when milk is heated, water eveporate at the sorface of the milk causing erreversible protein denaturation and awhite cream layer is formed and it is called as Ramasdan effect

When I meant lactin, I was meaning lactose. Anyways, I'd got my answer. Another related question - When we boil Rice we get a similar thick layer. Rice endosperm being mostly constituted by sugars and proteinaceous part mostly present in the germ, do you think this small part is responsible for the thick layer?

As for rice, the starch in it gels up on cooling to form the thick layer on top.

Hey but i heard that fat also accumulates in the top layer when milk cools. Is this true?

The cream is what sits on top of heated and cooled milk. Its a mixture protein and fat globules.

In other words, starch gelatinization.

In case of milk .. it bring me to the third question, why does fat accumulate at the top when we heat the milk - is this primarily because of the tendency of misfolded protein to attract and gel with the hydrophobic substances in the vicinity..?

No it's the fat! You get the cream acumulating on the top :)

So its just the cream.. no involvement of protein..?!!! But why does this cream accumulates on heating alone..!

when milk is heated, water eveporate at the sorface of the milk causing erreversible protein denaturation and awhite cream layer is formed and it is called as Ramasdan effect

Adding to what Suneel said...I also would like to add that any kind of fat / oil floats on water which is why the protein and fat end up together :)

Hmm that apparently makes the most perfect sense. Thanks Suneel.

Found a lucid illustration of the creaming effect, thought of sharing http://www.answers.com/topic/dairy-product-2

Also wasn't able to reference anything to Ramasdan effect. Can you provide any links on tat.

Had another doubt, protein denaturation causes protein aggregation which will make it heavier and should rather sink or settle down rgt, how does this stay along with the fat creamy layer, is the density of this creamy layer much less than than that of water or rest of the whole milk.

Interesting question ... what you are seeing here is I suppose focculation of protein and fats and this is the reason for it having lower density than rest of the milk

when the enzyme chymosin is added to milk, the protein part will coagulate and settle down leaving out the rest of the milk. What we observe from here is that the protein part will sink, but in the creaming effect the protein part aggregates/ flocculate at top why? How do we differentiate both scenarios having the same substrate at hand.

well lets see it this way you have milk which have both protein and fat content...

The fat is present in form of globules. Before heating (depending on source of milk) most fat globules have heavy load of protein and hence only cream layer containing most of the fats is formed with some little amount of fat globules loaded with less casein or few heavy fat globules with some casein.

When you heat treat the milk this causes casein and fat globules to aggregate. This forms heavy fat globules loaded with aggregated casein micelles molecules and hence now you get a cream containing fat and good amount casein

The casein protein consist of 2 parts - chymosin cleaves at position 105-106 seprating these 2 parts ... hence what you will see sendimenting is the acidic hydrophillic peptide part of the casein as it can no longer form micelles the hydrophobic peptide part also aggregate together or is dispersed in fat phase.

If collect this surface and try to melt it using a very low flame , you will get pure fat and a sediment of proteins. This I think indicates the nature of this top surface. Incedintaly we call the fat collected by this method in my part of the world SAMNA and the protein part MORTA which is realy delecious

Also to add to all of your above comments, fat has a lower specific gravity hence it floats compared to the milk serum.

Thank you all for sharing your thoughts and inputs, I have a fairly good understanding on this topic now.