Laboratory Activity 3: Student Version

Continued

Part 2: Removal of Milk Protein

  1. Add approximately 100 mL vinegar to the aqueous layer from Part 1, Step 8. Stir sample slowly until solid clumps appear in the liquid.
  2. Weigh a piece of filter paper and record mass in your data table.
  3. Place the filter paper in a Buchner funnel fitted to a suction flask, wet it with a little distilled water and suction filter the coagulated material in the beaker. Continue to apply suction for about 5 min after all the liquid has passed into the suction flask; this strategy will help in drying your coagulated sample.
  4. Carefully remove the filter paper and contents from the Buchner funnel and place on a watch glass that has been previously marked with your initials. Allow the filtered material, which consists largely of milk protein, to dry overnight.
  5. Dispose of the liquid in the suction flask as directed by your teacher.
  6. Thoroughly wash your hands before leaving the laboratory.

Day 2

Part 3: Determination of Percentage of Milk Protein

  1. Weigh filtered, dried milk protein plus filter paper and record mass in data table.
  2. Calculate the percent protein in the original milk sample.

Part 4: Determination of Percentages of Water and Carbohydrates in Milk

  1. Weigh a small evaporating dish (the size should be large enough to be seated on top of a 250-mL beaker) and stirring rod together to +/- 0.1g. Record mass in data table.
  2. Using a 10-mL graduated cylinder, add 5 mL fresh milk to the evaporating dish and weigh again. Record mass in data table.
  3. Set up a ringstand, ring, wire gauze and burner. Half fill a 250-mL beaker with water, add a few boiling chips, and place on the assembly.
  4. Place the evaporating dish and contents on top of the 250-mL beaker from Step 3. Heat the water in the beaker to slowly evaporate the liquid from the milk sample. As a thin layer of scum forms on the milk, break it up with the stirring rod, making sure that you keep the stirring rod inside the dish. Stir the milk gently and continuously to avoid scorching.
  5. As the milk dries, it becomes pasty. When the paste consistency appears to remain constant for a few minutes, stop heating. Allow the dish to cool. Wipe the condensed water from the outside of the dish. The dish now contains the milk solids.
  6. Weigh the dish, stirring rod and milk solids, and record the mass in your data table.
  7. From these data, calculate the percent water in the original milk sample.
  8. Knowing the percentages of fat, protein, and water contained in milk, calculate the percent of carbohydrate by difference. Assume that the major components of milk are fat, protein, water, and carbohydrates only.
  9. Thoroughly wash your hands before leaving the laboratory.
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