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Milk skin or lactoderm refers to a sticky film of protein that forms on top of milk and milk-containing liquids (such as hot chocolate and some soups). Milk film can be produced both through conventional boiling and by microwaving the liquid. It is caused by the denaturation of proteins such as beta-lactoglobulin (whey protein). Contrary to some sources, casein, which is the main constituent in cow's milk, is not thermolabile (does not react to heat) and is stable at as high as 100 °C.
When milk is boiled, soluble milk proteins are denatured and then coagulate with milk's fat and form a sticky film across the top of the liquid, which then dries by evaporation. The layer does not need to be discarded and can be consumed, as protein's nutritional value is unaffected by the denaturation process. Milk film is often considered to be desirable and is used in several recipes for various foods.
In various cultures
In France, a type of rice pudding called teurgoule employs an extreme version of lactoderm where the milk-containing dish is left to cook for many hours.
In Iran it is called "Sarshir" [فارسی:سرشیر] literally meaning "top of the milk". It is used as a breakfast dish, usually mixed with honey or jam and spread on flat bread.
In some regions of India it is referred to as "paaladai" (பால் ஆடை) in Tamil Nadu, "kene" (ಕೆನೆ) in Kannada, "malai" in West Bengal, it is known as "shawr" and often spread on slices of bread as a substitute for butter. However, "malai" actually means cream. The milk skin is sometimes confused with the layer of cream which rises to the top of whole, untoned, unhomogenised milk as it cools.
In Nepal this skin is referred to as turr and many people enjoy consuming the skin along with the milk.
In Cyprus, milk skin in called tsippa and is used as a filling for a pastry called tsippopitta (literally "milk-skin pie").
- Cantani, Arnaldo. Allergy, Asthma and Immunology. Google Books: Springer. p. 615.