MadSci Network: Biochemistry

Re: making plastic from milk and vinegar

Date: Fri May 21 13:26:31 1999
Posted By: Carl Custer, Staff, Office Public Health & Science, Scientific Research Oversight Staff , USDA FSIS OPHS
Area of science: Biochemistry
ID: 925946604.Bc

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 
	[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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