Because the 3D structure of proteins is caused by many different,
sometimes very weak interactions, proteins are very stress-sensitive.
Their structure in its native state can be distroyed - we say denatured -
by temperature, pH, removal of water, presence of hydrophobic surfaces,
presence of metal ions and high shear, etc. When a protein is
denatutred its complex 3D structure unfolds and takes on another shape. Denatured proteins
have very different properties from their original native state. Heat
e.g causes the egg white’s proteins to denature and coagulate, resulting
in the formation of a solid mass. If an acid, like lemon juice or
vinegar is added to milk, the protein casein denatures and coagulates,
forming curds.
In food processing denaturation can be desired, as in the examples
above, or as described in the Maillard reaction, where proteins react
with carbons if exposed to heat and produce a delicious crust on baked
or fried goods.
In some cases however, denaturation can also lead to unwanted changes
in texture and flavor. Frozen eggs or dairy products e.g. become lumpy
and unpalatable when thawed as a result of denatured proteins.
Understanding the science behind protein denaturation is therefore
important for successfull food processing.
While the tertiary and secondary structure of proteins, which depends
on hydrogen or ionic bonds and hydrophobic interactions, is broken up
during denaturation, the primary structure, which is made up of covalent
peptide bonds, is not altered. These peptide bonds therefore play an
important role in protein stability. When the protein is denatured, it
unfolds, but the amino acid sequence remains the same. This leads to the
fact, that denatured proteins can in some cases return from their
denautred, unfolded state back to their original state. This usually
depends on the type and intensity of the denaturing stress. An eggwhite
e.g. gets temporarily denatured when whipped into egg white foam, but
liquifies again if left to stand for a while (reversibly denatured).
An egg white that has been denatured by hot temperatures during
boiling, coagulates by coiling up, a state that cannot be reversed (irreversibly denatured).
The egg white foam, even if it liquifies again, will not return 100% to
its original state. Part of it will remain denautered. Beating an egg
white foam a second time therefore never works as well as the first
time.
