E STIMATION OF P ROTEINS AND L ACTOSE IN M ILK. 376BCH Nov 2008

OBJECTIVES To estimate the total protein content of milk. To estimate the lactose concentration in milk.

INTRODUCTION major constituents of milk are lactose, fats and proteins The proteins include casein (a phosphoprotein and 80% of total protein), lactoalbumin and a little lactoglobulin There are also important amounts Calcium phosphorus vitamin A, B 2 B 1, C, D3 and or iron

INTRODUCTION C ONT.., The white colour of milk is due to emulsified liquid and to the calcium salt of casein. The fat content of human milk is similar to that of cow’s milk but there is more lactose and less protein, calcium and phosphorus

INTRODUCTION C ONT.., Colostrum is more yellowish and contains about twice as much protein including immunoglobulins The protein content falls with time reaching average levels after about one month. Fat and lactose change little.

PRINCIPLE ( C ASEIN ) Total protein in milk is estimated as casein by the Biuret reaction. Under strongly alkaline conditions, such compounds give violet coloured complexes with cupric salts, which may be spectrophotometrically estimated.

PRINCIPE ( L ACTOSE )  The diluted milk is added to sodium sulphate-copper sulphate solution and protein precipitated by the addition of sodium tungstate solution. The mixture is centrifuged, and the supernatant liquid is then added to alkaline tartrate.  Lactose present reduces the copper sulphate to cuprous oxide.  Addition of phosphomolybdic acid solution oxidizes the precipitated cuprous oxide to cupric oxide and is itself reduced to molybdenum blue.  The absorbance of the resulting blue colour is taken at 680 nm. From this the concentration of lactose in the diluted milk may be found if parallel tests using standard lactose solutions are carried out.