denature, exposing their hydrophobic and hydrophilic parts. The proteins can then place themselves at
the air-water interface, because their hydrophilic parts stay in contact with the water, while their
hydrophobic parts, which do not like to contact water, tend to place themselves in contact with the air.
Blood is full of many proteins, and both blood and egg yolks can be used as thickening agents. Gentle
heating will denature the proteins, and they will unroll into their long strands. These strands can then
prevent the water molecules present from easily flowing around each other, in a similar way to starch
molecules, and the liquid will thicken.
Proteins denature at various different temperatures – thus knowledge and understanding of the
denaturation temperatures of various different proteins (i.e. egg proteins, meat proteins) can be very
useful in cooking.
If the mixture is heated further after denaturation, the added heat will cause the denatured proteins to
move around much more quickly. These unfolded protein chains will be attracted to each other if they
contact each other, and will form bonds between their chains at particular hydrophobic or hydrophilic
regions where they are attracted to each other.
As protein strands join together, a protein network is formed. This is known as coagulation. This
process is also responsible for the loss of transparency of raw meat, fish and eggs on heating them –
the network formed will not let the light pass through the strands as easily as when the protein bundles
were separated, so transparency is reduced.
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This network will trap the water molecules that were originally located between the unravelled protein
strands, reducing the fluidity of the mixture, and eventually forming what is known as a “gel”.
Uses of coagulation:
Coagulation can be both useful or a real nuisance in the kitchen. A custard becomes lumpy because
the egg proteins have been heated to too high a temperature and the denatured proteins have begun
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