So
what is it milk that lets us make plastic from it? Well milk is
a very complex mixture of water, fat, proteins, vitamins, minerals,
acids, enzymes and gases. 80% of the total milk protein is casein
and it is this component of milk that we separated out in this experiment.
Most casein proteins exist in milk as casein micelles (blobs
of casein separate from the water- similar to how oil forms droplets
in water). Because the sole purpose of milk is nutrition, the
function of the casein micelle is to carry large amounts of protein
to the stomach of the young animal where it clots to allow more
easy digestion. If a baby has ever been sick on you, you may have
noticed that although they only drink liquids, they puke up solids.
This is because babies produce an enzyme called Rennet that aids
the coagulation of casein in the stomach, the acidic stomach contents
also helps with this. Both of these factors act to break up the
micelles, leading to aggregation of the casein protein into milk
curds.
Caseins are fairly insoluble in water, hence the formation of
micelles, and are very insoluble at acid pH. This is how we managed
to separate the casein from the milk in this experiment. Heat
and acid are two of the factors that can lead to the aggregation
of casein - we heated the milk and added acidic vinegar at the
same time. The lumps you saw separating from the milk were aggregated
casein or milk curds. This is a natural plastic as the casein
molecules are associated together in long chains. It wouldn't
be much use to make plastic objects as it would be too expensive
to produce in very large quantities but casein is used to produce
some glues.
