Milk Glue

Michelle Jones                 Morgan Park High School                       
6701 S. Crandon 12A            1744 West Pryor Ave.
Chicago IL 60649               Chicago IL 60643
(312) 955-2406                 (312) 535-2550

Objectives:

The objectives of this intermediate lab are to practice filtration and observe a 
precipitation reaction and a neutralization reaction. 

Materials Needed:

Each group will need the following materials:

1. Skim milk, 125 mL                        2. Beaker, 250 mL
3. Vinegar (acetic acid), 25 mL             4. heat source 
5. Sodium bicarbonate (baking soda), 1g     6. Funnel and filter paper
7. stirring rod                             8. water, 30 mL
9. Graduated cylinder, 125 mL

Strategy:

 1. Place 125 mL of skim milk in a 250 mL beaker.
 2. Add 25 mL of vinegar, which is an acidic solution.
 3. Gently heat milk and stir constantly until small lumps begin to form.
 4. Remove beaker from heat and continue to stir until no more lumps form.
 5. Allow lumps to settle.
 6. Filter the solid (curds) from the liquid (whey) using funnel.
 7. Gently press the filter paper around the curds to squeeze out the excess 
    liquid through the filter paper.
 8. Return the solid to the beaker
 9. Add 30 mL of water to the solid and stir
10. Add 1/2 tsp. of baking soda to neutralize vinegar, bubbles should appear.  
    Add a little more baking soda until no more bubbles appear.
11. The substance in the beaker is glue.  Test the adhesive properties, 
    including waterproof property.

Performance Assessment:

Questions and answers.
1. What is the purpose of the vinegar in making glue?  What is the purpose of 
   the heat?   The vinegar and the heat act to change the protein molecules so 
   they will precipitate. 
2. How is the casein (protein) separated from the milk?  The casein coagulates 
   and then is separated by filtration. 
3. What is the purpose of the baking soda?  The baking soda neutralizes the 
   acid. 

Conclusions:       
   
The students studied the effects of heat and acid on the protein casein in milk.  
The process produced a substance that was sticky like a glue.  Milk is 87% 
water, 4.8% carbohydrate, 4% fat, 3.5 % protein, and 0.7% minerals.  The 
carbohydrate is mostly the dissaccharide sugar, lactose.  The fat is in the form 
of globules that reflect light and give milk some of its whiteness.  Some 
vitamins are dissolved in the fat.  The proteins are complete, meaning they have 
all essential amino acids.  The most abundant protein is casein.  The 
precipitation takes place when the milk becomes acidic, near pH 4.6. 

References:

Chemical Activities by Christie L. Borgford and Lee R. Summerlin