In this experiment, casein was isolated from milk by means of isoelectric precipitation. A percent yield of % was obtained by the group.
Milk is an opaque white or bluish-white liquid secreted by the mammary glands of female mammals, serving for the nourishment of their young. This liquid, as secreted by cows, goats or certain other animals are used by humans as food and as a source of dairy products such as cheese and butter. Milk composition differs widely among species. Factors causing these variances include: the type of protein; the proportion of protein, fat, and sugar; the levels of various vitamins and minerals; and the size of the butterfat globules, and the strength of the curd. On average, cow milk contains 3.4% protein, 3.6% fat, and 4.6% lactose, 0.7% minerals and supplies 66 kcal of energy per 100 grams. Bovine milk normally contains 30-35 grams of protein per liter. Of which, 80% is arranged in casein micelles.
Figure 1. Model of Casein Supramolecule
Containing a fairly high number of proline residues, which do not interact and no disulfide bridges, casein has, as a result, relatively minimal tertiary structure. It is comparatively hydrophobic, resulting to its poor solubility in water. Showing only limited resemblance with surfactant-type micellae in a sense that the hydrophilic parts reside at the surface and are spherical, casein is found in milk as a suspension of particles called "casein micelles." On the other hand, the interior of a casein micelle is highly hydrated. The caseins in the micelles are held together by calcium ions and hydrophobic interactions. Casein’s isolelectric point is 4.6. It has a negative.
The isoelectric point (pI) is the pH of a solution at which the net primary charge of a protein becomes zero. At a solution pH that is above the pI the surface of the protein is predominantly negatively charged and therefore like-charged molecules will exhibit repulsive forces.
References: Adams, C. (2011). Casein. The Columbia Electronic Encyclopedia. Sixth Edition. Columbia University. Goff, D. (2010). Introduction to Dairy Science and Technology: Milk History, Consumption, Production, and Composition. Dairy Science and Technology. University of Guelph. Ladisch. (2001). Bioseparations Engineering. New York: John Wiley & Sons, Inc. Nelson DL, C. M. (2005). Lehninger 's Principles of Biochemistry (4th ed.). New York: W. H. Freeman and Company. Pavia, L. K. (n.d.). Introduction to Organic Laboratory Techniques: A Microscale Approach. Walker JH, W. K. (2000). Principles and Techniques of Practical Biochemistry. Cambridge, UK: Cambridge University Press, 287–89.