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Chemistry

Meringue

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meringue-rack

A wire rack covered in freshly baked French meringues

Week 6 of the Bakeoff was the never-before-seen botanicals week, devoted to baking with plant life. For their signature bakes the contestants were asked to make citrus meringue pies, and the audience were treated to a great variety of both fruity fillings and styles of meringue. Meringue is a light, fluffy sweet eaten on its own or incorporated into puddings to add sweetness. There are few other foods which resemble the distinctive soft, airy texture of meringue so here we will look at the chemistry used to stabilize this edible foam.
1.          There are three types of meringue – French, Italian and Swiss – and all three featured in this week’s signature challenge. French meringue, probably the most familiar to home bakers, is made simply by beating together egg white and sugar. Italian meringue requires the sugar to be made into a syrup, which needs careful control over the temperature, before being added to the beaten egg whites, and in Swiss meringue the mixture is heated over a pan of boiling water (called a bain marie) and then beaten while it cools.

raspberry-cream-meringue

Raspberry cream meringue

Meringues of many different shapes and sizes can be found in pudding recipes. A large one topped with cream and fresh fruit is a pavlova, while the meringue acts as the topping for lemon meringue pie and queen of puddings. Smaller meringues are used to make the French classic îles flottantes (floating islands) and macarons – leftover meringues can then be broken up and mixed into Eton mess.
As well as making different sorts of meringues, the bakers differed on their approaches to cooking the meringues – some put them into the oven along with the pies, whereas others simply browned the tops of their meringues with a blowtorch. Generally speaking those which had been baked held their shapes more effectively when cut than those which had only been blowtorched, perhaps due to caramelisation or the Maillard reaction between sugar and protein.

2.          Making a foam by trapping air bubbles in water can be achieved by some surprisingly simple chemistry – it relies on the presence of molecules with parts that are hydrophilic (attracted to water) and other parts that are hydrophobic (repelled by water). This will form a foam because the hydrophilic parts will bind to the water but the hydrophobic parts will bind either to each other or to any air bubbles. A familiar example of this is soap – soap molecules have a hydrophilic “head group” and a hydrophobic “tail”. This brings together water and fats or oils, which do not normally mix, by a part of the soap molecule interacting with each component of the mixture to give an emulsion. However, the hydrophobic tails can interact with air bubbles as well as fat molecules and thus soap will form a bubbly lather.

soap-diagram

Diagram showing how soap binds both water and fats

In meringues the foam is stabilized by exploiting the properties of the globular proteins found in egg white. Proteins are made up of amino acids, each of which has a “side chain” which is not part of the main protein backbone, and these side chains may be either hydrophilic or hydrophobic. Ordinarily these globular protein molecules sit in such a shape, or “conformation”, that the hydrophilic residues point outwards, interacting favourably with the water in the egg white, whereas the hydrophobic residues point inwards, interacting only with each other. However, by beating and warming the whites the proteins become “denatured”, changing their conformation, so that some of the hydrophobic residues are exposed. Tiny pockets of air, introduced by whisking, are then trapped by the hydrophobic residues which do not interact favourably with water, so the meringue expands and forms a stable foam.

hydrophilic-hydrophobic-protein

A small protein segment containing hydrophilic and hydrophobic side chains

By considering the examples of soap and egg white, we can see why it is so important to keep your whites completely free of yolk when making meringues. Egg yolk is around 25% fat, so if it gets into the white the hydrophobic side chains of the egg white proteins will bind to the fats rather than air bubbles. This will prevent air from being incorporated into the mixture and making meringue will be impossible.

sliced-meringue

A French meringue cut in half, exposing the fluffy interior

 

The last thing to consider on this subject is the role of the sugar in the meringue other than improving the flavour. Sugars have many hydrophilic hydroxyl (-OH) groups and so interact preferentially with the water in the foam. An effect of this which is useful to bakers is that this encourages water to stay in the foam, rather than seeping out, so the meringue is more stable. In fact the hygroscopic (water-attracting) nature of sugar is why meringues left in humid spaces “weep”, forming water droplets on their surfaces.
In all, the formation of meringues is based on a few basic principles of physical chemistry put to use on a regular basis by bakers everywhere. Those wishing to improve their key lime pies might be well served by this highly informative review paper, as well as consulting a recipe book.

Contributors: Harry Morgan (writing), Anna Morgan (baking, photographs)

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