Science Buddies: "Ask an Expert"

My science fair topic is: How does the amount of distilled white vinegar added to whole milk affect the yield of casein plastic? I have tested using 10, 15, 20, 25 and 30 ml of vinegar to 250 ml of warmed whole milk. My test results showed 15 ml to yield the most. At 25 and 30 ml, I did not get as much casein plastic. It was more crumbly than the other tests resembling little pieces of rice. It was also harder to stick together into a ball. What happened? I thought that more vinegar would get more plastic. Does too much vinegar break down the new bonds that have formed? Thank you for any help you can give me!

Hi Chrisboo,

I'm glad to see you investigating science this way. This is a great experiment!

I must apologize that I am not a chemist. I am a Mechanical Engineer. I am not a specialist in polymer chemistry, which this experiment is teaching, but I do know the basics of chemistry, and I do good research on the internet. And I did not want your question to go unanswered any longer.

What is happening here is that the vinegar is an acid. This means it has free hydrogen atoms available to attach to something else. The milk contains casein. This is a protein chain that contains some special carbon and oxygen atom chains, called molecules. These molecule chains really like to attach themselves to hydrogen atoms. When you put the vinegar in with the milk you give those carbon and oxygen chains the chance to combine with the free hydrogen atoms in the vinegar. Once they have all combined, it makes no difference if there are more hydrogen atoms available, there just aren't any more carbon and oxygen molecules left to attract the hydrogen. So, once you have put in "enough" vinegar, putting in more won't change anything (except make the solution more acidic and smell worse). You won't get more casein plastic.

One thing you might try to gain a better understanding of this is to use vinegars of different acidic concentrations. Normal cooking vinegar is in the range of 4% to 8% acid, while pickling, or canning vinegar is up as high as 18%. You will be able to see this on the label of the product. If you experiment with different concentrations, you will discover how the optimum amount of vinegar needed to create casein plastic changes.

I hope this helps. Keep up the good work and...

Have fun with science!

Thanks Mr. Neu! This makes a lot of sense from looking at my research. Your explanation was very clear and helpful. I think I'm going to try to find a different concentration of vinegar and extend my experiment like you suggested. Thanks for taking the time to help me!

I'm an Electrical Engineer and not a bio-chemist either; however, the previous answer doesn't take into account some factors that Wikipedia has on casein and vinegar:
1) Whole cow milk is typically pH 6.6.
2) The casein in cow milk has an isoelectric point of 4.6
3) Apple cider vinegar is typically between pH 4.5 and 5.0
4) "Distilled vinegar 5 to 8% is typically pH 2.4

Some components of cow milk will act as a pH buffer which means the initial addition of small amounts of an acid will not change the pH of the solution. casein is one component that definitely will utilize free H+ ions and is likely one of the pH buffer components of milk.

Should you change the pH of the solution (milk plus vinegar) below 4.6, the state of the casein will definitely change dramatically.
Your observations of crumbly casein at 25 and 30ml of vinegar may correlate to a pH below 4.5.

So precisely what vinegar did you use? Do you know its pH or the concentration of acetic acid?
How much milk did you use? Did you use whole milk? 1 and 2% milk does not have as much casein as whole milk.

Without knowing both the quantity and pH of the starting components, it is impossible to predict the range of pH that might result and/or how many hydrogen atoms that were available to be absorbed by the buffer components of milk.

Thanks for your information, Craig. I did use distilled white vinegar (5%) and I added it to 250 ml of whole milk warmed to 49 degrees centigrade. I kept the amount of milk constant for all of my tests.

Hi Chrisboo,

Ed Neu and Craig Bridge have given an excellent explanation of the chemistry for your project. Here is some additional information that should be helpful:

It sounds like you are doing this excellent project on the Science Buddies website:

https://www.sciencebuddies.org/science- ... p101.shtml

Proteins such as the casein found in milk are composed of amino acids that may have both positive and negative charges. The isoelectric point of a protein is the pH at which all of the charged groups have an overall net neutral charge. In general, proteins are more soluble at a pH above or below their isoelectic point and least soluble at the isoelectic point.

Craig had provided the key information for explaining your results. The isolectric point of casein is 4.6. The pH of milk is about 6.8. The pH of vinegar is about 2.4 to 3.4, depending on the concentration. So the casein is very soluble in milk because the pH is above the isoelectric point; the vinegar is used to lower the pH to the isoelectic point to precipitate it. If additional vinegar is added to lower the pH below the isoelectic point, then the casein starts to dissolve again.

If you repeated your experiment and measured the pH of the samples, the 15 ml sample would probably be closest to pH 4.6. You can find more information on this by doing internet searches for isoelectic point, casein, amino acids, and vinegar/acetic acid.

I’m sure the Science Fair judges will be impressed if you can explain what happened in your experiment on your project board, so do let us know if you have any more questions.

Donna Hardy

Thank you Ms. Hardy. That was very interesting and helpful! I will do some research on the isoelectric point of casein. I did repeat my experiment 4 times with approximately the same results: 15 ml of vinegar made the most casein plastic.

Thank you for taking the time to write!

Chris