Why are eggs so useful in baking, and are there any substitutes?
Aquafaba is made from the viscous leftover from cooking (or soaking) chickpeas, and it's a vegan substitute for eggs.
Eggs contain over 40 different proteins, which hold the key to their culinary success. The long protein molecules fold and coil so that water-hating (hydrophobic) sections are tucked safely away. Loose bonds hold each protein molecule in a tight unit, but heat or whisking causes the chains to unwind or denature. This allows different proteins to link their hydrophobic regions together and form a strengthening 3D network. The coagulation is irreversible and transforms eggs into a semi-solid or solid state.
Whisking egg whites unravels proteins and adds air bubbles. The proteins link together and collect at the surface of the bubbles to hide their newly exposed hydrophobic parts from water in the egg white. The proteins prevent the air bubbles from popping, even during cooking. In baking, egg proteins combine with wheat proteins in flour to form a strong network of trapped air bubbles that can expand in the oven.
Meanwhile, egg yolk contains an excellent emulsifier called lecithin. Emulsifiers keep oil droplets dispersed in water, or water droplets in oil, preventing them from separating.
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It’s hard to find a total substitute for eggs. Egg emulsifiers are perhaps the easiest to replace. Plant-derived lecithins such as soya lecithin make good substitutes. Flax or chia seeds, bananas or mustard can also be used to stabilise emulsion droplets. Eggs are harder to replace as thickeners, although ground flax or chia seeds work quite well, especially in cookies or muffins.
What is aquafaba?
Replacing eggs’ foaming ability is tricky. This is where aquafaba comes in. Aquafaba is a temperature-resistant foamer made from the water in which legumes such as chickpeas have been cooked. The liquid contains enough protein, starch and fibre to work in tricky applications such as egg-free meringues.
To obtain aquafaba, either use the liquid in canned chickpeas or boil your own and collect the cooking water.
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Asked by: Claire Hill
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Dr Emma Davies is a science writer and editor with a PhD in food chemistry from the University of Leeds. She writes about all aspects of chemistry, from food and the environment to toxicology and regulatory science.