Brandon- This will be a nice project to figure out by experiments. Itís easier to figure out from general principles what the fat will do to the temperature
at which the milk starts to freeze. The answer (I predict) will be: not much at all. The reason is that the fat really isnít dissolved as separate molecules in the milk, but is found instead as droplets, each containing many molecules. So the total number of droplets is much less than the total number of molecules of different solutes, and thatís mainly what counts in lowering the freezing temperature. We discuss that in other answers.
I bet you will find variations in the freezing temperature, but you will have to measure carefully. The main factor will be how much salts and sugars (there are different types) are dissolved in the milk. Incidentally, because these molecules are dissolved in the milk, the freezing temperature drops as some ice forms because they become more concentrated. So thereís a range of temperatures over which the ice forms, not just one sharp freezing temperature.
My guess is that the freezing times wonít be much different, but sometimes there are tricky ways that things can affect rates, so you'd have to experiment to find out.
(published on 10/22/2007)