Foams are a type of colloids very well represented in the kitchen. Can you think about some examples? Yes, meringues! What a better way to recycle albumen?
Actually, whipped egg whites find much more applications than simple meringues: mascarpone creams, mousse, soufflé...But, regardless the final outcome, whipped albumen are still colloids. Dispersing phase? Water. Dispersed phase? Air. Stabilizing agents? Proteins.
We already discussed about albumen in one of the first posts: it is mainly made by water and proteins, and some mineral salts. Unlike yolk, it doesn’t contain fats and cholesterol.
How is it possible to create those frothy, soft meringues starting from simple albumen?
Colloids do not form spontaneously. We always need to provide energy, either mechanically or thermally. What do you usually do to whip egg whites or make mayo? You beat the eggs manually (how brave! :)) or with the aid of a whiskers (for mayo, a blender is often used), right? This mechanical action denatures proteins that change from globular (a kind of ball shape) to a more untangled conformation. The hydrophobic residues (=that have no affinity for water) take contact with air, while the hydrophilic ones with the watery phase. As you keep beating, the proteins will get more and more untangled and will incorporate more air bubbles.
Sugar should be added little by little when the mass is a bit frothy or it will delay the formation of a stable foam.
This frothy system is not stable forever: whipped albumen must be either cooked (soufflé, meringues) or stored in the fridge (cold augments stability, think about a mousse).
What makes this system unstable? In other words: what prevents the liquid albumen to become a soft, frothy delicacy?
Did you notice how hard it is to whip the albumen if a little bit of yolk is present? This happens because fats in the yolk compete with air for the hydrophobic residues of the proteins. Always use clean bowls and avoid plastic.
It denatures proteins. Didn’t we say that, actually, we have to denature proteins in order to obtain a meringues? Yes, but if we add salt proteins denature right away. By beating the albumen, we further denature the proteins that start to coagulate and associate with each other. If that happens you will observe a phase separation (what a news! ;)).
On the other hand, the stabilizing factors are:
Adding citric acid (e.g. lemon) or tartaric acid neutralizes the negatives charges on the proteins (acids releases H+ ions*). In this way, proteins stop repelling each other and associate in stable nets that increase the stability of the foam.
Apparently hydrocolloids as gelatin, pectin, agar agar stabilize the meringue. This substances form nets, which prevent the foam collapsing. Honestly, I have never tried hydrocolloids to stabilize meringues (frankly, neither citric nor tartaric acids), basically because I never had problems with whipping albumen. Anyway, this is an extra aid to whip your albumen.
Using albumen at room temperature helps with the whipping, because it shortens the time required to denature the proteins. You can certainly use cold albumen, but it will take you longer to whip them up.
And with this, I believe I gave a comprehensive background on whipped egg whites. In the next post, I will pass you a recipe, as usual. Not meringues though, I am a kind of sick and tired about them. Instead, I will give you the recipe of a classic French tarte. Let’s see if you guess :)
*this definition is a bit simplified. Actually, there are acidic compounds that do not release H+, but in this specific case we can consider the definition as valid.
W. J. Stadelman, D. Newkirk, L. Newby, Egg science and technology, Fourth Edition, CRC Press
Bovšková et al., Factors influencing egg white foam quality, Czech J. Food Sci., 2011, 29, 322-327