Q. What do you mean by Milk Foams?
The proteins and water in milk are extended into thin films by agitation. These thin films enclose small air bubbles to make foam in which the protein and water provide the continuous network of the colloidal dispersion, and the air is the discontinuous or dispersed phase. This arrangement is possible because the native proteins in milk have a low surface tension and low vapour pressure. The low surface tension makes it possible to spread the liquid proteins into thin films, and the low vapour pressure reduces the likelihood that evaporation will occur.
In fluid milks, the concentration of protein is too low to permit the production of foam with any stability. However, evaporated milk can be whipped into foam with a very large volume. The increased protein and fat concentrations of undiluted evaporated milk make it possible for the foam to form, and the foam will even have some limited stability. Foam formation and stability are enhanced if the undiluted evaporated milk is chilled until ice crystals start to form in it. This condition causes the fat to be rather firm, which concentrates the protein in the remaining unfrozen water and also helps to give some rigidity to the cell walls in the foam.
Stability can be achieved by adding lemon juice because the acid promotes precipitation of the milk proteins to give more strength to the cell walls.