19 December 2005

Soup Skins

Matt in Phoenix asked why "skin" forms on the top of soup and cocoa. I am not sure why that forms, although my guess it has something to do with the warmer liquid hitting the cooler air and congealing. Maybe someone reading this weblog could shed some light on it.


Daphne said...

Why do they call those teensy-weensy candy-bars you get at Halloween "Fun Size"?
That isn't a fun size. It's a downsize.
A 2-pound bar--THAT would be "fun-size"!
Why do they do that?

Monkey David said...

Well, for food questions such as this the first reference would be Harold McGee's amazing food science book "On Food and Cooking: The Science and Lore of the Kitchen." Alas, my copy is missing. But wait! Barnes and Noble has the whole first chapter!

"Whether fluid milk is used to make a soup or a sauce, scalloped potatoes or hot chocolate, the tendency of its proteins to coagulate can cause problems. The skin that forms on the surface of boiled milk or cream soups is a complex of casein and calcium, and results from evaporation of water at the surface and the subsequent concentration of protein there. If you skim off the skin, you remove significant amounts of valuable nutrients. Skin formation can be minimized by covering the pan or whipping up a little foam; both actions slow evaporation. Milk scorches easily because relatively dense complexes of casein micelles and whey proteins fall to the pan bottom, stick, and burn. A moderate flame or a double boiler is the best safeguard against scorching. Large foreign molecules -- starch, vegetable or fruit cellulose and hemicelluloses, sugars, fats -- can also cause some general coagulation, and so a curdled appearance, by providing sites at which the milk proteins can gather and begin to clump, And acid in the juices of all fruits and vegetables, and phenolic compounds in such things as potatoes and tea, make the milk proteins more sensitive to coagulation and curdling at cooking temperatures. Fresh milk and careful control of the burner are the best weapons against curdling."

Christopher said...

Monkey David: Thank you for the concise answer. I always wondered why milk was heated on a double boiler.