Question:
After milk is boiled why why does a layer of fat/cream form on top of it.?
amit
2007-04-05 06:20:46 UTC
After milk is boiled why why does a layer of fat/cream form on top of it.?
Twelve answers:
dymps
2007-04-05 06:27:53 UTC
Because when the temperature is raised the high temprature seperates the creamy part of the milk from the rest of it; just like the effect of when oil and water is mixed together.
2016-03-29 04:50:13 UTC
It's called 'the skin' and its made up of solid proteins that combine with the milk’s fat molecules, which begin to evaporate as the milk is heated. These proteins, casein and beta, clump together when the liquid reaches a temperature of around 45 to 50 degrees Celsius. As the heating continues, the soft protein layer begins to dry out, which is why the milk forms a skin on the liquid’s surface. This layer of skin forms a hard barrier, causing steam to build up beneath it and increase the liquid’s temperature. When left alone, this often causes the milk to boil over. Though milk forms a skin when heated in most cases, there are several ways to prevent this skin from forming. If you plan to heat the milk over the stove, frequent stirring will break up the protein and fat molecules, so that the membrane will not develop. If you are heating milk in a microwave, you can place a wax paper lid known as a “cartouche” on top of the container, which will slow the evaporation process and maintain the milk’s liquid form. The milk forms a skin only on heated milk that contains fat. If you are heating skim milk, there is no danger of a skin forming on top. Because skim milk contains no fat, the protein molecules have nothing to bond with, and are unable to coagulate.
J.L. S
2007-04-05 10:25:10 UTC
As the fat is lighter than other part of liquid milk. After boiling or churning the milk fat floats on the top.
2007-04-05 06:32:21 UTC
Hi i hope u r a College Boy or read Science also... but ok now ur questions answer is this .......... when we boiled the milk that time all fat material melting in Oil or liquid, which have more surface tension so its come on the upper surface on milk then it get contet with atmosphere which have low temperatur than milk so its again gain some solid formation but its lower surface in contect with hot milk so its make creamy layer ...............
thirumoorthi m
2007-04-05 06:33:42 UTC
If some chemical problem cream for on top of boiled milk.
cnsone
2007-04-06 08:27:50 UTC
Why only Milk?



When we boil any liquid the Cream or Scum will always come to top!
2007-04-05 15:53:29 UTC
Milk is an emulsion of an aqueous product (whey) and a non-aqueous product (curds). They will eventually separate, with the cream rising to the top due to it's lighter density. Heating it just speeds up the separation.
2007-04-05 06:29:42 UTC
oleo substances have the tendency to form a layer on the top any liquid medium, as they are not soluble in them. also they are light. hence, the fat/cream gets accumulated on the top
surnell
2007-04-05 06:41:20 UTC
Because cream is denser than the base, it tends to float.
satishfreeman
2007-04-06 17:55:14 UTC
So that its easy to lift it up with your fingers and eat the same easily. You can also eat the same with a little sugar.
2007-04-06 02:07:36 UTC
It is natural.

You can ask: How does a layer....
2007-04-05 08:46:09 UTC
Types of dairy products



* Milk, after optional homogenization, pasteurization, in several grades after standardization of the fat level

o Cream, the fat skimmed off the top of milk or separated by machine-centrifuges

+ Sour cream, cream that has been fermented by the bacteria Streptococcus lactis and Leuconostoc citrovorum

+ Crème fraîche, slightly fermented cream

+ Smetana, Central and Eastern European variety of sour cream

o Cultured buttermilk, fermented concentrated (water removed) milk using the same bacteria as sour cream

o Milk powder (or powdered milk), produced by removing the water from milk

+ Whole milk & buttermilk

+ Skim milk

+ Cream

+ High milk-fat & nutritional powders (for infant formulas)

+ Cultured and confectionery powders

o Condensed milk, milk which has been concentrated by evaporation, often with sugar added for longer life in an opened can

o Evaporated milk, (less concentrated than condensed) milk without added sugar

o Khoa

o Infant formula, dried milk powder with specific additives for feeding human infants



