DETERMINATION OF SPECIFIC GRAVITY OF MILK

DETERMINATION OF SPECIFIC GRAVITY OF MILK

Specific gravity or density of milk is the ratio between the weight of a certain volume of milk at the standard temperature of 15°C (60°F), and the weight of the same volume of water at the same temperature. It is found by dividing the former weight by the latter weight. The specific gravity of water is taken as 1 and since milk is slightly heavier than water, its specific gravity is more than unity. It, however, varies in different samples of milk. It depends upon the amount and nature of the solid substances present in milk and since the composition of milk from different animals varies within wide limits, especially in so far as the fat content is concerned, the specific gravity of different samples of milk also correspondingly varies considerably. The average specific gravity of buffalo and cow milk is 1.033 and 1.032, respectively.

The removal of fat or skimming increases the specific gravity and the percentage of solids-not fat, while the addition of water reduces the specific gravity and decreases the percentage of total solids of a sample of milk. The richer the milk in fat, the lower is its specific gravity.

Taken alone specific gravity may be thoroughly unreliable and misleading but in conjunction with other tests especially the determination of fat and total solids, it is of great value in examining market milk. The influence of temperature on density is great and important, Freshly drawn milk if immediately tested, records a considerably lower specific gravity.

The specific gravity of milk may be determined by any one of the following methods:

1.                  By a lactometer

2.                  By a specific gravity bottle or pycnometer, and

3.                  By a Westphal’s balance

Since for routine determination of specific gravity a lactometer is considered convenient as well as accurate, therefore, the details given below only pertain to the use of lactometer.

DETERMINATION OF SPECIFIC GRAVITY OF MILK BY A LACTOMETER

A lactometer is a hydrometer specially constructed for the testing of milk. The instrument is based on the fact that when a solid body floats in a liquid it displaces an amount of liquid equal in weight to the weight of the floating body. Lactometer consists of a long narrow graduated glass stem which is enlarged below into a hollow glass bulb weighted with mercury and lead shots. Numbers registered on the lactometer scale range from 10 or 15 to 40, which record Quevenne’s degrees.

Lactometer

Apparatus                   Cylinder

Thermometer

Procedure

The given sample of milk is thoroughly mixed and brought to temperature between 10 and 21°C (50 and 70°F). It is then poured into a high glass cylinder by its sides aud the lactometer is slowly lowered into the fluid until it floats. There it is allowed to remain in

position for about half a minute. The lactometer must not be allowed to touch the sides of the cylinder or its bottom. When it becomes stationary one records: (i) the scale reading, and (ii) temperature. The scale reading is taken from the line on the scale which is in level with the surface of the milk and not from the top of the meniscus. If the temperature is above or below the standard, (15°C or 60°F) the lactometer reading is corrected according to the following rule: add 0.1 to the lactometer reading, or 0.0001 to the specific gravity for every one degree Fahrenheit above 60°F, and subtract 0.1 from lactometer reading or 0.0001 from the specific gravity for every one degree Fahrenheit below 60°F.

The specific gravity of the sample of milk is then calculated by dividing the Quevenne’s degree by 1000 and adding one.

For example, if the lactometer reads 31.9 at 63°F., the corrected lactometer reading becomes 31.9 plus 0.3, or 32.2, and by further calculation, specific gravity equals 1+32.2/1000 or 1.0322.

Precautions

●             Milk should not be examined until an hour after milking.

●             Mix the milk sample thoroughly before examining it for specific gravity.

●             The lactometer must not touch the sides and bottom of the cylinder.

●             Read the lactometer about half a minute after its becoming stationary.

●             Record the temperature correctly.

●             Duplicate observations be recorded.

●             Clean the lactometer after having used it.

EXERCISES

1.                  Explain briefly the principle involved in determining the specific gravity of milk by a lactometer.

2.                  Find the specific gravity of the given milk samples A and B by using a lactometer.

3.                  Why must temperature corrections be made?

4.                  What would be the specific gravity of a sample of milk having lactometer reading of 31.1 at 57°F?

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