AP Biology Milk Analysis Lab

PURPOSE

In this lab, our group was experimenting with milk, acetic acid, and filter paper in order to determine how much protein is in skim milk. With our data received from our experiment, we compared it to the measurements given on the milk carton. Through denaturing of the proteins in the milk, we were able to conduct this experiment.

INTRODUCTION

Proteins are chains of amino acid molecules that are bonded together by peptide bonds. In milk, there are nine amino acids. Through denaturation, proteins or nucleic acids lose their secondary, tertiary, and quaternary structures from their native state when they are presented with external stimulations of stress such as big changes in temperature or strong acids and bases. Denaturing proteins in milk allows for digestion due to the easy access to the protein change from enzymes as well as increasing the ability for water to bond to proteins which is used in making yogurt. 

 

 

METHODS

 

Using a coffee filter, we allowed the acetic acid to drip through while the curdled milk was contained in the paper.

In the above picture, you can see globular proteins where the acetic acid has curdled the milk.

 Here is the nutrition facts label for Prairie Farms non-fat milk. 80 calories and 8 grams of protein can be supposedly be found in every carton.

 In the solution found beneath the filter paper, we used Biuret to test for presence of proteins. In the above picture, you can see a purple substance within the test tube, which means the test came back positive. This meant the filter paper did not filter out all of the protein.

 Here is the data table explaining the masses we found throughout the milk lab. By subtracting the mass of the filter paper from the dry protein, we are able to determine that 4.5 grams of protein were found after curdling the milk with acetic acid.

 

In reality, however, our calculations showed that only .508g of protein were to be found in the milk. The reasons for this error are explained below.

 

 

DISCUSSION

Our result of 4.5 grams of protein was four whole grams of protein off compared to our math result of .5 grams, which is an 800% error. 800% is a pretty big error, and it is safe to say this error was the highlight of our lab. Instead of moping over our results, a much wiser choice would be to go through our lab and list the multiple sources of error. At first glance, it may seem very strange that our measured protein was greater than the calculated amount of protein. How is this possible, if protein was detected in the filtered milk graduated cylinder via the biuret? Logically, we should have calculated less protein. The reason for this strange error comes from the presence of excess water in our filter paper. Our original filter paper was taking too long to filter the milk, so we decided to use a coffee filter. We let the coffee filter filter the milk overnight, but in the morning there was still a considerable amount of liquid left over in the filter:

 

 

As you can see above, there is a considerable amount of liquid still left in the filter paper. So, when we weighed the "mass of dry paper and protein" we were actually weighing the "mass of dry paper and protein sludge." The mass of the liquid gained dwarfed the mass of the solid protein lost through the filter paper into the biuret graduated cylinder; this is why our protein amount was so high. The main error stemmed from liquid mass. A way to correct this would be to let the protein dry completely before weighing it. However, because this was our second time filtering, we had no time to let the filter paper dry completely. Other minor errors include scale zeroing problems and too much milk measured, both of which can be easily fixed with more diligence. After explaining these errors, it is easy to state that our results can easily be refuted.

 

 

CONCLUSION

Although our experiment wasn't prefect, we were still able to finally finish the lab! The purpose of this lab was to ultimately see whether or not the amount of protein that we found within our lab matched with or at least came close to the amount said on the milk carton. As a result we still found denatured proteins within the beaker after being filtered through with the help of the biuret to locate these proteins. According to our math, we found that there were less proteins present from our lab results than what was stated in the label on the milk carton.

REFERENCES

http://www.milkfacts.info/Milk%20Composition/Protein.htm

Why does milk curdle smell bad? & What causes milk to curdle?

http://foodreference.about.com/od/Dairy/a/Why-Does-Milk-Curdle.htm

What are caseins? - The proteins found in milk

http://ansci.illinois.edu/static/ansc438/Milkcompsynth/milkcomp_protein.html

Procedures of how to make yogurt!

http://milkfacts.info/Milk%20Processing/Yogurt%20Production.htm

1. Milk ingredients are mixed together 

2. The milk is pasteurized at high temperatures to denature the why proteins to form a gel-like consistency 

3. The yogurt is then homogenized to mix all ingredients 

4. The milk is then cooled 

5. Starter cultures are put into the yogurt 

6. Then it is held at a certain temperature until the pH reaches to 4.5 

7. After cooled, fun flavors are added for better taste and then packaged to stores for consumers! :D