This is a great question!
The exact freezing point of milk (also called the melting point) varies slightly according to the individual cow, the breed, the time of day / season that the milk is collected, the type of feed that the cow receives, etc. According to this very nice article
, the majority of cows produce milk with a natural freezing point of -0.5250 to -0.5650 °C, with an average of about -0.5400 C. When you think about all those factors I just listed, tho, this may seem like a surprisingly narrow range. And it is remarkably narrow. The reason is this...
Milk is produced by a part of the cow called the mammary gland. (All mammalian species use mammary glands to produce milk.) One of the ways that the mammary gland works is to produce milk with the same overall "osmotic pressure" as the animal’s blood has.* Osmotic pressure is basically a measure of how much stuff is dissolved in a liquid, which determines the liquid’s freezing point. Because of the important role of blood in the body, the body works very hard to keep its osmotic pressure at an ideal level, and that doesn’t vary much from animal to animal - all cows have basically the same physiological requirements for their blood.
An interesting way that the freezing point of milk is used is to determine if milk is watered down - either intentionally by the producer or naturally as a result of the cow being unhealthy. The freezing point of pure water is exactly 0 °C. The more stuff you dissolve in the water, the lower the freezing point gets - this is called Freezing Point Depression. Milk that has been watered down contains more water and less solutes, so its freezing point is closer to 0 °C. Most milk processors will conclude that milk has been watered down if the freezing point is anywhere above -0.250 °C.
* Water flows naturally to establish equilibrium of osmotic pressure. That means that if two chambers are connected so that water can flow between them but solutes can’t, the water will naturally move so the osmotic pressure is the same on both sides. In a mammal, the blood vessels and the mammary glands are the two chambers - water can flow back and forth between them, but (most of) the stuff dissolved in the blood/milk can not. The water in milk comes directly from the cow’s blood.
(republished on 07/25/06)