Methods of analysing milk?

Question

Methods of analysing milk?

Answer 1

Students A and B

Label the following items:

1-250 mL beaker – "fat"

2-weighing boats – "protein" and "Ca3(PO4)2"

Weigh these labeled containers and record the data in your notebook. These containers will be used to store your isolated products until the next lab period. Keep them in your drawer until you are ready to use them.

Place a clean, dry 150 mL beaker on the top loading balance. Tare the balance and add about 50 mL of milk to the beaker. Record the exact mass.

Add ~1 mL of 8 M acetic acid to the milk and stir while heating on your hot plate (at a medium setting) until the mixture almost boils. The protein and adhering fat should completely precipitate in about 5 minutes.

Cool this mixture thoroughly (~10 minutes) in an ice water bath (use a large beaker to make your ice water bath). While the mixture is cooling, take care not to disturb its contents.

To filter out the protein and fat, place the center of a clean cheesecloth loosely over a 250 mL beaker and pour the mixture through the cloth into the beaker. Gently squeeze the cheesecloth containing your precipitate to allow the excess liquid to drain into the beaker. Remove the precipitate from the cheesecloth and place it in a clean 150 mL beaker.

At this point, Student A should take the precipitate and proceed with the separation of the protein and fat. Student B should use the filtrate to remove the phosphate from the lactose.

Student A: Protein and Fat

To remove the adhering fat from the protein (casein), add 35 mL of acetone to the precipitate. Break the solid into small pieces with your stirring rod and place the beaker inside your hood on your hot plate. With a moderately fast stirring rate (to avoid "bumping" your solution) SLOWLY heat your mixture on the lowest setting. Because acetone evaporates quickly, you should periodically replenish the acetone to its original volume.

When the solution comes to a boil, remove the beaker from the hot plate to a paper towel on your lab bench (still under the hood). After allowing it to cool for about 2 minutes, reheat just to boiling and cool as before, repeating this process for a total of 3 times. This should be sufficient to remove the fat.

Filter the casein using your Büchner funnel and wash with a small amount (~5 mL) of acetone. Continue drawing air through the casein until it is dry and easily removed from the filter paper (about 5 minutes). Place the casein in the weighing boat labeled "protein" and carefully place it in your lab drawer to dry until the next lab period.

Transfer the filtrate into the beaker labeled "fat". Try to remove as much fat from the flask as possible by rinsing twice with ~3 mL acetone. Add this to the filtrate in the beaker. Carefully place the beaker in your lab drawer. During the week all, or most, of the acetone will evaporate.

Student B: Phosphate and Lactose

To the filtrate from the first part (containing phosphate and lactose) add ~7 mL of 1 M NaOH. Stir the solution and test with litmus paper to make sure it is basic. If it is not basic, add an additional mL of NaOH until it becomes basic.

Add 10 mL of 0.5 M Ca(NO3)2 and heat on a medium setting. While stirring with your magnetic stir bar, continue heating until the volume has been reduced by about one-third (~15 minutes). Allow the solution to cool undisturbed in an ice bath for about 5 minutes.

Filter the solid Ca3(PO4)2 using a Büchner funnel. Wash with two 2 mL portions of cold water, two 2 mL portions of ethanol, and one 5 mL portion of acetone. Dry the precipitate until it is easily removed from the filter paper (~5 minutes) and scrape it into the weighing boat labeled "Ca3(PO4)2".

Use the filtrate (containing lactose) to perform the class test for reducing sugars.

Benedict’s Test for Reducing Sugars

Set up a boiling water bath on your hot plate and prepare two test tubes. To both test tubes, add 5 mL of Benedict’s solution. To the first test tube, add 8 drops of the prepared 1% glucose solution. Glucose (a reducing sugar) will give a positive test result and will be used to compare with the results obtained from your lactose solution. To the second test tube, add 8 drops of your lactose solution. Mix the contents of both test tubes well and place them in the boiling water bath for 3-5 minutes. Note the results in both test tubes.