|
Sci Tech
Creamy layer
Why does a layer of cream appear on hot milk kept for a few minutes whereas it does not appear on cold milk or lukewarm milk?
Srinidhi Joshi
Bidar, Karnataka
Milk, as has been described many times on this forum, is basically an intimate mixture of certain proteins (mainly casein), carbohydrates (mainly lactose sugar), lipids, phospholipids, etc among others suspended in a large pool of water.
Some inorganic salts (mainly calcium), vitamins and cholesterol are also present at low concentrations. The actual composition of the ingredients depends mainly on the mammalian species.
The size of the suspended entities is at micron scale and many of these particles have electrical charges on them and thus get mutually repelled from one another while avoiding coagulation. Such a system of mixture wherein the constituents are stabilized at microscopic sizes and suspended uniformly in a large volume of another matrix, is called a ‘colloid’.
Depending on the natural state of the guest (suspended particles), called the ‘disperse phase’ and the host, called the ‘disperse medium’ (in the case of milk, it is water), we have many names for the colloids. On this basis milk is primarily an ‘emulsion’ (liquid guest in liquid host). Colloidal particles of fat are known as lipids.
Cream in milk is chemically a fat. Fat consists of thread-like fatty acid molecules each of which possesses a long hydrophobic (water-hating) hydrocarbon tail at the tip of which there is a hydrophilic (water-loving) carboxylic group.
Forced by their hydrophobicity and assisted by mutual attraction among themselves, the tails of the fatty acid molecules converge inward and away from water pool while due to mutual electrostatic repulsion and their hydrophilicity, the carboxylic ends face outward into the water pool.
Thus, a host of tiny spherical shaped colonies of colloidal particles of fat, called lipid micelles are distributed uniformly throughout the bulk of the milk.
Adjacent micelles also repel one another and do not coagulate because of identical charge on their surfaces. This is the situation of the milk when it is cold or lukewarm.
When the milk is heated to near-boiling, the fatty acid molecular tails get thermally agitated and become conformationally dislocated from their micellar orderly arrangement and lose their colloidal characteristics.
The fatty acid molecules get coagulated into a mass of ‘cream’. Since cream’s density is less than water’s, it floats over milk, spreading like a layer with the polar carboxylic acid ends sunk into the milk’s water pool.
Prof. A. Ramachandraiah
Department of Chemistry, National Institute of Technology, Warangal, Andhra Pradesh
Printer friendly
page
Send this article to Friends by
E-Mail
Sci Tech
|
