REACTIONS OF CASEIN

 

REACTIONS OF CASEIN

 

                Casein is the chief protein of milk. It is a phosphoprotein. It is insoluble in water but soluble in dilute alkali. It gets precipitated at its iso-electric pH 4.6. It is precipitated by half saturation with ammonium sulphate. It is not a heat coagulable protein.

 

                Casein being an animal protein is rich in most of the essential amino acids and is therefore a protein of high biological value. Sulphur containing amino acids are absent in casein.

 

 

(1) Precipitation at Iso-electric pH 4.6:

                To 5ml of casein solution add 3 drops of bromocresol green and adjust the pH to 4.6, according to the procedure given below. A thick precipitate will be observed proving that casein is precipitated at its isoelectric pH.

 

a)       BCG gives blue colour above pH 4.6 a green colour below pH 4.6 a pale green colour at pH 4.6.

b)       Add to the given solution few drops of BCG ,If a blue colour is formed the pH is above 4.6 and bring it down by adding dil acetic acid drop by drop till a pale green colour is observed.

c)       On adding the BCG If you get a green colour the solution is below pH 4.6 and bring up pH, by adding drop by drop and with shaking dilute sodium carbonate till the colour is pale green.

d)       If on adding BCG a pale green colour is got, the solution is below pH 4.6.

 

(2) Half Saturation Test:

                Perform half saturation test with ammonium sulphate. Draw your conclusion.

 

(3) Neumann’s Test:

                This test is for organically combined phosphorus. Principle of this test to convert the organic phosphorus into inorganic phosphate by heating with hot cone. nitric acid. Inorganic phosphate thus formed is detected by ammonium molybdate.

                In this test you are using hot conc. acids, so be careful while heating and water for dilution.

 

PROCEDURE:

 

                Take 5ml of casein + 1ml of conc. Nitric acid warm cool add 1ml of ammonium molybdate solution, formation of canary yellow color of ammonium phosphor molybdate confirm that casein is a phsophoprotein.

 

(4) Perform all the colour reactions of proteins and draw your conclusions.

 

Note:

While performing the ninhydrin test add few drops of 1% aceticacid to neutralise the NaOH and then boil with ninhydrin reagent. (Casein in dissolved in NaOH).