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).
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