On Food and Cooking

Fresh out of the refrigerator, the various yolk
particles move only sluggishly and don’t coat
the fat droplets as quickly and completely.
When yolks are cooked, the proteins unfold
and coagulate, thus ending their usefulness as
flexible surface coatings. Hard-cooked yolks
are sometimes used instead of raw yolks to
make emulsified sauces; their disadvantage is
that the proteins have been coagulated in
place and phospholipids probably trapped in
the coagulated particles, so they have far less
emulsifying power, and the yolk texture can
give a subtle graininess.
And egg whites? They’re a less
concentrated source of protein, and designed
for a fat-free, watery environment, and
therefore of little help in coating fat droplets.
However, the white proteins provide some
viscosity thanks to their large size and loose
associations with each other, so they have
some value as emulsion stabilizers.