Milk is the probably the most nutritionally complete food found in nature. Whole milk contains vitamins (principally thiamine, riboflavin, panthothenic acid and vitamins A, B12 and D), minerals (calcium, sodium, phosphorus, potassium, and trace minerals), proteins (which include all the essential amino acids), carbohydrates (mostly lactose), and lipids (fats). Whole milk is an oil in water emulsion, containing approximately 4% fat dispersed as very small (micron sized) globules. The fat emulsion is stabilized by complex phospholipids and proteins that are absorbed on the surface of the emulsion. Since the fat in milk is so finely dispersed it is more easily digested than fats from any other source.
Proteins and Casein
A protein is a naturally occurring, unbranched polymer in which the monomer units are amino acids. More specifically a protein is a peptide in which at least 50 amino acids residues are present. Proteins can be classified into two types: fibrous and globular. Fibrous proteins are proteins in which peptide chains are arranged in long strands or sheets. Globular proteins are proteins that tend to fold back on themselves into compact spheroidal shaped units. Globular proteins do not form intermolecular interactions between protein units and are more easily solubilized in water as colloidal suspensions than fibrous proteins are. They are "complete proteins" or "storage proteins" because they contain all the amino acids essential for building blood and tissue, and can sustain life and provide normal growth even if they are the only proteins in the diet. Milk contains three kinds of proteins: caseins, lactalbumins, and lactoglobulins, all of which are globular proteins. The main protein in milk is casein. Casein is a phosphoprotein which has phosphate groups attached to the hydroxyl groups of some of the amino acids side-chains. Casein exists in milk as a calcium salt, calcium caseinate. It is actually a mixture of three similar proteins, alpha, beta and kappa caseins which form a micelle. Alpha- and beta-casein are both insoluble in water and are solubilized by the micelle surrounding them. The kappa-casein which has a hydrophilic portion is responsible for solubilizing the other two caseins by promoting the formation of and stabilizing the micelles. Calcium caseinate has an isoelectric point of pH 4.6. This means it is insoluble in solutions with a pH less than 4.6. The pH of milk is 6.6, therefore, casein has a negative charge at this pH and is solubilized as a salt. If an acid is added to milk, the negative charges on the outer surface of the casein micelles are neutralized, by protonation of the phosphate groups. The casein micelles are destabilized or aggregate because the electric charge is decreased to that of the isoelectric point (pH at which there is no net charge because there are equal number of positive and negative charges present). The casein micelles disintergrate and the casein (the neutral protein) precipitates because it is no longer polar, with the calcium ions remaining in solution. Ca+2Caseinate + 2HCl -> Casein + CaCl2
All amino acids found in proteins have the basic structure, shown below, differing only in the structure of the R-group or the side chain.
Amino Acid Structure and pH
An amino acid can have several forms depending on the pH of the system. At low ph or acid conditions, the amino group (-NH2) is protonated by the addition of a proton (H+) from the acid. At high pH or basic conditions, the carboyxlic acid (-COOH) is deprotonated by the removal of a proton.
In an aqueous solution at a certain compound-specific pH, this structure may change so that a proton from the COOH, carboxylic acid group, transfers to the NH2, amino group, leaving an ion with both a negative charge and a positive charge, resulting in a net neutral charge because the number of protonated ammonium groups with a positive charge and deprotonated carboxylate groups...
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