPLING OF MILK
Milk is subjected to different methods of examination in order to find out the chemical composition like fat, protein, lactose, mineral matter along with the detection of developed acidity, presence of adulterants or preservatives which might be used to unscrupulously keep the milk from spoilage, estimation of bacterial count, presence of sediments etc.
Proper and accurate sampling is the corner stone in getting the perfect result in analyses of milk. Careless preparation of samples will jeopardize the results than the actual performance of the test itself.
EQUIPMENT / IMPLEMENTS / DEVICES REQUIRED FOR SAMPLING
It is an equipment used to mix the milk or cream to make it uniform in composition throughout the container or can in which the product is kept.
It is usually made up of stainless steel or aluminium or any metal which will not adversely react with the milk or any other dairy product. It consists of a disc containing several perforations. A long handle is fixed to it at the centre which helps in its to and fro movement in the milk or dairy product.
This device consist of a small cup fixed to one end of a long handle and is mainly used to collect the sample from the container. The capacity of the dipper is usually 50 ml.
The tube samplers are advantageous in that
A representative sample can be obtained regardless of how long the milk has stood before sampling
A column of milk which represents the milk from top to bottom of the container is collected as sample.
McKay sampler or McKay equity sampler
It consists of two parts and the principle involved in the sample collection is the same as that of tube samplers. The sample of milk in proportion to the quantum of the container is collected.
IMPORTANT POINTS TO BE KEPT IN MIND WHILE SAMPLING MILK
The whole body of the milk from which a sample is drawn should be uniform throughout its composition. The sample of milk drawn from the container must be a truly representative one.
- The constituent of the milk that is more variable when the milk is standing still is fat. Hence extra care has to be taken to see that it is present uniformly throughout the length and breadth of the container. Other essential factors are freezing and souring of milk.
- When milk is allowed to stand for some time in sample bottles, the fat rises to the top by virtue of its specific gravity. Under such circumstances, the bottle may be immersed in water bath kept at a temperature of 40°C to keep the fat under molten condition before thorough mixing.
- Violent mixing of milk may be avoided since viscosity of milk will not allow air bubble to rise to the top. Slow but steady churning of fat globules will occur when milk is transported over long distance because of charging of the fat globules with air bubbles.
- Thorough mixing of milk sample may be ensured by stirring the milk with a long handled plunger if the container is bigger one. If the container is small, the milk can be poured from one to another container or simply shaking it gently.
- Extra care has to be exercised when sampling is done from frozen milk because it is not uniform in composition. The frozen portion is rich in water whereas the liquid portion is rich in solids. In order to get a true and representative sample from milk, heating to a temperature of 40°C is resorted to in a water bath.
- Curdled milk samples pose another problem in sampling due to the developed acidity. Hence, they are treated with a strong solution of caustic soda or ammonia (5-10% to the volume of milk) and a correction factor has to be worked out for the quantity of ammonia or caustic soda added.
- It is necessary to examine the samples as early as possible since physical, chemical and microbiological changes affect the quality of milk.
- Transport of milk over long distance entails its own problem. In such cases, they have to be preserved by adding suitable preservatives and this kind of addition is permissible only for chemical analysis.