An egg yolk follows a different set of temperatures:- At 145°F: The yolk proteins begin to denature, thickening
the liquid yolk. - At 158°F: The egg yolk is firm, able to hold its shape and
to be cut with a fork or knife. Its appearance is still dark
and translucent, with an almost fudge-like texture. - Between 158° and 170°F: The yolk becomes firmer and
firmer until eventually it suddenly shifts from translucent
and fudge-like to pale yellow and crumbly as tiny
spherical chambers invisible to the naked eye separate
from each other. - Above 170°F: The yolk becomes increasingly crumbly as
the temperature goes up. The sulfur in the white rapidly
reacts with the iron in the yolk, creating ferrous sulfide,
tingeing the outside of the yolk an unattractive green.
Boiling eggs is all about balancing the differences
between the way the whites and the yolks cook.
SOFT-BOILED EGGS
For me, the ideal soft-boiled egg has a white that’s
completely opaque, but not to the point of rubberiness
(somewhere in the range of 155° to 180°F), and a yolk
that’s pretty much 100-percent liquid (no hotter than 158°F).
In this way, with each spoonful, you get tender bites of soft,
velvety-smooth white bathed in a sauce of glorious, bright
golden, rich, flavorful yolk.