PHYSICAL CHEMISTRY OF MILK

Lesson 13 COLLIGATIVE PROPERTIES OF DILUTE SOLUTIONS - 3

Module 6. Collegative properties

Lesson 13
COLLIGATIVE PROPERTIES OF DILUTE SOLUTIONS - 3

13.1 Introduction

The physical properties which are influenced by the concentration of the soluble components in the system are known as collegative properties. These properties play an important role in the processing of milk and preparation of milk products. Among these properties for milk, the freezing point and boiling point are important in detecting the adulteration of milk.

13.2 Boiling Point

Boiling point of a solution is the temperature at which the vapour pressure of the liquid equals to the external (Atmospheric) pressure. The water content in fluid milk is highest and hence the boiling point of milk will be close to that of water. But being a collegative property it is influenced by the dissolved substances like lactose, minerals etc and hence it is slightly higher than that of water. Since the vapour pressure of a solution is always less than the vapour pressure of the pure solvent it follows that the boiling point of a solution will always be higher than that of the pure solvent. Pure water boils at 100 ^oC. Milk constituents are responsible for the elevation of the boiling point of milk. The normal Ionic ↔ molecular ↔ colloidal equilibrium is altered due to heating. Addition of water lowers the concentration of dissolved substances responsible for the elevating the boiling points. As such adulterated milk with water boils at a lower temperature than the normal milk. The boiling point of normal milk is 100.15 ^oC.

13.3 Freezing Point

The freezing point of milk, like that of any aqueous systems depends on the concentration of water soluble components. The mathematical relationship between depression of freezing point and concentration of the solute was determined by Raoult and is expressed in the below mentioned equation.

Tf = Kf M

Where Tf is the difference between the freezing points of the solvent and the solution, Kf is the molal depression constant (1.86 ^oC for water) and M is the molar concentration of the solute. This relationship is valid only for dilute solution of undissociated solutes. The freezing point is a property controlled by the number of particles rather than the kind and nature of the particles in the solvent.

In the dairy field, the objective of freezing point measurement is virtually restricted to determination of the water content of the product in order to detect the illegal addition of water. Its value rests on the fact that the freezing point of authentic bovine milk varies within very narrow limits. Since the depression of freezing point is directly proportional to the number of particles in solution it is obvious that it is primarily determined by the major constituents of low molecular weight, the lactose, and the salts and is nearly independent of variation in the concentrations of colloidal micelles and fat globules. Since there is complimentary relationship between the lactose and sodium chloride in milk such that the osmotic pressure and hence the freezing point is maintained within a narrow range.

The temperature at which the solid, liquid and vapour pressure exists in equilibrium with each other a solution freezes at a lower temperature than the pure solvent and the depression of the freezing point depends on the concentration of the dissolved substances. The freezing point depression is the function of osmotic pressure as that of the solution. Since milk is having same osmotic pressure as that of the blood on physiological grounds and has fairly constant value.

Since the concentration of the total dissolved substances influences the freezing point and as milk is a physiological secretion of the mammary gland it is the most constant physical property. The mean freezing point for milk being -0.545oC. The addition of water will decrease the concentration of the dissolved substances the freezing point will raise (below – 0.53 ^oC). This property will help in detecting the adulteration of milk. The freezing point of milk can be determined by using Hortvet cryoscope. Although the Hortvet method has been accepted as the official method a number of modifications have been developed such as replacing the ether cooling system with mechanical refrigeration and adding mechanical stirring and tapping devices. Cryosopes employing thermistors in place of mercury –in – glass thermometers have been developed. Regardless of the method used, determination of the freezing point of solutions is empirical. As such it is necessary to use standardized equipment and technique. The Hortvet method involves measurement of the differences between the freezing point of a standard solution and the freezing point of milk. Any systematic error should be reflected in both the observed values and should thus to be eliminated.

The average freezing point of bovine milk is usually within the range– 0.512 to – 0.550 ^oC the average value is close to -0.522 ^oC, buffalo milk -0.560 ^oC, sheep milk -0.588 ^oC and goat milk -0.575 ^oC. Freezing point depression of milk is inversely proportional to the amount of water added. Colostrum is having a freezing point -0.605 ^oC which is slightly less than normal milk. The composition of milk is altered by the udder diseases like mastitis, but in order to maintain the osmotic pressure in equilibrium with blood the lowering of the lactose content is compensated by increase in the sodium chloride. As such the milk obtained from the animal affected with mastitis will not alter the freezing point.

The major components affecting the freezing point are lactose and soluble salts. 75 to 80% of the depression of the freezing point is due to these two constituents. Fat, protein, colloidal calcium phosphate, casein colloids and fat globules have negligible effect on the freezing point of milk because of their high molecular weight. Souring of milk, fortification of milk with lactose or non fat solids or addition of sugar will increase the depression of the freezing point.

Environmental factors associated with the variation in freezing point have been studied by several workers. Some of the variations have been attributed to seasonal effects, feed, water intake, stage of lactation, breed of the cow, heat stress and time of the day (i.e. morning or evening milk).If the net number of osmotically active particles are changed due to the handling of milk from the time it is drawn from the udder and the freezing point depression will also be changed.

The general principle employed for the determination of the freezing point is to super cool a sample slightly to induce crystallization and then to observe the maximum temperature attained. The temperature of cooling bath must be controlled otherwise the rate of heat loss will be greater than the rate of heat transfer to the bath by the heat of fusion of the solution and the observed freezing point will be too low. Control of temperature of super cooling and seeding techniques is extremely important. If it varies the amount of solvent that crystallizes out and consequently the observed freezing point also varies. Attempts have been made to apply a correction factor to the observed freezing point in order to enable calculation of the true freezing point.