Name: N Alexander C.
Recently I warmed up some milk in a saucer and then poured some vinegar in
it. I filtered the fat out of it and shaped it into a ball. I let it sit
for a couple of days and it turned into plastic.
Will you explain what reaction took place and why?
You precipitated the casein (a protein) out of the milk by effect of the acid
(vinegar) and heat. Acids denature and coagulate proteins. The plastic-like
substance you obtained was very likely rather sticky. In fact, in bygone days,
a reaction such as you describe was a common and a popular means to produce an
early form of glue -- now supplanted by good old Elmer's carpenter glue which
is very different in composition and far superior to casein glue.
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Update: June 2012