Re: what is the chemical formula of heated milk and vinegar to make plastic?

Brian,
This is a great question, however, I'm not sure it has an answer.  Let me 
explain:  Milk has 3 main ingredients: lactose (a sugar), protiens and fats.  
Skim milk, if that's what you used, only has lactose and protiens.
Given the stucture for lactose and acetic acid (vinegar) and what I know 
of organic chemistry, I can't find a way the two would react to form a 
plastic.  So, my conclusion is that the vinegar is reacting with the 
protiens.  Proteins are polymers of amino acids, and can differ greatly in 
structure.  Since there are many different proteins in milk, there is no 
way to predict what reactions will occur without very sophisticated 
instruments.  My hypothesis is that your plastic is actually a mixture of 
several different polymer structures, none of which can be named at this 
time.

Of course, I may not be right. The source your teacher used may have more 
information.   Have you tried contacting the authors of the experiment?  
If they insist there is an answer, they probably know it!  I did find the 
experiment posted on the web at a site developed by polymer scientists.  They 
might be able to help.  Here's the address:  http://www.psrc.usm.edu/macrog/demos/index.htm
It's under the K-4 demos.  it's called mooglue.

If you still aren't satisfied, you can always try re-entering the question 
to Madsci and someone else will take a stab at it.  Sorry not to give you a 
definate answer, but I hope this answers some questions anyway.
Organic Jen

Admin note:
The protein component that forms the "plastic" is called casein.  For more
on this, check out the following previous answers:

Making Plastic from Milk?

Making plastic from milk and vinegar

What would be a good lab experiment with polymers?