Baking is chemistry. Tasty, tasty chemistry. One of the things I love most about baking (apart from eating the end results, obvs), is watching the transformation of ingredients from their starting appearances to their finished product.; the ways in which interacting with each other changes their properties, and how adding heat can completely change the taste and texture of them. One of the most dramatic, beautiful and tasty transformations in baking is that of liquid egg whites into crunchy, chewy, glorious meringue.
Egg white is a gloopy mixture of proteins and water. Proteins are made up of individual amino acids, likes beads on a string, forming a long chain. This chain is folded, like a long beaded necklace jumbled in a pile, except proteins aren’t folded randomly like my messy jewellery, instead they fold in order. Some of the amino acid building blocks are hydrophilic, they love to be near and interact with water molecules, and others are hydrophobic, they are repelled by water. When proteins are in a water-based environment, like egg white, they fold in such a way that the hydrophobic amino acids are curled up inside, hidden safely away from the surrounding water.
When you whisk egg whites, you introduce huge quantities of air to the proceedings, as well as battering the proteins around a bit. Bubbly pockets of air are forced into the egg white gloop, and these pockets allow the proteins to unfold, helped along by the physical battering they’re receiving from you wielding your whisk. The tentative water-shy hydrophobic core of the protein uncurls and the previously hidden amino acids expose themselves fearlessly to the air, whilst the hydrophilic bits continue to make love to water molecules.
Once the proteins are unfolded, they are able to see each other properly for the first time, sections that were hidden away have faced the air, and are able to interact with other nearby proteins, forming lifelong bonds that create a network of protein, a protein scaffold, which traps the air and allowing the mixture to expand into a big fluffy pile of white. Some recipes suggest that you add a small amount of lemon juice or vinegar at this point; the acid slows down the process of the proteins bonding, which allows more air to be forced in before the network is locked into place, as it were.
The next step in making a gorgeous meringue is to add sugar to the mixture. Sugar doesn’t simply make the meringue taste far more delicious, it also allows more air to be incorporated to the mixture. It dissolves into the water and interacts with the proteins, preventing them from stretching too far and collapsing on themselves, leaving you with a sad flat meringue (although I have ended up with sad flat meringue before, and it does still taste pretty awesome). Adding sugar too soon can also lead to sad flat meringue, if air hasn’t already been incorporated, the sugar just prevents the proteins from unfolding and interacting properly. If you add it once plenty of lovely air is already there, however, sugar does strengthen the mix, allowing more air and giving you the chance to create huge, stiff, fluffy mountainous peaks of meringue.
The final stage in the making of a meringue is the baking. Heating causes the air in the pockets to expand, the water to evaporate, and the protein structure to solidify. If you heat the meringue too quickly, the sugar on the surface browns before all the water has had a chance to evaporate, leaving you with a soggy middle. However, if you heat it at a lower temperature, the process occurs evenly throughout, leaving you with the perfect crunchy-chewy meringue. Yet another reason to truly love science.