|MadSci Network: Chemistry|
Milk is a colloidal mixture of water, lipids, carbohydrates and proteins. When you heat this mixture, evaporation goes on and the upper layer of milk gets supersaturated, high temperature denatures milk proteins and the colloid collapses. The "film" hence is a layer of clotted proteins and long chain carbohydrates, interpenetrated and fused by milk fats. (Consider the fact that almost all proteins have hydrophobic inner core which in normal state is not exposed to the solvent. When protein denatures, the hydrophobic core gets exposed and fats penetrate it, completing the process of "film" formation).
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