Colligative properties of milk

Colligative properties are those physical properties which are governed by the number, rather than the kind, of particles present in solution. The important colligative properties of milk are its freezing and boiling points (c. —0.522 and 100.15°C, respectively) and its osmotic pressure (approxi- 

The freezing point of an aqueous solution is governed by the concentration of solutes in the solution. The relationship between the freezing point of a simple aqueous solution and concentration of solute is described by a relation based on Raoult s law  

The freezing point of bovine milk is usually in the range —0.512 to —0.550°C, with a mean value close to -0.522°C (Sherbon, 1988) or — 0.540°C (Jenness and Patton, 1959). Despite variations in the concentrations of individual solutes, the freezing point depression of milk is quite constant since it is proportional to the osmotic pressure of milk (approximately 700 kPa at 20°C), which is regulated by that of the cow s blood. The freezing point of milk is more closely related to the osmotic pressure of mammary venous blood than to that of blood from the jugular vein. 

However, it is now recommended that thermometers be calibrated in °C. More recently, thermistors have been used instead of mercury thermometers. Cryoscopes based on dew point depression have also been approved for use. These latter instruments also use thermistors and are based on changes in osmotic pressure. Thermistor cryoscopes are now used more widely than Hortvet instruments. 

Measurement of the freezing point depression of milk is used to estimate the degree of adulteration of milk with added water. Assuming an average freezing point of 0.550°C, the amount of added water can be calculated from  

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