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Milk skin or lactoderm refers to a sticky film of protein that forms on top of dairy milk and foods containing dairy milk (such as hot chocolate and some soups). Milk film can be produced both through conventional boiling and by microwaving the liquid, and as such can often be observed when heating milk for use in drinks such as drinking chocolate. It is caused by the denaturation of proteins such as beta-lactoglobulin (whey protein).
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
Milk skin is typically known as "śōr" (Bengali: সর) in Bangladesh. It is used to make tea and milk dishes rich in taste and consistency.
Milk skin is called "nǎi pí”(奶皮) in Chinese. Considered to be of high nutritional value, Milk skin is often made into various desserts and delicacies. Milk skin is also favoured in regions such as Inner Mongolia and Tibet, as milk products are of more importance in regular diet. Double skin milk (雙皮奶) is a Cantonese dessert developed in Shunde that has milk skin on the surface.
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 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 West Bengal the upper part of milk is called Sor(দুধের সর), a Bengali word. Sarpuria and Sarbhaja are the two sweets of Krishnanagar, West Bengal. It is also referred to as "Aadai" in Tamil, Tamil Nadu, there are various other regional references to milk skin too, like “saay” (साय) in Marathi, "malai" in Hindi, "paada" in Malayalam, "thari" in Ahirani, "meegada" in Telugu. "kene" (Pron: K-nay) in Kannada, "Baave" (pron: Baa-way) in Tulu (Coastal Karnataka).In West Bengal, it is known as "Sor" 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 Japan, a dairy product called "so" was made from layers of milk skin during the 7th-10th centuries. So was further processed to make "daigo". The dairy usage in Japan dwindled during the Heian era, and the technique was later adopted to produce yuba from soy milk.
In Nepal this skin is referred to as turr or chhali and many people enjoy consuming the skin along with the milk. Some people prefer to use in making curd for it produces higher amount of "nauni" (butter) while stirred with the help of "madani".
The milk skin is referred to as "penka" ("little foam") in Russian, and is infamous as the least favorite thing among children, but is used as an ingredient in some haute cuisine dishes like Guriev porridge. The skin that forms on baked milk is higher regarded and is preferred for those dishes.
In Cyprus, milk skin in called tsippa and is used as a filling for a pastry called tsippopitta (literally "milk-skin pie").
In Pakistan, milk skin is called "ملائی."
Spain, Portugal, Latin America
In many countries with Portuguese or Spanish ancestry, "milk skin" is translated as "nata", and lends itself to many baked goods and foodstuffs.