Percent Casein Protein in Cow Milk Name:___________________________
Lab Partner Name:_________________________Date: ____________ Block:______
Purpose: To determine the percent casein protein by mass in skim milk.
Pre-lab Discussion:
Separation of the proteins, The ISOELECTRIC Point
If you add acid to milk you will start to precipitate out the protein dissolved in milk. **You
have reached the isoelectric point of the protein called casein. At this point, the protein
molecule has a net charge of zero and it changes its shape by denaturing.
This point occurs at the pH of 4.6 for the casein molecule. At this point the molecule is
insoluble and it precipitates to form a soft curd. This curd forms the cheese.
What is left in the solution is the whey protein molecules along with the lactose and
dissolved substances such as water-soluble vitamins and minerals. Whey will
sometimes separate out when milk is heated too quickly. The whey molecule settles to
the bottom of the pan and forms a layer that will burn.
Procedure:
1. Obtain 200.0 mL of skim milk. Place in a 400 mL beaker.
2. Using pH paper, and a stirring rod, test the pH of the milk.
Place the pH in the space provided.
pH of milk: ________________
3. If the density of milk is 1 g/mL and you have 200 mL of milk, determine the mass of
the skim milk in your 200.0 mL of milk. Mass of Milk: ___________________
4. Slowly heat up your milk on a hot plate stirring with a stirring rod until condensed
water vapor comes off the surface of the milk. Take the temperature of the milk and
when the temperature is around 70C, remove the beaker from the hot plate and place
on the tabletop. (The top of the beaker should not be hot, but you may use beaker
tongs if you desire.)
5. Slowly add 30 mL of lemon juice to your beaker. Observe what happens to the milk.
Observations:
6. Test the pH of the solution with pH paper and record the pH in the space provided.
pH of the mixture when it begins to curdle:_______________
7. Filter the mixture using the funnel and cheese cloth. Collect the filtrate in a large
beaker. Allow the curds to dry overnight and obtain the mass of the curds tomorrow or
the next class day and record the mass in the space provided.
Mass of cheese cloth + curds:_____________ Mass of cheese
cloth:________________
Calculation of the mass of curds:
Calculations: 1. Calculate the percent by mass of casein protein in your skim milk.
Show all calculations. Use the percent composition equation.
2. Using the information from your pre-lab reading, calculate the theoretical amount of
casein protein found in your 200.0 mL sample. The reading stated that 3.5% of the milk
should be protein and of that protein, 82% should be casein protein. Show all of your
work.
3. Using your answer in calculation #2 as the true value, calculate the percent error in
your experimentation.
Questions:
1. What substance is makes up the largest component of milk?
____________________
2. What substance in milk makes cheese? _______________
3. Do you need fat in milk to make cheese? ________________
4. Using the pH of milk, determine whether milk is acidic, basic, or neutral. Explain your
answer: