INSIDE THE MILK
Even though milk looks white and uninteresting, it contains a mixture of components that are interesting per se and they all contribute to the milk's properties. First of all, we have the fat that is dispersed in small floating globules (small sphere-like objects) that all together are opaque and therefore do not allow the light to go through the milk. For this reason, the milk is white. Fat also contributes to the body of milk, and carries the life-important fat-soluble vitamins A, D, E, and K. Of special importance in cooking are the fat globules themselves. They are tiny fat balls, coated with special kinds of fatty acids and proteins that repel other similar fat globules and make sure they are dispersed in milk (and not clog together). However, if we let fresh, unhomogenized milk stand for some hours in cold, these fat globules will float to the surface and accumulate there, since they are lighter than water. This phenomenon is called creaming, and this fatty fraction of milk is used to produce cream and butter. Nowadays, thanks to technical advantages, we can use centrifuges to speed up the creaming process. So what has all this to do with cooking? Some milk proteins start to stick to the surface of the fat globules as we heat up the milk. As a result, the fat globules become more stable and heat-tolerant, which is why we can cook away much of the water in milk without making the fat "fall out". On the other hand, the very same fat globules can be easily broken by frozen water crystals, resulting in fat sippers. So, don't freeze and then heat up the milk, or you'll end up with a whitish substance with creamish pools of fat on top.
Milk proteins constitute another major part of milk biology. They can be divided into two classes - whey proteins and caseins - and they can be separated by acidic foods. If you add acid (lemon juice, vinegar) to your milk, the caseins will aggregate together and form a curd, which is the very substance used in production of many cheeses. Whey proteins on the other hand will remain soluble in the liquid fraction of milk. If acid can separate our proteins, what influence has the heat? Apart from lots of foods, caseins and whey proteins are very heat-tolerant, at least when it comes to coagulation. They don't precipitate when heated, unless we have some acid in our milk. This is another reason that we can reduce the milk's water content simply by boiling it, and without spoiling it.
Milk sugar is a very uncommon one among foods. Lactose, a disaccharide made of glucose and galactose, provides the sweet taste in milk. Being a slight nightmare to some of the people (as we discussed on another page), the fact that it's an uncommon sugar makes milk less prone to bacterial infestations. Not only that, we can use special lactose-digesting bacteria to make certain milk products, including yoghurt and other fermented milks.
Besides these three major components (fat, proteins, sugar) milk also contains lots of calcium, some vitamins and minerals. Therefore, the nutritional value of milk is high. However, we should not focus too much on milk products, as our food needs to be varied and give us a little from each kind of food.
Here is a short fact sheet on what cow milk contains:
87.3 % water
3.9 % milk fat
8.8 % solids-not fat
3.25 % protein ( ¾ casein)
4.6 % lactose
0.65 % minerals (Ca, P, citrate, Mg, K, Na, Zn, Cl, Fe, Cu, sulfate, bicarbonate)
0.18 % acids (citrate, formate, acetate, lactate, oxalate)
enzymes, gases, vitamins