When we talked about making pavlovas last week, one thing we didn't really get into was the best kind of egg whites to use for the meringue. Fresh or old, room-temperature or cold - does it make a difference? You bet it does!
Although they stay edible for quite some time, eggs begin aging as soon as they're laid. Both the white and the yolk start off slightly acidic and with their chemical structures and membranes fully intact. In the white, the proteins are tightly folded and tend to cluster. As it ages, the egg - and especially the white - becomes more alkaline. This causes the proteins in the white to repel each other rather than hold together, making the egg white runny. The proteins are still folded, but tend to be looser.
This has consequences when whipping up egg whites for meringues and soufflées, some good and some not so good. Foam is made when the mechanical action of whisking the whites forces the individual proteins to unfold and then re-combine in a new structure around the air bubbles.
When the egg whites are fresh, it takes more time and more force to whip those tight proteins into a foam. However, the foam that is created ends up more stable with small, strong, uniform bubbles. Cold temperatures also help keep the proteins rigid and stable.
On the other hand, whites foam up much more easily and with greater volume when they're older, but the resulting foam has larger bubbles and a less stable structure. If not used right away, the foam will become runny and begin collapsing. Eggs whipped at room temperature will get the same result.
We recommend using fresh, cold egg whites for meringues. They make a foam that is easier to work with, and the baked meringues have a more delicate and uniform texture. Since most of us are whipping egg whites with a mixer instead of by hand, it's not such a big deal if it takes a little longer to whip them into a foam.
That said, older and room temperature whites will still make a perfectly usable foam. If that's all you have or if you're whipping by hand, this is a fine option.
Do you have a preference for the kind of egg white used for meringues?