Freezing Point

Freezing point of water is a constitutive property i.e. it depends upon the nature of the molecules of the liquid i.e. water. But the freezing point of a solution or a liquid such as milk is a colligative property which depends on the total number of solute particles. If the solvent content is the same any variation in the solute concentrationwill affect freezing point of the solutions. The solvent of milk is water while solute consists of molecules like sugars, minerals, proteins, milk fat in suspension, etc.

When ice and water are in equilibrium with one another that is called freezing. At this temperature they have the same vapour pressure. The addition of solute to water lower the vapour pressure. The reason is that it delays the escape of the vapours from the surface. Thus the freezing of a given solution is always lower than that of water. The amount of depression of the freezing point is proportional to the fraction of the total number of solute particles, which are solute molecules.Also freezing point depression is a function of the osmotic pressure of the solution.

The osmotic pressure of milk is believed to be the same as that of blood physiologically.Freezing point of milk: Freezing point of milk ranges from – 0.530 0 to - 0.550 0 C.In order to obviate negative sign the term freezing point depression is used. Thus 0.540 refers to the freezing point depression of milk. This eliminates the negative sign. Addition of water lowers this value. Values below 0.530 indicate the addition of water. Since skim milk also has the same freezing point as milk adulteration of skim milk cannot be detected by this method.

Souring of milk lowers the freezing point as the number of molecules increases due to breakdown of lactose, which affect the osmotic pressure of milk. This results in an increase in freezing point depression Determination of freezing point depression of milk: Freezing point is a fairly sensitive and constant property of milk. It is principally used to detect adulteration of milk with water. Since the freezing point depression of milk differs from water with only 0.5 0 C, highly sensitive and accurate thermometers are required for its measurement, which can read up to 0.001 0 C. Two methods are used to determine freezing point of milk

1. Hortvet Cryoscope method: It was one of the earliest method to determine freezing point of milk by an instrument called Hortvet cryoscope. It was developed and used as early as 1923. This instrument has a number of drawbacks.Major difficulty being the cumbersome of the operation of cryoscope and limitations in getting reproducibility of results. This instrument uses ordinary thermometer called freezing point thermometer.

2.Thermistor Cryoscope method: Due to the development of probes are special type of thermometers based on the measurement of changes in electrical resistance with variation in temperatures. Thermistor cryoscope are vary popular in determining freezing point depression of milk.“Fiske Cryoscope” and “Advanced Milk Cryoscope” are the important cryoscope instruments which are used for determining freezing point of milk. Official methods of Analysis of the Association of Official Analytical Chemists (AOAC) has recommended the method for determining freezing point depression of milk using thermistor probes.

In order to determine percentage of water added it can be calculated by the formula -

Minimum percentage of added water by mass = F . P . =((0 . 530 − Δ T)/(0 . 530))× ( 100 − SNF )

Where WT is the freezing point of sample

F.P = Freezing point

SNF = % Solids- not-fat of milk sample

(0.530 0 is the freezing point depression of genuine milk)

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