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.
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:
- The given sample of milk is thoroughly mixed and brought to a temperature between 50-70°F.
- It is then poured into a high glass cylinder by its sides and 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 float freely without touching the walls of the cylinder or its bottom.
- When it becomes stationary records:
- Scale reading &
- Temperature
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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