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. In milk, which has a pH of about 6.
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 major metabolic end-product of carbohydrate [lactose in milk] fermentation.
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What is cultural absolutism? Study Guides. Trending Questions. Still have questions? In this process, one molecule of glucose is converted to galactose and joined to another of glucose. Galactose is thought to be needed by developing infants to build brain and nervous tissue. It is more stable to metabolic oxidation than glucose and affords a better material for forming structural units in cells.
The digestion of lactose involves the enzyme lactase, which hydrolyzes the disaccharide into its two component sugars. In the first part of this experiment, we will isolate casein and lactose from cows' milk and carry out a few chemical tests on the isolated casein and lactose. As implied above, these are rather simple operations to carry out.
Casein is precipitated by simply adjusting the pH of the milk to be sufficiently acidic that the protein is insoluble, taking care not to acidify too much so that the lactose does not hydrolyze. The other proteins remain water-soluble in acidic solution, but they can also be precipitated and isolated by merely heating the acidic solution and filtering.
The isolated casein is insoluble in water, alcohol, and ether, but dissolves in alkaline and some acidic solutions. Once the casein is removed, lactose can be isolated as the alpha-anomer by addition of ethanol and crystallization from the resulting water-ethanol mixture at room temperature.
Casein is isolated from milk commercially and is industrially important because after dissolving in alkaline solutions and drying, it becomes a sticky substance that can be used in glues, the coating of paper, and the binding of colours in paints and wallpaper. It is also used as a coating for fine leather, and is cured with rennet to produce a plastic material used for buttons. When isolated under sanitary conditions and dissolved in alkaline solutions, casein is also employed in the manufacture of pharmaceutical and nutritional products.
In the test section of the experiment, we will carry out a few chemical tests on the isolated casein and lactose, as well as on test samples of other representative amino-acids and carbohydrates. Historically, these tests were designed for the purpose of structure elucidation.
Since we already know the structures of these substances, we will use the chemical tests to demonstrate various aspects of the chemical reactivity of the protein casein. Of course, these tests depend on the specific structural features present in the molecules. While the tests and the chemistry involved are described briefly below, you should read Ege, Chapters 25 and 26 for a more complete introduction and the necessary background.
Procedure: Weigh out 5 grams of powdered non-fat dry milk and dissolve it in 20 mL of warm water in a mL beaker. Do not add the acid too rapidly. Continue the acid addition slightly less than 2 mL will be required , keeping the beaker on the hot plate, until the liquid changes from milky to almost clear and the casein no longer separates. It is important not to add too much acid, because it may hydrolyze some of the lactose in the milk and reduce your yield in Experiment 11B.
Stir the precipitated casein until it forms a large amophous mass; then remove it with a stirring rod or tongs and place it in another beaker. Immediately add 0. The separation of lactose should be done as soon as possible during the same laboratory period.
Collect the casein by suction filtration to remove as much water as possible. Press the solid with a spatula. Stir the casein in the ether for a few minutes, decant the ether, and repeat the process with a second 5 mL portion of ether.
After the second washing with ether, suction filter the product. The ether washings remove any small quantities of fat that may have precipitated with the casein. Place the casein between several layers of paper towels to help dry the product, and let it stand in the air for minutes.
Divide the wet product in half, and weigh the two portions. Place one portion in a mL Erlenmeyer flask with 35 mL of water and 0. You may carry out the chemical tests for the protein during this lab period if you have time, or in your next lab period. Allow the second portion to dry in your locker over the following two weeks. When dry, weigh this portion and calculate the total yield of casein from the powdered milk. Show your calculations. Procedure: Gently boil the original liquid to which the calcium carbonate was added after isolation of casein.
Bumping will not be a problem so long as you stir the solution constantly and vigorously with a glass rod. The solution will foam somewhat as it refluxes. This procedure precipitates the remaining proteins lactalbumin and lactoglobulin. Suction filter the hot mixture to remove the proteins and calcium carbonate, and transfer the hot, slightly yellow filtrate to a mL Erlenmeyer flask.
Concentrate the filtrate to a volume of about 5 mL by heating with constant swirling, again being careful to avoid bumping. Foaming can be controlled by heating the liquid less vigorously and gently blowing onto it. Put this mixture aside and prepare a slurry of about 1 gram of Celite and 7. Suction filter the slurry into a Hirsch funnel containing a correct sized filter paper to obtain a filter pad of Celite, and discard the alcohol in the filter flask. To the slightly cooled ethanol mixture containing the lactose, add 1 mL water.
Suction filter the mixture through the Celite filter pad, making sure the filtrate is clear.
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