Isolation of Casein from Milk
Isolation of Casein from Milk In this experiment, casein was isolated from milk by means of isoelectric precipitation. A percent yield of % was obtained by the group. Introduction 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 f 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.
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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 omparatively 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 (p’) is the pH of a solution at which the net primary charge of a rotein becomes zero. At a solution pH that is above the pl the surface of the protein is predominantly negatively charged and therefore like-charged molecules will exhibit repulsive forces. Likewise, at a solution pH that is below the p’, the surface of the protein is predominantly positively charged and repulsion between proteins occurs.
However, at the pl the negative and positive charges cancel, repulsive electrostatic forces are reduced and the attraction forces predominate. The attraction forces will cause aggregation and precipitation. The pl of most proteins is in the pH range of 4-6. Mineral acids, such as hydrochloric and sulfuric acid are used as precipitants. The greatest disadvantage to isoelectric point precipitation is the irreversible denaturation caused by the mineral acids. For this reason isoelectric point precipitation is most often used to precipitate contaminant proteins, rather than the target protein.
The precipitation of casein during cheesemaking, or during production of sodium caseinate, is an isoelectric precipitation. tive charge in milk since milks pH is 6. 6. 1 Results and Discssion Casein was isolated from milk by means of isoelectric precipitation. A percent yield of % was obtained by the group. Table 1 presents the data and results obtained from the experiment which includes: (1) the weight of powdered milk, (2) the initial pH, (3) the tinal pH, (4) the volume ot the acetic acid used, (5) the weight ot casein and 6 percent yield.
In order to compute for the percent yield, the weight of the isolated casein was divided by the weight of the powdered milk, and then multiplied by 100%. Data and Results Values weight of powdered milk initial pH final pH olume of the acetic acid used weight of casein percent yield Table 1 . Data and results of the experiment: Isolation of Casein from Milk. Before autoclaving, the isolate was a white-yellowish solid with smooth texture. After autoclaving, it turned into a brown solution with black precipitate.
The filtrate was a yellowish solution. The general principle behind this experiment is that when casein is at its isoelectric point, it is generally at the pH where it is least soluble. As a result, casein precipitates at this PH. To explain further, casein is present in milk as calcium alt, calcium caseinate. It is a mixture of alpha, beta and kappa caseins to form a cluster called micelle. These micelles were responsible for the white opaque appearance of milk.
The casein, as proteins, is made up of many hundreds of individual amino acids, each of which may have a positive or a negative charge, depending on the pH of the [milk] system. At some pH value, all the positive charges and all the negative charges on the [casein] protein will be in balance, so that the net charge on the protein will be zero. That pH value is known as the isoelectric point IEP) of the protein and is generally the pH at which the protein is least soluble. For casein, the IEP is approximately 4. and it is the pH value at which acid casein is precipitated. In milk, which has a pH of about 6. 6, the casein micelles have a net negative charge and are quite stable. During the addition of acid to milk, the negative charges on the outer surface of the micelle are neutralized (the phosphate groups are protonated), and the neutral protein precipitates. The same principle applies when milk is fermented to curd. The lactic acid bacillus produces lactic acid as the ajor metabolic end-product of carbohydrate [lactose in milk] fermentation.