Ask A Scientist© Chemistry Archive

name        N Alexander C.
status      other
age         18

Question -  Hi
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?
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N Alexander,

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.

Regards,
ProfHoff
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You isolated the protein called casein.  Adding vinegar to the milk caused
the milk to "curdle", that is, the casein, because if its mix of surface
functional groups, is not as soluble in acid as in neutral conditions.  So,
it "precipitated" out of the milk.

This material was used to makle aircraft windows in World War II, before
there were many synthetic plastics available.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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You precipitated the milk protein by addition of vinegar. Letting is stand
for a couple of days evaporated a significant amount of water from the curd.
The milk protein, having a high molecular weight, forms a plastic-like
solid.

The same result should occur if you start with non-fat cottage cheese, which
is just the milk protein minus fat coagulated with acid, usually HCl, washed
free of the acid.

Vince Calder
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