Science Buddies: "Ask an Expert"

My daughter Lyndie is in 4th grade and for her science project we turned milk into plastic using vinegar.
They are required to do 10 trials of only 2 dependant variables. We made 10 plastic pieces using 1 cup whole milk and 2 teaspoons vinegar. Then we did 10 trials of making plastic pieces with 1 cup whole milk and 8 teaspoons vinegar. After allowing all 20 pieces to dry thoroughly, we weighed them all to the nearest 10th gram. When we average all the results, the 10 pieces made with the 2 teaspoons vinegar weigh an average of 1 gram more than the average of the 10 pieces made with 8 teaspoons vinegar. I am thoroughly confused as I thought More acid (vinegar) would cause the protien molecules of milk to unfold more and form more chains making more plastic than the smaller amount of acid (vinegar). I have been all over the internet trying to find out why our result seems to be opposite of what was supposed to happen. Can the 8 teaspoons of acid (vinegar) cause the milk to become TOO acid, therefore not allowing the chains to form? Does the amount of vinegar actually matter? (Meaning any amount of acid (vinegar) forms the plastic?) I cannot find any information on the internet as to if the milk can become TOO acid. Or if a smaller amount of acid works better by not causing the milk to be too acidic? I would appreciate any help you could give. My daughter has worked very hard on her project and is very proud of her plastic pieces. For her science project we have to explain the results and we are completely at a loss as to how to explain our results. Thank you so much for your time and help, Mary [contact information removed by administrator]

Here is a quote from the background information:

"How much vinegar is needed to give you the most plastic? Without enough vinegar the casein molecules do not unfold well, making it difficult for them to link together into a polymer. Of course, if you were manufacturing you would be thinking about both the amount of plastic you can make and the cost. The more of any ingredient you use the more expensive the end product is. The "best" recipe will have the highest yield (make the most plastic) for the smallest amount of vinegar."

Each casein molecule needs a given amount of vinegar to unfold into plastic, so I agree with you. The expected result is that more vinegar should yield more plastic. Then once each molecule is paired up. The excess vinegar is just wasted.

Could it be that some other variable was not controlled? milk temperature? milk volume? The instructions suggest at least four vinegar concentrations: 1, 2, 4, and 8 teaspoons.

I would suggest to try the other two concentrations. It may be that with the full data set you will see that one of the original two points that you have is outside a trend. This is fine and a big part of science. Understanding un-expected results.

As I re-read your note I see that you write: "... 10 trials of only 2 dependent variables...".

Did you identify your dependent variables? vinegar concentration is an independent variable, i.e. one that is changed, and as I say, is best to vary this variable in more than two steps.

Hope Lyndie got to do something fun or artistic with the plastic.

Good luck