Above: Lemon meringue tartlet (image © istockphoto.com/alfernec)
Is dessert your favourite part of a meal? Do you sometimes indulge in a piece of lemon meringue pie? Meringue is also used to give mousses, white buttercream frostings, and macarons their light and fluffy textures.
Did you know? Meringue must be fully cooked to avoid the risk of salmonella contamination.If you’ve never made it yourself, you might be surprised to learn that egg whites and a little sugar are what gives meringue its wonderful texture. Let’s have a look at the science behind transforming egg whites into fluffy clouds of tastiness.
Proteins and amino acids
An egg white is about 90% water. The rest is almost all protein, with traces of minerals and vitamins. The cells of all living things contain proteins. For example, skin, muscle, feathers, and egg whites all have proteins in them.
|Figure 1. General structure of an amino acid. The amino group is shown in blue and the carboxyl group in red. A generic R group, which is different in each amino acid, is shown in orange. (Edwin Wong)
Proteins are essentially long chains of hundreds or thousands of molecules called amino acids (Figure 1). There are only 20 common amino acids. However, they can be combined in many different ways to produce proteins that can build anything from the lens of your eye to a spider web to an egg white.
The order of amino acids in a protein cause it to fold up into a blob-like structure, a bit like a ball of yarn. Also, some amino acids are hydrophobic (water-fearing). Instead of dissolving, they clump together in water. Others are hydrophilic (water-loving), which means they are attracted to water. Proteins that usually make their home in water fold up so that most of the hydrophilic amino acids are on the outside of the blob. That way, the hydrophobic amino acids are hidden away inside.
Did you know? One of the most common human allergies is to eggs.When you whisk egg whites, you create foam by introducing air bubbles. At the same time, the wires of the whisk cause some of the proteins to unravel. This is called unfolding or denaturing. It exposes some of the hydrophobic amino acids, which move to the air bubbles to get away from the water in the egg white (Figure 2). As proteins coat the air bubbles, some of the hydrophobic amino acids at the surface react with each other. This results in some of the proteins linking together to form nets, which help stabilize the bubbles and keep them from popping.
Sugar is the other key ingredient in meringue. Adding sugar to foamy egg whites transforms them into a thick and glossy foam that doesn’t disappear when you stop whisking. Food scientists believe that sugar helps more proteins gather on the surface of the air bubbles, making the bubbles even more stable.
|Figure 2. Unfolded proteins coat an air bubble in the egg white, making it more stable. The hydrophilic parts of the protein chains stay in the egg white. The hydrophobic parts move to the surface of the air bubble to escape the water in the egg white.
You need to be careful when separating the egg white from the egg yolk. Yolks contain fat molecules, which are attracted to air bubbles and push some of the proteins away from the surface. But unlike proteins, fat molecules do not help stabilize the air bubbles. Instead, they cause them to pop, making it difficult to make a meringue. Other fats, such as cooking oil and butter, will have the same effect. So it is important to make sure your whisk and bowl are spotlessly clean!
It is also possible to whisk a meringue too much. It will eventually split into a grainy solid and a runny liquid. This happens when too many proteins join together and form a net that is so tight that it starts to squeeze the water out of the egg white.
Did you know? The amino acids found in all living things on Earth are left-handed.You can help prevent over-whisking by using a copper bowl or by adding cream of tartar. Small amounts of copper ions from the bowl will prevent some of the proteins from linking together, so the nets do not become too tight. Cream of tartar is an acid, which also prevents some of the proteins from linking together.
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Now that you know the science behind meringue, why not head into the kitchen and make some yourself? Cooks have many superstitions about how to make the fluffiest meringue. For example, they might insist on cold or room temperature eggs, old or new eggs, or a plastic or stainless steel bowl. Try putting these superstitions to the test and see if any of them make a difference!
Websites with general information on how foam is created from eggs:
Scientific article on research related to making egg white-based foams: