Determination of specific gravity by lactometer
Introduction
- A lactometer is a hydrometer especially made for the testing 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 records Quevenne’s degrees.
Apparatus
- Lactometer
- Cylinder
- Thermometer
Procedure
- The given sample of milk is thoroughly mixed and brought to a temperature between 10-21°C (50 and 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 not be allowed to touch the sides of the cylinder or its bottom.
- When it becomes stationary one records:
- the scale reading
- temperature
- 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.
- Subtract 0.1 from lactometer reading or 0.0001 from the specific gravity for every one degree Fahrenheit below 60°F.
- The specific the 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.
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