* Butter, mostly milk fat, produced by churning cream

o Buttermilk, the liquid left over after producing butter from cream, often dried as livestock food

o Ghee, clarified butter, by gentle heating of butter and removal of the solid matter

o Anhydrous milkfat



* Cheese, produced by coagulating milk, separating from whey and letting it ripen, generally with bacteria and sometimes also with certain molds

o Curds, the soft curdled part of milk (or skim milk) used to make cheese (or casein)

o Whey, the liquid drained from curds and used for further processing or as a livestock food

o Cottage cheese

o Cream cheese, produced by the addition of cream to milk and then curdled to form a rich curd or cheese made from skim milk with cream added to the curd

o Fromage frais



* Casein

o Caseinates

o Milk protein concentrates and isonates

o Whey protein concentrates and isonates

o Hydrolysates

o Mineral concentrates



* Yogurt, milk fermented by Streptococcus salivarius ssp. thermophilus and Lactobacillus delbrueckii ssp. bulgaricus sometimes with additional bacteria, such as Lactobacillus acidophilus

o Ayran

o Lassi



* Gelato, slowly frozen milk and water



* Ice cream, slowly frozen cream and emulsifying additives

o Ice milk

o Frozen custard

o Frozen yogurt, yogurt with emulsifiers that is frozen

* eggs but only when cooked

* Other

o Kumiss/Airag

o Viili

o Kajmak

o Kephir

o Filmjölk

o Piimä

o Vla

o Dulce de leche

Cream is a dairy product that is composed of the higher-butterfat layer skimmed from the top of milk before homogenization. In un-homogenized milk, over time, the lighter fat rises to the top. In the industrial production of cream this process is accelerated by using centrifuges called "separators". In many countries, cream is sold in several grades depending on total butterfat content. Cream can be dried to a powder for shipment to distant markets.



Cream produced by cows (particularly Jersey cattle) grazing on natural pasture often contains some natural carotenoid pigments derived from the plants they eat; this gives the cream a slight yellow tone, hence the name of the yellowish-white colour cream. Cream from cows fed indoors, on grain or grain-based pellets, is white.



Types of cream



In the United States, cream is usually sold as:



* Half and half (10.5–18% fat)

* Light, coffee, or table cream (18–30% fat)

* Medium cream (25% fat)

* Whipping or light whipping cream (30–36% fat)

* Heavy whipping cream (36% or more)

* Extra-heavy or manufacturer's cream (38–40% or more), generally not available at retail.



Not all grades are defined by all jurisdictions, and the exact fat content ranges vary. The above figures are based on the Code of Federal Regulations, Title 21, Part 131 [1][2] and a small sample of state regulations.



Sour cream in the U.S. is cream (18% or more milk fat) that has been subjected to a bacterial culture that produces lactic acid (0.5%+), which sours and thickens it.



Crème fraîche is a heavy cream slightly soured with bacterial culture, but not as sour or as thick as American sour cream. Mexican crema (or cream espesa) is similar to crème fraîche. Smetana is a Central and Eastern European sour cream.



In the UK, clotted cream (similar to Indian malai) is a very high-fat (55%) product processed with heat. For cooking purposes, both single and double cream can be used in cooking, although the former can separate when heated, usually if there is a high acid content. Most UK chefs always use double cream or full-fat crème fraîche when cream is added to a hot sauce, to prevent any problem with it separating or "splitting". In sweet and savoury custards such as those found in flan fillings, crème brûlées and crème caramels, both types of cream are called for in different recipes depending on how rich a result is called for. It is useful to note that double cream can also be thinned down with water to make an approximation of single cream if necessary.



Butter is made by churning cream.



[edit] Whipped cream



Cream with 30% or more fat can be turned into whipped cream by mixing it with air. The resulting colloid is roughly double the volume of the original cream as air bubbles are captured in a network of fat droplets. If, however, the whipping is continued, the fat droplets will stick together destroying the colloid and forming butter; the remaining liquid is buttermilk. Confectioner's sugar is sometimes added to the colloid in order to stiffen the mixture and to reduce the risk of overwhipping.



Whipped cream may be sold ready-to-use in pressurized containers. Nitrous oxide is used as a propellant, and when the cream leaves the nozzle, it produces four times the volume of whipped cream, i.e., twice the volume produced by whipping air into it. Using this technique, it may also be prepared in reusable dispensers, similar to a seltzer siphon bottle, using inexpensive disposable nitrous oxide cartridges. However, the whipped cream produced with nitrous oxide is unstable, and will return to a more or less liquid state within half an hour to one hour. Thus, the method is not suitable for decorating food that will not be immediately served.



Chantilly cream (French: crème Chantilly) is whipped cream with sugar and vanilla.


This content was originally posted on Y! Answers, a Q&A website that shut down in 2021.
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