The most constant property of milk is typically recognized as its freezing point value1. Freezing point is affected by solutes in its solvent. Water without solutes will freeze at zero degrees C. The presence of any solutes will depress freezing point below zero degrees C. The freezing point of milk depends upon the concentration of water-soluble components. As milk is more diluted, the freezing point will raise closer to zero.
Lactose and milk salts are the key components dissolved in the milk that affect the freezing point2. Changes in the composition of milk, including levels of fat, protein, and total solids, do not cause significant changes in freezing point. While milk components are dependent upon natural deviations determined by the cow's breed, feeding, age, region, season, and other environmental factors, the freezing point value is highly preserved even when collected from a larger population of animals and pooled before further processing3.
Historic and Regulatory Perspectives
The freezing point value is considered to be the best single index of the presence of added water in milk and has been the basis of an official method since 19234. Thus the dairy industry and regulatory bodies have been using freezing point determination to detect the presence of added water in raw and pasteurized milk for almost a century5. Any added water constitutes adulteration of milk and regulatory agencies have established freezing point standards for “water-free” milk (that is, no added water). These limits are based on data of authentic milk from different regions or countries, and are designed for whole-herd, bulk-tank samples or processed milk samples, and not for samples from individual cows or individual quarters. The freezing points of milk are controlled at different levels depending on national regulations in order to prevent the addition water to milk either on the farm or during processing6.
Why Test Milk for Added Water?
Why would someone want to measure milk’s freezing point? Producers have been known to intentionally add water to their bulk tank to increase the volume of milk they ship. They are paid, in part, on the total mass of milk they ship to the processing plant. Water may also be introduced accidentally into the milk whenever milk processing takes place. This can occur because of the presence of water in the milking machine pipes, bulk tanks, or any other dilution with water at the milk producer’s site.
Potential Impact on Quality Premiums
Added water is illegal and results in an economic loss to the dairy industry. In addition to the regulatory mandates, precautions should be taken and more programs implemented to prevent accidental or intentional entrance of added water in milk. Contamination of milk with water may lead to off-flavor and quality problems as water supplies may contain bacteria, chemicals, and minerals. Many plants have incorporated the freezing point value into the quality measurements used for quality premium programs in an attempt to reduce incidental added water in milk which can increase milk procurement costs, reduce product yields, and increase processing costs.
There are many reasons why the addition of water to milk must be monitored. In addition to the legal requirements involving composition and regulations, there are practical reasons relative to the shelf-life and flavor of dairy products.
Standard Method More Accurate
The thermistor cryoscope method, as approved and standardized under ISO/IDF (5764), is the most common method used to measure the freezing point of milk7. Its principle of operation is as follows: 1) a sample is supercooled below its freezing point whereupon a vibrating wire induces crystallization; 2) crystallization causes a rise in temperature within the sample, and 3) the freezing point reading is taken when the temperature of the sample plateaus for typically 20 seconds. Prior methods and instruments have been mostly phased out due to the improved accuracy and precision demonstrated with the thermistor cryoscope.
1 Hennisgson, R.W. 1969. Thermistor cryoscopic determination of the freezing point value of milk produced in North America. J. Assoc. Offic. Anal. Chem. 52: 142-151.
2 Wendorff, W.L., Kaiser R.M., Bradley, R.L. 1992. UW Dairy Alert, Technical Update, University of Wisconsin-Madison.
3 Buttel,B., Fuchs,M., Holz, B. 2008. Freezing point osmometry of milk to determine the additional water content- an issue in general quality control and German food reulgation. J. Chem. Cent. 2:6.
4 Hortvet, J. 1923. J. Assoc. Offic. Anal. Chem. 6: 422.
5 Jackson, L.C., Rothera, A.C.H. 1913. Milk – Its Milk Sugar, Conductivity and Depression of Freezing Point. pp. 1–27.
6 Rohm, H. 1993. Determination of the freezing point of milk-an analytical myth? Z Lebensm Unters Forsch. 197: 558-561.
7 Wehr, M. ed. 2004. Standard Methods for the Examination of Dairy Products. 17th ed. Am. Public Health Assoc., Inc., Washington, DC.