DETERMINATION OF PERCENTAGE OF FAT IN MILK

DETERMINATION OF PERCENTAGE OF FAT IN MILK

The determination of fat in milk is important for the following reasons:

1.                  The payments for milk and several milk products are mostly made to the producers according to the fat contents of milk. The recent trend, however, is that payments to producers are made on the basis of solids-not-fat content of milk.

2.                  Its determination is very useful in the manufacture of butter and cheese.

3.                  It helps to detect adulterations like watering and skimming of milk.

The percentage of fat in milk can be determined using the following methods:

a)                  By Centrifugation (Gerber’s and Babcock’s methods)

b)                  By Ether extraction (Soxhlet’s method)

c)                  By Calculation (Mayer’s and Fleishmann’s formulae)

For the purposes of routine determination of fat content, Gerber’s method based on the principle of centrifugation, may be used. The details of this method are given below:

Centrifugation

The centrifugal force is exerted when any substance is rapidly swung round a center and tends to throw the whirling body as far from the center as possible. When two bodies of different densities are swung together, the heavier one is acted upon more strongly by this force and is forced to the outside. Similarly, when two liquids such as sulphuric acid and an oil, having different densities, are swung in the same bottle, the heavier one (sulphuric acid) will be driven to the end of the bottle farther from the center round which revolutions take place and oil being lighter will remain at the innermost end of the bottle, i.e., the end of the bottle near to the center.

Gerber’s method

Gerber’s method for the estimation of fat content in milk is named after its inventor, Dr. Gerber, a Swiss scientist. With this method the milk fat is separated from milk by strongly acidifying and rotating the milk in an especially constructed Gerber’s acid-butyrometer in the Gerber’s centrifugal machine. The method is simple, quick and accurate if carefully carried out. In this method the casein of milk is dissolved by means of 90 percent sulphuric acid and separation of fat is facilitated by the addition of amyl alcohol. This prevents the partial charring of fat and sugar by the sulphuric acid, keeps the fat clear and renders the separation of fat easier.

Procedure

The test-bottles are placed in a stand, open end upwards. Ten ml of sulphuric acid are drawn up into the acid pipette and run into the test-bottle by placing the pipette obliquely through the opening of the test- bottle until it comes in contact with the side of the test-bottle. Care should be taken that no acid deposits on the spiral groove on the inner side of the opening, otherwise rubber stopper screwed in will not hold well and will easily come off causing accidents. Then, slowly add 11 ml of milk by carefully letting it flow down the side of the bottle so that it will not mix with the acid. Finally, 1 ml of amyl alcohol is added carefully so that in the end the three fluids float in three distinct layers. The rubber stopper, dry and with no cracks, is then carefully inserted. The stopper having been well pushed home, the fluids in the bottle are mixed by slowly inverting the bottle one way after the other, until a perfect solution is obtained and all white curdy material disappeared (see the figure on the right).

The bottles are placed in the machine with the wide stoppered end near the circumference, opposite each other in order to maintain balance and whirled for three to four minutes at the rate of one thousand revolutions per minute. On removing the bottles from the centrifuge, they are submerged in an upright position, with the stoppered end downwards, in a water bath at about 140°F (60°C) and the reading taken. The fat collects in a clear yellow column at the top of the neck and to make the reading easier, the position of the fat column is adjusted by pressing in or slightly withdrawing the rubber stopper in order to bring it in level with one of the main divisions of the scale. It is then read from the bottom of the fat column to the lower border of the meniscus at the top. After completion of the test the butyrometers should be emptied at once and cleaned thoroughly for future use.

Precautions

●             The sampling of milk must be accurate and representative.

●             Acid should not wet the spiral opening of the butyrometers.

●             Amyl alcohol must be pure and free from all traces of fat.

●             The three fluids should be carefully added so that they arrange themselves in three distinct layers.

●             The rubber stoppers should be dry and without cracks.

●             Before centrifuging test-bottles, see that there is no curdy white material left undissovled.

●             The centrifuge must be properly balanced.

●             The reading should be taken accurately and the test performed in duplicate.

●             After use empty and clean the test-bottles.

EXERCISES

1.                  Make a drawing of Gerber’s butyrometers.

2.                  Why is it essential to mix the milk thoroughly before testing it by the Gerber’s method?

3.                  What precautions must you bear in mind while using pipettes?

4.                  What is the average percentage of fat in buffalo and cow milk?

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