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We can make meringue by whisking egg whites with sugar- this can be piped out and baked as it is; or used to make macarons, cake layers, souffles, sponges, mousses, and many other products. It is the proteins contained within egg white that give it the ability to form a stable foam- the main protein being ovalbumin.
The proteins, when whisked, begin to unfold and so are able to move freely throughout the liquid. As we whisk the egg whites, we also incorporate air bubbles and the unfolded proteins move to surround the surface of these bubbles. Once there, the proteins aggregate or bond with each other forming a network that is strong but flexible. This then strengthens the air bubbles, preventing them from collapsing, and hence a foam is formed.
We can stable a meringue further by bearing in mind the following factors:
1. Sugar – adding sugar prevents over whisking as it slows down the unfolding and aggregation of protein strands. It also dissolves once added to the white and forms a thick syrup which coats the air bubbles protecting them from collapsing. This is why it is so important to add the sugar gradually so that there is time for it to dissolve, otherwise the sugar crystals weigh down the foam or prevent the whites from whisking and forming a foam alt all.
2. Fats – Ever wondered why even the tiniest amount of egg yolk can prevent an egg white from whisking into a foam? That’s because of the fats and emulsifiers it contains- particularly lecithin. What they do is coat the proteins, preventing them from unfolding and bonding. Not only this, they also compete with the proteins and surround air bubbles. However, unlike proteins they cannot form a strong network and so any bubbles that form only then collapse once they begin to expand.
3. Acid – Cream of tartar is an acid often added to egg white when making meringue, but we can also use lemon juice or vinegar. These lower the pH which helps stabilize the meringue. The optimum pH is between 4.6 and 4.8, and the best results come from adding the acid early on.
4. Temperature – Egg whites whisked at a slightly higher temperature, ideally room temperature (21 degrees centigrade) will produce a foam with a higher volume.
5. Thickness – Older eggs will have a thinner white and will whisk to a higher volume, however fresh eggs with a thicker white make a more stable foam. So it really depends on what is more important in the recipe we are using- volume or stability.
6. Whipping time – If we whisk eggs for too long, the film that surrounds each air bubble stretches and becomes rigid. Eventually the foam collapses and looks as if it has curdled. This is because the proteins separate out into clumps which float about in the remaining liquid.
7. Equipment – The type of materials we use affect stability. For example, a copper bowl will release tiny particles of copper which improve the flexibility of the protein network, and will also give the meringue a slightly golden colour.
So there we have it, seven factors that affect the stability of an egg white foam, keep note of these next time you’re baking to achieve the perfect meringue!