LPT 311: Milk and Milk Products Technology (1+1)

Determination of specific Gravity by using Lactometer

DETERMINATION OF SPECIFIC GRAVITY BY LACTOMETER

Materials required

• Sample of milk
• Lactometer
• Measuring cylinder
• Dairy thermometer

Lactometer

• The lactometer is a special type of hydrometer. It is constructed and graduated so that the lactometer reading is related to the specific gravity of milk on the ratio of the milk to water weight of a unit volume at a specified temperature.

Principle

• The lactometer is based on the principle that a freely floating body displaces a quantity of liquid of the same weight as the floating body and that density of a solution depends on its total solids content. Of the several types of lactometer in use, the most accepted lactometers are,
• The quevenne lactometer - designed to determine the specific gravity of milk at 15.5^ºC (60^ºF).
• Zeal Lactometer – designed to determine the specific gravity of milk at 29^º C at 84^º C.
• The Watson lactometer – is applied at a temperature of 39^ºC.
• ISI lactometer calibrated at 27^ºC is used. For temperature differences the correction factor is added.
• Measurement of the lactometer reading at a temperature below the melting point of milk fat leads to erroneous result.

Procedure

• The milk must be kept cold (40-50^ºF) atleast 1-2 hour before being tested with the lactometer.
• The milk should be thoroughly mixed by being poured from one container to another until a homogenous mixture is obtained.
• The milk is then poured into a measuring cylinder having the same temperature.
• The diameter of the cylinder should be atleast 1 inch greater than the largest diameter of the lactometer and the capacity should be sufficient to float is the lactometer.
• The cylinder should be filled to such a point that when the lactometer is placed in the milk, the cylinder will over flow.
• All bubbles should be brown from the surface particularly around the stem.
• Note the reading from the lactometer when it becomes stationary.

Specific gravity = 1 + CLR/1000 (CLR- corrected lactometer reading)

Uses of specific gravity assessment

• Used to control composition of milk during manufacture.
• Used to estimate total solids and non-solid porition of milk.
• Used to screen sample for addition of water.

Interpretation

The specific gravity of

• Cow milk ranges from 1.028-1.030. and
• Buffalo milk is 1.030 to 1.032.