Specific gravity is the ratio of density of any substance to the density of standard substance (water) at 4ºC.
Specific gravity of normal milk is 1.028 to 1.032. Specific gravity of water is one. Hence, addition of water to milk tends to decrease the specific gravity of milk.
Fat content reduces the specific gravity since fat is the lighter portion.
|Specific gravity of different substances||Value|
Determination of milk specific gravity
Specific gravity is the relation between the mass of a given volume of any substance and that of an equal volume of water at the same temperature.
Since 1 ml of water at 4°C weighs 1 g, the mass of any material expressed in g/ml and its specific gravity (both at 4°C) will have the same numerical value. The specific gravity of milk averages 1.032, i.e. at 4°C 1 ml of milk weighs 1.032 g.
Since the mass of a given volume of water at a given temperature is known, the volume of a given mass, or the mass of a given volume of milk, cream, skim milk etc can be calculated from its specific gravity. For example, one litre of water at 4°C has a mass of 1 kg, and since the average specific gravity of milk is 1.032, one litre of average milk will have a mass of 1.032 kg.
( L / 1000 ) + 1 = specific gravity (sp. gr.)
Thus, if L = 31, specific gravity = 1.031.
A tall, wide, glass or plastic cylinder.
A thermometer – the lactometer may have a thermometer incorporated.
Heat the sample of milk to 40°C and hold for 5 minutes. This is to get all the fat into a liquid state since crystalline fat has a very different density to liquid fat, and fat crystallises or melts slowly.
Mix the milk sample thoroughly but gently. Do not shake vigorously or air bubbles will be incorporated and will affect the result.
Place the milk in the cylinder. Fill sufficiently that the milk will overflow when the lactometer is inserted.
Holding the lactometer by the tip, lower it gently into the milk. Do not let go until it is almost in equilibrium.
Allow the lactometer to float freely until it reaches equilibrium. Then read the lactometer at the top of the meniscus. Immediately, read the temperature of the milk.
Determination of total solids (TS) and solids-not-fat (SNF) in milk
SNF = TS – % fat
The only accurate way to determine TS is by evaporating the water from an accurately weighed sample. However, TS can be estimated from the corrected lactometer reading. The results are not likely to be very accurate because specific gravity is due to water, material less dense than water (fat) and material more dense than water (SNF). Therefore, milk with high fat and SNF contents could have the same specific gravity as milk with low fat and low SNF contents.
It should be noted that the relationship between Lc and TS varies from country to country depending on milk composition. The above formulae are called the Richmond formulae and were calculated for Great Britain.