Determination of specific gravity of milk

Specific gravity is the ratio of the density (mass of a unit volume) of a substance to the density (mass of the same unit volume) of a reference substance. The reference substance is nearly always water for liquids or air for gases. Or in simple words, the specific gravity refers to the ratio of the density of a solid or liquid to the density of water at 4 degrees Celsius.

The density of milk is the result of the densities of its components complicated by variations in the ratio of solid to liquid fat and in the degree of hydration of proteins. While density refers to the mass of a certain quantity of material divided by its volume, specific gravity is the ratio of the density of a substance to the density of a standard substance. The history of temperature fluctuations and processing treatments affects the specific gravity. The density of milk varies within the range of 1027 to 1033 kg at 20°C, corresponding to a specific gravity range of 1.027 to 1.033.

Table of Contents

Significance: -

Used in conjunction with the fat test for estimating total solids content.
To convert volume into mass and vice versa.
To estimate other physical properties, e.g., viscosity.
To detect adulteration of milk.
To maintain conformity with the laid-down legal standards.
To differentiate between different types of milk.

Methods of Measurement:

Determination of the extent to which on object sinks in a filed e.g. hydro meters, lactometer or series of beads of graded densities.
Weighing or given volume e.g. Pycnometer.
Hydrostatic weighing of an immersed bulb e.g. Westphal or analytical balance.
Measuring the volume of a given weight of a products e.g. dilatometer.
Measuring the distance that a drop of products falls in a density gradient column.

The choice of method demands that a balance be struck between precision on the one hand and speed and convenience on the other. Use of lactometers for determination of specific gravity is the most common method world over.

Two types of there are lactometer officially recommended and generally used.

i) Quevenne lactometer: This is calibrated at 15. 5 C or 60 F. It gives accurate reading in the temperature range of 60+ 10 F. variation in the temperature can be corrected from the correction table. We can also calculate this correction factor approximately by adding 0.1 subtracting to lactometer reading for ever degree increase diereses from the caliberation temperature of lactometer . After correction the reading is reffred to as corrected lactometer reading Quevenne and is designated as CL Rq.

ii) Zeal lactometer: This is calibrated at 29 C or 84 F. It gives accurate reading in the temperature range of 84 + 10 F subject to correction factor from the table. We may also calculate this correction factor approximately. For every 1 C change in the temperature there is a corresponding change of -0.3 in the lactometer reading . After application of the correction factor the reading is known as correction lactometer reading zeal and designated as C L R z.

Principle: A lactometer works on the Archimedes principle that anything sinks in a fluid unless it displaces an amount of the fluid equivalent to its weight.

Apparatus required:

Lactometer: A lactometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. It usually contains a scale inside the stem, so that the point (graduation mark) where the surface of the liquid touches the stem can be recorded.

Zeal Type: Calibrated at the temperature of 84^oF/29^oC.
Quevenne type: Calibrated at the temperature of 60^oF/15^oC.

Cylinder.
Thermometer.
Water bath.
Beaker.

Procedure:

Warm the milk sample to around 40 °C.
Cool it down to the approximate calibration temperature of the lactometer.
Pour the sample in a cylinder after proper mixing without incorporation of air or formation of froth or foam.
Put the lactometer in the cylinder and record the lactometer reading while it is steady and not touching the sides.
Record the temperature of milk with the help of a thermometer.
Calculate the specific gravity of milk from the observations (lactometer reading, temperature, and corrected lactometer reading).

Precautions:

The sample should be thoroughly mixed by pouring it from the sides of the container.
The milk to be tested should be 2-3 hours old after milking. This will allow air and gases to escape from this sample. (Recknagel Phenomenon)
Use a standard lactometer.
Do not allow the lactometer to remain in milk longer.
Read the lactometer reading in ½ to 1 minute.
Lactometer should not touch to the sides of the jar/cylinder.
While recording the lactometer readings, come to the level of the lactometer so as to eliminate the parallax effect.

Observation and Calculation:

Specific gravity = 1 + CLR
                                    1000
   (Where CLR= Corrected Lactometer reading)

Note: Estimation of Corrected Lactometer reading:

When temperature is recorded in centrigrades then for every degree decrease in temperature from calibrated temperature, subtract 0.3 from lactometer reading while as for every degree increase in temperature from calibrated temperature, add 0.3 to lactometer reading.

CLR = (LR ± ΔT x 0.3), where LR= Lactometre reading, ΔT= difference in temperature of milk from calibrated temperature of lactometer.

When temperature is recorded in degree fehrenhites then for every degree decrease in temperature from calibrated temperature, subtract 0.1 to lactometer reading while as for every degree increase in temperature from calibrated temperature, add 0.1 to lactometer reading.

CLR = (LR ± ΔT x 0.1), where LR= Lactometre reading, ΔT= difference in temperature of milk from calibrated temperature.