Why is milk solution heated to 40 degree celcius during isolation of casein?

Isolation and Purification of Casein


Make a dilute solution (approx. 10%) of acetic acid by adding 1 mL glacial (100%) acetic acid to 10 ml


distilled water in 10-mL-Erlenmeyer flask. Mix thoroughly and set aside.


Place 4.0 g of powdered nonfat milk and 10 mL of water into a 50- or 100-mL beaker. Heat on a sand


bath to about 40掳C ( top of sand bath at about 50掳C). Monitor the temperature of the milk solution with


a thermometer.


When the mixture has reached 40掳C, add the dilute acetic acid dropwise to the warm milk. Do not add


all of the dilute acetic acid at one time! Maintain the solution at about 40掳C and after every 5 drops,


stir the solution gently using a small spatula. Using the spatula, push the precipitated casein onto the


side of the beaker so that most of the liquid drains from the solid (you will have to hold it on the side of


the beaker to drain the liquid).


Then transfer the congealed casein to a 20-mL vial in small portions. The casein will stick together and


be hard to transfer if you use large pieces. If any liquid separated from the casein in the vial, use a


Pasteur pipet to transfer the liquid back into the reaction mixture.


Slowly continue the dropwise addition of the 1 mL of dilute acetic acid solution to the beaker to


complete the casein precipitation. Remove as much casein as possible from the beaker and transfer it


to the vial. Avoid adding an excess of acetic acid to the milk solution, as this will cause the lactose in


the milk to hydrolyze to glucose and galactose.


When most of the casein has been removed from the milk solution, add 0.2 g of calcium carbonate to


the milk in the beaker. Stir this mixture for a few minutes and save it for use in the isolation of lactose


below. (The mixture should be used as soon as possible during the current lab period.)


Transfer the casein from the 20 mL vial to a Hirsch suction filter funnel. Draw a vacuum on the casein


for about 5 min to remove as much liquid as possible, pressing the casein with a spatula during this


time. (The liquid contains the albumins and lactose--so a great loss of liquid at this point will result in


decreased yields of these other two components.)


Transfer the casein to a 7- to 10-cm piece of filter paper, fold this over onto the casein, and press gently


to absorb any remaining liquid. Place the solid on a tared watch glass, let air dry until the next lab


period, and weigh. Casein is used to make white glue, so it is important that you don鈥檛 leave it on the


filter paper or it will become glued to it! Calculate the weight percent of casein isolated from the


powdered milk.


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