Milk Notes

 The role of milk in nature is to nourish and provide immunological protection for the mammalian young.  Milk is also a complex food with over 100 000 different molecular species found.

Composition of milk:
    87.3 % water        
    3.9 % milk fat
    8.8 % solids-not fat
    3.25 % protein ( ¾ casein)
    4.6 % lactose
    0.65 % minerals (Ca, P, citrate,  Mg, K, Na, Zn, Cl, Fe, Cu, sulfate, bicarbonate)
    0.18 % acids (citrate, formate, acetate, lactate, oxalate)
    enzymes, gases, vitamins

 Milk Fractions:
     Plasma (skim milk or milk minus fat)
    Serum (plasma minus whey [casein micelles])
    SNF (solids-not fat [proteins, lactose, minerals, acids, enzymes, vitamins])
    Total Milk Solids (fat plus SNF)

 Milk can be described as:

§         an oil-in-water emulsion (fat globules dispersed in continuous serum phase0

§         a colloid suspension

§         a solution of lactose, soluble proteins, minerals, vitamins, other components

 Milk Fat:

 The fat content of milk is of economic importance because milk is sold on the basis of fat.  The main milk lipids are triglycerides which are comprised of a glycerol backbone binding up to three different fatty acids.  The fatty acids are composed of a hydrocarbon chain and a carboxyl group.  Saturated fatty acids (no double bonds) make up two thirds of milk fatty acids. 

 Physical Properties of milkfat:

 At room temperature, the lipids are solids, therefore, are correctly referred to as “fat” as opposed to “oil” which is liquid at room temperature.  The final melting point of milkfat is 37° C.  This temperature is significant because 37° C is the body temperature of the cow and the milk would need to liquid at this temperature.  Milk fat provides lubrication.  They impart a creamy mouth feel. The spreadable range of butter fat is 16-24° C.  Unfortunately butter is not spreadable at refrigeration temperatures.  Milk fat provides energy (1 g = 9 calories) and nutrients (essential fatty acids and fat soluble vitamins).

   Milk Proteins:

 The primary structure of proteins consists of a polypeptide chain of amino acids residues joined together by peptide linkages.  Amino acids contain both a weakly basic amino group, and a weakly acid carboxyl group both connected to a hydrocarbon chain.

 The nitrogen content of milk is distributed among caseins (76 %), whey proteins (18 %), and non-protein nitrogen NPN (6 %).

 Precipitation at pH 4.6 – separates caseins from whey nitrogen

Precipitation with sodium acetate and acetic acid (pH 5.0) – separates total proteins from whey NPN

 The casein content of milk represents about 80 % of milk proteins.    The distinguishing property of all caseins is their low solubility at pH 4.6.

 Most, but not all, of the casein proteins exist in a colloidal particle, known as the casein micelle.  Its biological function is to carry large amounts of highly insoluble CaP to mammalian young in liquid form and to form a clot in the stomach for more efficient nutrition.  Colloidal calcium phosphate acts as a cement between hundreds or even thousands of submicelles that form the casein micelle.  Binding may be covalent or electrostatic.

 Lowering the pH leads to dissolution of calcium phosphate until, at pH of 4.6, all phosphate is dissolved and the caseins precipitate.  Acidification causes the casein micelles to destabilize  or aggregate by decreasing their electric charges.  At the same time, the acidity of the medium increases the solubility of minerals so that organic calcium and phosphorus contained in the micelle gradually become souble in the aqueous phase.  Casein micelles disintegrate and casein precipitates.  Aggregation occurs as a result of entropically driven hydrophobic interactions.

 Heat treatment causes the whey proteins to become adsorbed, altering the behavior of the micelle.  At temperatures above the boiling point casein micelles will irreversibly aggregate.  On heating, the buffer capacity of milk salts change, carbon dioxide is released, organic acids are produced, and tricalcium phosphate and casein phosphate may be precipitated with the release of hydrogen ions.

 Whey Proteins

 The proteins appearing in the supernatant (filtrate) of milk after precipitation at pH 4.6 are collectively called whey proteins.  These globular proteins are more water soluble than caseins and subject to heat denaturation.

 Enzymes

 Enzymes are a group of proteins that have the ability to catalyze chemical reactions and the speed of such reactions.  Some people suffer from lactose intolerance; they lack the lactase enzyme, hence they cannot digest lactose, or dairy products containing lactose.

 Vitamins

 Milk includes fat soluble vitamins A, D, E, and K as well as several water soluble vitamins (B1, B2, B6, B12)

 Minerals

 All 22 minerals considered to be essential to the human diet are present in milk. 

 Density

 Density ranges from 1.027 to 1.033 kg/m³ at 20° C.

 Viscosity

 Milk and skim milk, excepting cooled raw milk, exhibit Newtonian behavior, in which the viscosity is independent of the rate of shear.  The viscosity depends on temperature (temperatures above 65° C increase viscosity due to the denaturation of whey proteins) and pH.   Cooled raw milk and cream exhibit non-Newtonian behavior in which the viscosity is dependent on the shear rate.

 Optical Properties

 Light scattering by fat globules and casein micelles causes milk to appear turbid and opaque.  Light scattering occurs when the wavelength of light is near the same magnitude as the particle.  Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light (blue) more than the red.

Information from the University of Guelph Dairy Chemistry and Physics Areas

http://www.foodsci.uoguelph.ca/dairyedu/chem.html