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Determination of specific gravity of milk

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Specific gravity:

Specific gravity is the ratio of the density of a substance to the density of a reference substance; relative density.

Lactometer:

A lactometer is just like a hydrometer, consisting of a long narrow graduated (10 or 15 to 40; Quevenne scale) glass stem which is enlarged below into a hollow glass bulb weighted with mercury and lead shots. 

Lactometer for Specific gravity of milk test

Principle:  

 

When a lactometer floats in a milk sample it displaces an amount of milk equal in weight to the weight of the floating lactometer. 

Apparatus: 

Lactometer, Glass cylinder, and thermometer.

Procedure: 

  1. The given sample of milk is thoroughly mixed and brought to a temperature between 50-70°F.
  2. It is then poured into a high glass cylinder by its sides and the lactometer is slowly lowered into the fluid until it floats.  
  3. There it is allowed to remain in position for about half a minute.
  4. The lactometer must float freely without touching the walls of the cylinder or its bottom.
  5. When it becomes stationary records: 
  6. Scale reading &
  7. Temperature
Cyliner for lactometer test

Calculations:

 If the temperature is above or below the standard, (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 the standard temperature.
  • Subtract 0.1 from the lactometer reading or 0.0001 from the specific gravity for every one degree Fahrenheit below the standard temperature. 
  • The specific gravity of the milk is then calculated by following the formula;

                     Specific Gravity of Milk=1+(CLR)/1000 

                     CLR= Corrected lactometer reading

EXAMPLE:

 The lactometer reading for a milk sample of a cow is 31.9 at 63°F, calculating its specific gravity (1.029-1.032).

Solution: 

    CLR          = 63°F – 60°F                 =3

                         = 3 x 0.1                            =0.3

          CLR          = 31.9+0.3                         = 32.2

Formula:

Specific Gravity   =1+(CLR)/1000 

    SG                   = 1+ 32.2 / 1000

                          = 1+ 0.0322   

                  =1.0322.

Values for specific gravity of different milk:

For Cow’s milk: The value is between 1.026 – 1.032.

For Buffalo milk: The value between 1.030 – 1.034.

For Skim milk: The value is between 1.034 – 1.036.

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