Determine of Specific Gravity of Milk
To determine the basic nature of milk.
To decide the nature of adulteration of milk.
Lactometers are used for rapid determination of specific gravity. The method is based on law of floatation which states that when a solid is immersed in a liquid. It is subject to upward thurst equal to the weight of the liquid displaced by the body and acting in upward direction.
The types of Lactometer Generally used:
1. Quevenne’s type:
This is calibrated a 15.5 0 C or 60 0 F, it gives accurate reading in the temperature ranges of 60 +/- 6 0F subject to temperature correction factor. The correction is calculated as follows.
For every 10 0 F change in temperature there is corresponding change of 1.0 lactometer reading. After correction factor the reading is known as corrected lactometer reading.
2. ISI lactometer:
This new lactometer recommended by Indian standard Institution is calibrated at 27 0 C
Milk sample is kept at 27 0 C and reading of the lactometer is noted.
i. The milk to be tested should be 2-3 hours old after milking. This will allow air and gasses to escape from this sample.
ii. The temperature of the sample should be adjusted between 50-80 0 F for accurate reading.
iii. Sample should be thoroughly mixed by pouring it from the sides of the container.
iv. Use standard lactometer Quevenne’s or ISI.
v. Do-not allow the lactometer to remain in milk longer.
vi. Read lactometer reading in ½ to 1 minute.
vii. Lactometer should not touch to the sides of jar/cylinder.
Whole milk, skim milk, partially adulterated milk.
3. Petri dish
4. Diary floating thermometer
i. Adjust the temperature of milk sample at 50-80 0 F
ii. Fill the clean, dry glass jar about 2/3 rd volume of it with milk, pour the milk down along the sides of the jar to avoid the incorporation of air.
iii. Lower the lactometer gently in the milk making sure that the lactometer floats freely without touching the sides of the jar.
iv. Add milk to brim of the jar.
v. Read the lactometer reading at the top of the meniscus within one minute.
vi. Record the temperature of milk.
Specific gravity of milk can be calculated by the following formula (for all type of lactometer).
Corrected lactometer reading
Sp. Gr. = ——————————————————— + 1
Corrected lactometer reading = LR + CF.
Where CF for Quevennes lactometer
CF (+) = 0.1 x difference in temperature above 60 0 F
CF (-) = 0.1 x difference in temperature below 60 0 F