MILK IN COOKING
Since milk can be used in many different dishes, it would be good to know a thing or two on how to use and not to use milk in cooking. Normally, milk is used as a liquid in doughs or batters because it provides better taste and body than water would do. It should be remembered though that milk is not as simple as water, and its myriad of nutritional molecules is affected by other ingredients that we use in our dishes. We already mentioned acids that coagulate the milk casein proteins. Acids can be found in many foods, even those that we never consider to be acid (e.g. coffee and some vegetables). Also, tannins can contribute to the coagulating effect, and they can be found in tea, coffee, or potatoes. To avoid coagulation/curdling, it is best to use milk that is fresh (less sour) and be careful with the heating temperature (higher temperatures promote curdling of the milk proteins).
Milk proteins can also curdle when heated, although they may look differently. For example, if we heat milk for a certain period of time, a thin layer of coagulated proteins, calcium and fat will appear floating on the surface of the liquid. This is due to evaporation of water, prominently at the surface of the milk, which creates a concentrated protein-calcium-fat cocktail that will easily aggregate into a skin. To circumvent the problem, the pan can be covered with a lid, so the evaporation is partially prevented. Another trick is to whisk the milk a little so a protective foam forms on its surface.
Evaporation is not the only problem though. We know that excessive heat will have a similar effect at the bottom of the pan, where proteins also become concentrated and start to adhere to the pan, which is accompanied by burned appearance and taste. A trick to avoid this is to coat the pan with a thick layer of water and let it heat up before adding milk. Of course, we should not use excessive heat either if we don't have to! Another good way to prevent burning of milk proteins (also in other milk products) is to use a double boiler, where we have a pan of boiling water and in it we put another pan that contains our milk. In this way, the boiling steam will heat up another pan, but it will not give us as high temperature as we would get from simply putting a pan on the heat plate.
Milk proteins can also be formed into a foam, as we can see on an espresso coffee. Milk foam is simply air bubbles trapped within a network of milk proteins. However, creation of milk foam may not be that simple, since it is far more fragile than foams made of egg whites or heavy cream. This is due to the low protein content of milk (about 3%) and no stabilizing function of fat like in a whipped cream. One trick to use when creating a milk foam is to heat up the milk (preferably fresh, not sour, so to prevent curdling!) and then whisk it. Some proteins will denature in heat and aggregate into networks in-between the air bubbles provided by the whisking force.