Melting Milk

Most recent answer: 10/22/2007

Q:
I read your question about ice melting faster in water than milk. I tried freezing milk cubes and water cubes, and putting them each in a glass and the milk cubes melted faster than water cubes. Is this becuase milk has fat and is more dense? The milk cubes were sticky and looked very dense and seemed to melt in my hand. The ice cubes were clear. Did I do something wrong?
- Andrew (age 8)
Chesterbrook Elementary, West Chester, Pa
A:
Andrew -

No, you didn’t do anything wrong. That’s just what frozen milk looks like. You’re right that milk is more dense than water, but this is not why it melts more quickly, although it is indirectly related. The reason that milk is dense is because it contains a lot of impurities - raw milk is about 3.7% fat and 3.2% protein (for Holstein cows; other breeds actually have more). I think that this is also the reason that frozen milk melts faster. Here is our theory:

In solid ice, the water molecules are bound together tightly, in a regular pattern. This regular pattern is part of what makes the chemical bonds holding the molecules together so strong. But if you add a lot of fat or proteins, the ice that forms when the water freezes will contain pockets of these impurities. When you take the milky ice-cube out of the freezer, you noticed that it was sticky. That was probably due to the fact that these pockets of impurities were not really frozen. They probably also help the ice melt since they provide lots of nucleation centers for the melting to start.

-Tamara & Mike

(published on 10/22/2007)