When adding eggs into creamed butter and sugar mix for a rich cake like a fruit cake, how will it affect the cake if the mixture curdles? Does it make the cake more dense if it curdles? I am a bit of an egg fan, so often if the recipe like fruit cake or egg tart calls for five eggs, I’ll add seven, just in case. Why does it curdle, anyway?
Firstly, I must say, as a recipe and cookbook writer, that if you are making a baking recipe that asks for five eggs, then please do not use seven. Baking and patisserie recipes are precise for good reason, and that’s because baking is the most scientific of all cooking procedures. If you add an extra clove of garlic or another sliced tomato to a savoury recipe nothing bad will happen, the flavour will be altered and that’s probably okay. However, add another few eggs, a cup of flour or a tablespoon of baking soda to a cake and you are courting disaster. The eggs contains protein and lecithin, among other things. Extra flour will thicken and make a dish stodgy, and the baking soda will help raise a cake if used judiciously but it’ll also make the cake taste soapy and unpleasant. So, before I address your question — please do not ever change a baking recipe until you’ve made the writer’s version initially. Once you’ve done that you can play around a little but not before. Okay, now that’s out of the way, I can deal with the rest of your query.
Curds. They’re not always bad and, in fact, without them we wouldn’t have cheese. In cheesemaking a "curdling" agent is added to milk and cream. The agent causes the proteins in the milk to curdle, creating lumps of "fat", from which we discard the whey and transform the curds into cheese. When you curdle an egg-based sauce such as hollandaise, the curdling happens when the egg mixture is overheated and the protein in the eggs sets, making it unable to absorb the butter that is added to it, no matter how slowly. The sauce is ruined unless it’s reintroduced into a new mother sauce where it is slowly added, over a lower heat, to more eggs. In drastic situations you can blend the mixture into raw egg yolks but this doesn’t always guarantee success.
In the recipe you’re describing, the "curdling" of the eggs into the butter mixture isn’t really a curdling. What generally happens is that butter sets at room temperature and below (think of a block of butter on your bench in winter) and causes the fat to lump together. Adding eggs (even though they contain lecithin, which works to emulsify fats and water) to a buttery mix simply cools the mixture down and suddenly the batter looks like it’s curdling. It’s not at all, it’s simply the reaction of a cold buttery mixture setting, having liquid added to it causing the fat to behave like ... fat.
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Add some flour and other dry ingredients and the mixture comes back together and forms a batter that, when baked, creates a deliciously soft crumbed cake or a heavy fruit cake. So, don’t worry too much about the cake batter curdling, as it isn’t doing that at all.
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