Name _________________________________ Period ________ Date Due ______________________
____/10 Points
Lab: Casein Glue
Directions
In this lab, you will separate a mixture and synthesize a new product – glue! Wear your safety goggles
and work steadily. Be sure to answer the questions below in complete sentences
Introduction
Homogenized cow’s milk contains 4.4% fat, 3.8% protein, and 4.9% sugar. It has been well-mixed to
prevent separation of these components. At the normal pH of milk (6.3 – 6.6; pH values between 6 – 8
are neutral; pH values between 1-6 are considered acidic; pH values of 8-14 are considered basic), the
protein, named casein, remains evenly dispersed in the solution. When an acid is added to the mixture,
the pH drops and the casein can no longer stay dissolved; it coagulates into an insoluble mass. This
coagulation of casein occurs at a pH of about 4.6.
Homogenized Milk = Fat + Protein + Lactose
Pre-lab Questions
1. Milk is a homogeneous mixture (a solution) and not a compound. Explain this statement.
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2. Is milk acidic or basic
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3. What is the purpose of adding vinegar (acetic acid) to the milk?
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4. Could other acids (hydrochloric acid, HCl, or nitric acid, HNO3) be used to separate the protein
from the solution?
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Procedure
1. Pour about 100 ml of nonfat milk into a 400 ml beaker. Add 15 ml of white vinegar (5%
acetic acid).
Name _________________________________ Period ________ Date Due ______________________
2. Place the mixture on a low (1-3) hot plate and heat – while gently stirring. Observe the
mixture carefully and stop when you see turbidity (solid curds floating in the beaker). Do not
overheat the mixture; the protein will denature and your glue will not function.
3. Filter the mixture through a coffee filter into an Erlenmeyer flask.
4. Discard the filtrate (the liquid) which contains the whey. Scrape the curds from the filter
paper back into the 400 ml beaker.
5. Add about 1 gram of NaHCO3 (baking soda) to the beaker and stir. (Observe and record what
happens.) Slowly, add drops of water, stirring intermittently, until the consistency of Elmer’s
white glue is obtained.
6. Use your glue to fasten together two pieces of paper and/or fasten together two wooden
splints (if available). Allow the splints to dry overnight and then test for strength.
Post-Lab Questions
1. What would happen if you used whole milk in this lab?
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2. Why do we heat the mixture? What would happen if you heated the mixture to 95 °C?
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3. How did you know when the milk was separated?
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4. How would the glue you created with nonfat milk differ from glue created from 2% milk? How
would it differ from glue created from whole milk?
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5. Why do we add baking soda to the casein? (Hint: what did you observe when you added the
baking soda to the casein?)
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6. In this experiment, what happened to the lactose and fat portions of the milk?
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