The freezing point of a pure water (0°C) depends on the structure and chemical properties of its molecules. Adding solutes to water will depress the freezing point of the solution. The freezing point of milk is lower than that of pure water due to the presence of milk solids.
Milk has a constant proportion of milk solutes hence will have a definite freezing point. Research has shown that milk freezes at temperatures between −0.53 and −0.56 °C. A deviation from this range indicates adulteration and explain why freezing point of milk gives accurate results for adulteration.
This is because the physiological conditions of a lactating mammal works to keep the osmotic pressure of blood and milk equal.
Milk adulteration plagues the dairy industry. It is the devaluation of milk quality by either addition of inferior/unacceptable product or removal of a valuable ingredient.
Is milk adulteration intentional or incidental?
Adulteration can be either intentional or incidental. Intentional adulteration happens when an individual decides to either remove content or add content to milk to achieve a certain goal. Incidental adulteration can happen at any point in the dairy value chain and may go unnoticed.
An adulterant is any extraneous matter that will make the food product unsafe or sub-standard.
Adulteration of milk by water addition will raise the temperature of milk towards the freezing point of water. Addition of solutes in milk will depress the freezing point of milk further.
Why would someone add water to milk?
Simple. Many producers have been shown to boost their milk volumes by dosing with water. This is partly due to the fact that they get paid by the mass of milk delivered.
Factors affecting the freezing point of milk
It is important to determine the freezing point of milk since it is easy to detect adulteration my water addition.
Freezing occurs when water and ice are in an equilibrium. This is due to constant vapor pressure between the two forms of water.
Here are the factors affecting the freezing point of milk:
1. Genetic predisposition
As we have already noted, the freezing point of milk vary due to some inherent factors. You will notice that milk from high producers tend to have a higher freezing point than that from low producers. This difference is attributed to the difference in solute concentrations between the two milk samples.
Low producers tend to produce milk with high concentrations of sugars, minerals, butter fat, and proteins. You can read the research findings here.
2. Stage of lactation
Milk composition varies at different stages of lactation. During early lactation, milk sugars are high in the colostrum milk while salts dominate in the late lactation.
Solutes present in the milk will disrupt the equilibrium and delay vapor escape from the surface of a freezing body. This will lower the freezing point of the milk sample.
3. Freshness of the milk sample
The osmotic pressure difference of the medium will also affect the freezing point of milk. Higher solute concentration will lead to a higher osmotic pressure, which will lower the freezing point of the sample.
Souring of milk leads to breakdown of lactose, which increases the number of solutes in the milk hence further depression of the freezing point.
4. Health of the lactating cow
As we have already noted, the osmotic pressure of a healthy cow is constant between the blood and the milk. Any deviation in the health of the animal will lead to disruption of the osmotic pressure equilibrium.
As a result, solutes will leak into the milk leading to higher solute concentration, which increases freezing point depression (lowers the freezing point further).
Mastitis plagues a large majority of dairy farms. Research has shown that mastitis will increase the freezing point depression of milk due to destabilization of milk solutes.
Watch the video explaining how the OptiFZP freezing point analyzer works. You can use this digital analyzer to determine the freezing point of milk.
Skimming of milk destabilizes solute balance hence will affect the freezing point.
Methods for determining the freezing point of milk.
1. Hortvet Cryoscope method
Uses the Hortvet Cryoscope to determine the freezing point of milk. One of the earliest methods developed in the early 920s.
Please note that the machine is quite taxing to use efficiently and it does not guarantee reproducibility of results.
The Horvet cryoscope uses an ordinary freezing point thermometer.
2. Thermistor Cryoscope method
This cryoscope has special probes that measures the changes in temperature as well as electrical resistance.
The Association of Official Analytical Chemists (AOAC) recommends the Thermistor cryoscope method for determination of milk freezing point depression.
It is also called Fiske Cryoscope or Advanced Milk Cryoscope.
You can get a complete Thermistor Cryoscope manual here.
How to calculate the percentage of added water by using the cryoscope.
By determining the freezing point of milk, it is possible to find out the amount of water of adulteration. This relies on the convention that pure unadulterated milk has a freezing point depression of 0.54°C.
The term depression is used to denote the negative sign (-). That is why we indicate the value in absolute figures.
Use the formula below to get the percentage of added water
Added water =((0.54 − ΔT)/(0.54)) × (100 − SNF)
ΔT is the deviation in temperature
SNF = % Solids-not-fat of milk sample
Further reading about the freezing point of milk
Henno, M., Ots, M., Jo˜udu, I., Kaart, T., & Ka¨rt, O. (2008). Factors affecting the freezing point stability of milk from individual cows. International Dairy Journal, 18(2), 210-215. doi:10.1016/j.idairyj.2007.08.006
J.O.L.King. (1969). The Effect of Mastitis on the Freezing-Point of Cows’ Milk. British Veterinary Journal, 125(1), 25-30. doi:https://doi.org/10.1016/S0007-1935(17)49160-3
Radewonuk, E., Strolle, E., & Jr., J. C. (1983). Freezing Points of Skim Milk Concentrates. 66(10), 2061-69. doi:http://dx.doi.org/10.3168/jds.S0022-0302(83)82051-7
Zagorska, J., & Ciprovica, I. (2013). Evaluation of Factors Affecting Freezing Point of Milk. International Journal of Nutrition and Food Engineering, 7(2), 106-111.