Casein in milk

Casein is a protein found in a number of animal and vegetable materials but only one source is of commercial interest, cow s skimmed milk. The amount of casein in milk will vary but a typical analysis of cow s milk is ... 

Many proteins and polymers have been analyzed on SynChropak GPC and CATSEC columns. Table 10.6 lists some of the published applications. The use of a surfactant to analyze the caseins in milk is illustrated in Eig. 10.12. Viruses have also been analyzed on SynChropak GPC columns, as seen in the chromatogram from Dr. Jerson Silva of the University of Illinois (Pig. 10.13). Dr. Nagy and Mr. Terwilliger analyzed cationic polymers on a series of CATSEC columns using differential viscometry as detection (Pig. 10.14) (9). 

Nitric add also reacts with proteins (casein in milk [test tube] and albumen in eggs) to give a characteristic yellow color. 

Because they occur as large aggregates, micelles, most (90-95%) of the casein in milk is sedimented by centrifugation at 100000 g for 1 h. Sedimentation is more complete at higher (30-37°C) than at low (2°C) temperature, at which some of the casein components dissociate from the micelles and are non-sedimentable. Casein prepared by centrifugation contains its original level of colloidal calcium phosphate and can be redispersed as micelles with properties essentially similar to the original micelles. 

Isolated as2-casein in solution is also very susceptible to plasmin eight peptide bonds are hydrolysed with the production of 14 peptides. Plasmin also hydrolyses as2-casein in milk but the peptides formed have not been identified, although at least some are included in the proteose-peptone fraction. 

You are cerlain lo be familiar with three common, pure proteins albumin in eggs, casein in milk, and gelatin. 

The estimation of casein in milk by refractometric techniques appears to hold some promise. The casein may be precipitated, washed, and redispersed to yield a solution suitable for refractometry (Brereton and Sharp 1942 Schober etal. 1954). Another method involves computation from the difference between the refractive indices of two samples, one made alkaline to dissolve the casein and the other treated with copper sulfate to precipitate it (Hansson 1957). Heating the milk... 

Schober, R., Christ, W. and Niclause, W. 1954. The refractometric estimation of casein in milk. Lebensm. Untersuch Forsch 99, 299-302. 

Stability of the complex protein system of milk or whey is decreased by concentration (Fox 1982 Muir and Sweetsur 1978 Sweetsur and Muir 1980B). In addition to closer packing of casein micelles and other proteins in concentrated milk, calcium phosphate is precipitated so that the pH decreases (Fox 1982). The pH effect causes protein which would be soluble at a normal solids concentration to precipitate. Casein in milk concentrated to three times its original solids level forms a flocculent after 1 to 3 weeks at -8°C (Lonergan 1978). 

Globular proteins - these have complicated three-dimensional structures and are soluble in water. They are easily affected by acids, alkalis and temperature increase, when they are said to be denatured. Casein (in milk), albumen (in egg white) and enzymes are examples of globular proteins. 

Other applications involve detection of adulteration of dairy products with soy, pea, and wheat proteins (Haasnoot et al., 2001), presence of asrcasein in milk (the detection threshold for this protein was 0.87pg/mL) (Muller-Renaud et al., 2005), [3-casein in milk and cheese (Muller-Renard et al., 2004), and detection of peanut allergenic proteins (the detection threshold for this protein was 0.7 pg/mL) (Mohammed et al., 2001). 

Muller-Renaud, S.P., Dupont, D., and Delie, P. 2004. Quantification of beta-casein in milk and cheese using an optical immunosensor. J Agric Food Chem 52 659-664. 

Percentage of casein in milk Chemical analysis of proteins Biuret test... 

Which of these metal ions gives a positive test with casein in milk Xanthoprotein test... 

The fact that milk and milk products arc rich in calcium is known by most people. Casein is the main protein of milk, accounting for about Sd% of milk protein. Casein has a molecular weight of about 23,000 and, thu.s, is a relatively small protein however, the casein in milk occurs as a large complex of casein molecules having an overall molecular weight of about 100 sc 10, Casein is phosphorylated on residues of serine. These phosphate groups bind calcium ions. The calcium ions help maintain the stability of the large complex. 

Different proteins have different physical properties. Some—such as casein in milk, ovalbumin in egg whites, and hemoglobin in blood—are water-soluble. Others—such as keratin in hair, fibroin in spider silk, and collagen in connective tissue—are flexible solids. 

Other properties which are of commercial value but are peculiar to more limited groups are as follows (a) the ability of cellulose derivatives, particularly carboxymethylcellulose to adsorb onto cotton libers, forming a barrier repellent to dirt and (b) the ability of carrageenans and quince-seed mucilage to alter the aggregation of casein in milk. The last two poly-sacolloids are the only ones that seem to be absorbed by skin tissues, making them particularly effective in cosmetic and pharmaceutical preparations for topical application. 

Egg albumin and casein in milk are examples of nutrient storage proteins. 

The appealing feature of this model is the occurrence of K-casein on the surface of the micelle where it would be available for the action of proteolytic enzymes. Presumably, proteases such as rennin would cleave the exposed macropeptide from the K-casein on the micellar surface yielding patches of p-K-casein on the surface of the micelle. After a certain portion of the coat units is modified, adjacent micelles would cohere, leading to the coagulation of the milk. Approximately 30% of the K-casein in milk is so-called "soluble K-casein (176) and is also... 

Precipitation with ethanol. The casein in milk coagulates at pH 6.6 on addition of ethanol to about 40% stability decreases sharply as the pH is reduced, and only 10-15% ethanol is required at pH 6. Ethanol-precipitated casein may be dispersed in a micellar form and has very good emulsifying properties. The commercial production of ethanol-precipitated casein is probably economically viable but the process is not being used commercially. 

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