In order to prevent raw egg white from leaving the egg by the prick when boiling pricked eggs or from
any cracks that form in the eggshell during boiling, vinegar is often added to the boiling water. Vinegar
will cause any egg white proteins that leave the egg via the prick or the crack to coagulate sufficiently
rapidly (see above explanation) to form a “plug” in the prick or crack, sealing the rest of the egg inside
The age of the egg affects coagulation temperatures and therefore cooking times. As eggs get older,
carbon dioxide (which is weakly acidic) leaks out through the small pores in the shell, making the egg
white less acidic, so coagulation of the proteins will take longer (less charged H+ ions to act as
bridges) and cooking takes longer.
Eggs are usually poached in water with vinegar. Because vinegar is acidic (it is a solution of many
percent of acetic acid, plus other acids such as malic acid), it contains a high number of positively
charged H+ ions. Dropping an egg in water containing such H+ ions will cause the outside of the egg,
which is in direct contact with the H+ ions, to coagulate and solidify rapidly, which will ensure the egg
stays whole during poaching. As explained above, in addition to reducing repulsion between
denatured proteins, H+ ions help break the bonds holding the folded proteins initially together,
increasing the formation of the denatured proteins.
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The part of the egg white in contact with the yolk tends to stay more liquid during frying an egg as the
rest of the white solidifies, which does not produce a very appetizing egg. To prevent the outside of a
fried egg from becoming too rubbery while these inner parts solidify, it is often suggested to salt the
part of the egg white in contact with the yolk to speed up the coagulation of these parts, reducing
overall cooking time and thus the drying out of the rest of the egg white. The yolk however should not
be salted – it favours the undesired hardening of the yolk.