Science Questions

Why does milk change colour when it freezes?

My son Robert has been wondering about this question: when we freeze milk it changes colour – from white to yellow. Why is this? Paul Dart

The reason for this is that milk isn't just a straightforward solution, it's not like dissolving salt in water.  Milk is what's called an emulsion.  It's a mixture of various things which are suspended in water.  Milk is about 5% fat and that fat is dispersed through the water as tiny globules.  They're cells and surrounded by little proteins.  They have a fat-loving part of the protein and a water-loving end.  The fat loving part of the protein sticks into the fat and the water-loving bit sticks into the water.  This helps to solubalise or keep suspended the fat molecules.  Also in the milk are these things called pepsinogens or proteins because milk contains a lot of protein.

The idea of milk is to nourish a growing animal and protein is what you need to grow more muscle and get bigger.  When you put your milk in the freezer all these things which are normally well-suspended and kept separate begin to become very close together.  The freezing process means that the water in the milk, 95% forms big chunky ice crystals and they don't want to have the proteins and the fat in them.  The proteins and the fat get squeezed out of the mixture.  They tend to form around this central core of ice.  So because you see all the fat in one place it looks yellow.  If you look where butter comes from, butter comes from milk and it's yellow.  That's where the yellow colour comes from.  It's all conglomerated in one place and you can actually, physically see it.

March 2008