Purpose: The purpose of the milk lab is to determine the percentage of protein in non fat milk. It was also beneficial in demonstrating how chemicals denature (change the shape of) proteins.
Introduction: In one cup (244g) of protein, there is said to be 8 grams of protein. Casein is a family of proteins that make up to 80% of the protein found in cow milk (1). Other milk proteins in milk are beta-lactoglobulin and alpha-lactoglobulin, which are found in the watery fluid remaining after milk curdles (2).
Methods:
 |
| Measuring 15 mL of skim-milk in a graduated cylinder |
 | |
| Massed an empty 50 mL beaker(26.75g) and then massed it again with 15 mL of skim milk(41.7g). So there was 14.95 grams of skim milk. |
 |
| Concentrated acetic acid was used to denature the proteins in the skim milk. |
 |
| 30 drops of acetic acid added and stirred into skim milk |
 |
| This is what the milk and acid mixture looks like after 2 minutes. The milk proteins begin to coagulate and curds are beginning to appear. |
 |
| A filtering system was created to separate the milk curds from the clear fluid. We put the filter paper on a beaker then put a rubber band to keep it in place. |
 |
| Pouring the coagulated milk into the funnel. |
 | ||
| This image really captures the texture and appearance of the milk curds. This is not what we think of when we drink milk. |
 | ||
| After the milk curds and milk filtrate were separated, Biuret(a base) was used to test of the milk filtrate contained proteins. |
 |
| Test Tube #1 containing 1 mL of distilled water. 1 mL of Biuret reagant was used to detect the presence of proteins. Test tube #1 turned purplish blue. This means proteins were present. |
 | ||
| Test Tube #2 containing 1 mL of the milk filtrate. 1 mL of Biuret reagant was used detect presence of proteins. This test tube was clearly purple, meaning that the milk filtrate contained proteins even though the milk curds had been separated from his filtrate. |
 |
| We waited until all milk curds were in the funnel and put the coffee filter in a box to dry. Final image of the milk curds. There was still some filtrate left in this image. |
Data:
Mass of 50mL beaker
|
41.75g
|
Mass of 50mL beaker w/ 15 mL milk
|
26.75g
|
Mass of 15 mL milk
|
14.85g
|
Mass of filter paper alone
|
.89g
|
Mass of paper and dry protein
|
1.6g
|
Mass of dry protein
|
.71g
|
Color of distilled H2O and biruet: blue
Color of milk filtrate and biruet: light purple
Discussion:
Our percent yield of protein in a 14.85g sample of milk was 4.7%. We found .71g of dry protein left in our coffee filter, which is slightly higher than the average of .51g. The fact that the milk curdled wasn’t surprising to us; we expected the acetic acid to denature the proteins. We just didn’t expect the value to be so high. Our number may have differed from the expected value for several reasons. There may have been dust particles that landed in our sample. The denatured proteins may have spread out and caused the mass to increase. Or the leftover acetic acid could have reacted with the coffee filter, and the precipitate of that reaction resulted in a higher mass. In the future, it may be better to give more time for the milk to drain, or use a less durable filter. That way students can be sure the all the acetic acid is in the filtrate, not the precipitate. It also might be helpful to put some type of cover over the drying samples, so as to make sure dust particles aren’t interfering.
We thought the milk filtrate would contain proteins as well, because not all proteins change shape when mixed with chemicals. As it turns out, we were correct. Casein was precipitated, but beta-lactoglobulin and alpha-lactoglobulin were not. Consequently, the milk solution turned slightly purple in the presence of Biuret. The control group (distilled water) did not turn purple.
Conclusion: While we did get a 4.7% yield of protein (.71g/14.85gx100), we can’t say for sure this is the correct value, especially as it is higher than the expected value of about 3%. It’s unusual, to say the least. However, we did learn that acid denatures some proteins (casein), but not all.
References:
