|MadSci Network: Biochemistry|
What I thought would be an easy question turned out to be tough because so much information is missing. It would be nice to know the recipe or procedure you used (only found some dead links on the web) but it's Friday so I'm going to guess; uh make a professional assessment). Adding vinegar to milk will cause it to "curdle". The curd is the casein (the primary protein in milk) that has precipitated i.e., it's come out of solution. [Most proteins are pretty big molecules so to stay in solution, they have a lot of water loving handles (hydrophillic moieties) sticking out. When you radically change the pH (acidity) or mess up the shape of the protein (such as with heating), enough handles get changed so they can't hold the protein in solution and it precipitates - imagine a caterpillar holding to a wall and it's legs beginning to slip - when enough go; it falls.] Precipitated casein, when dried, is pretty tough stuff. That is what I'm guessing your "plastic" is. An example of such an edible plastic is Parmesan cheese. Take a look at a block, not the grated stuff. Cheese is made from precipitated casein that has first been snipped by an enzyme called rennet. Casein is also used to make casein glue and "milk paint", a traditional but durable mix of milk and lime. Why didn't the goat milk separate and make plastic? Well, I don't know for sure. I suspect the amount of vinegar was too little to precipitate the goat's milk. More should have worked. However, there is the possibility that the goats milk casein had already been partially broken down by heating or proteolysis and thus, was small enough that it would not precipitate.
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