Prompted by a comment, I’ve been researching what happens when creamed butter and sugar become curdled with the addition of the eggs when making a cake. There are four things to think about here: why does it happen, does it matter if it happens, what can I do to prevent it happening, and if it does happen, can I fix it? I’ll take them in turn.
Why does creamed butter and sugar curdle when eggs are added?
It helps first to understand the point of creaming and what’s happening when this is done. Creaming, according to Paula Figoni, incorporates air bubbles into plastic fats (ie into the butter). The tiny fat crystals surround the air bubbles and keep them from popping. These air bubbles then help to leaven the cake.
When eggs are added to creamed butter and sugar they act as an emulsifier (if added correctly). They stabilize the creamed mixture, and then help it to blend with the other ingredients. What you’re creating here by creaming and then adding egg is a water-in-oil emulsion. There’s a great and detailed explanation of this at the excellent Using Mainly Spoons blog, with useful pictures. They wisely suggest thinking of the process of adding eggs to butter and sugar as like the stage of adding oil to eggs when making mayonnaise. I’ll just quote them as they explain it so well:
when you start to add the eggs, you are aiming for little droplets of the water from the eggs, suspended through the fat-and-sugar mixture that is already there. At some point, the liquid from the eggs can overwhelm the amount of fat, causing the bubbles of water to all join up and become the main part of the mixture – the continuous phase, as it’s called.
To prevent this happening, you need to ensure that the fat and sugar are able to hold as much liquid as possible – which means soft, but not melted. You also need to add the egg very gradually, so that it doesn’t overwhelm the mixture. This is the same principle as adding oil to mayonnaise – go slowly and incorporate each bit before you add some more.
Crafty Baking have even more detail on what’s going on when you’re creating the butter / sugar / egg emulsion.
Does it matter if my butter, sugar and egg mixture is curdled?
I’ve read and heard that it doesn’t matter, but I’m inclined to believe that it does. Figoni suggests that
A poorly emulsified batter bakes into a cake that may not rise properly and that has a coarser crumb.
Delia Smith, doyenne of British cookery, agrees, explaining that the curdling causes the mixture to ‘break up’ and this means that
some of the air incorporated at the creaming stage will escape and the finished cake will be slightly heavier.
Crafty Baking are a little more doomy, but I think they’re referring to really bad curdling when they say the result could be
a baked cake that is grainy or flat in texture, dry and flavorless, look uneven and may even sink.
How can I prevent curdling?
Curdling by adding eggs can happen for two reasons. First, the eggs are too cold, and this breaks the emulsion of butter and sugar. Second, the eggs are added too quickly, again breaking the emulsion. The result looks like little lumps of butter and sugar in liquid. Adding mix can also curdle a mixture. There are a few things, therefore, that you can do to avoid curdling:
Tip 1: Use eggs that are the same temperature as the butter and sugar (between 18 and 21C ideally for everything).
That said, Crafty Baking says that in fact you can use cold eggs if you’re using a stand mixer, and that even if it curdles a little, it will sort itself out as you beat, just go slowly. This matches up with my experience.
Tip 2: Gently whisk the egg yolk and white together before adding
Tip 3: Add slowly, in a steady stream.
If you’ve ever made mayonnaise, it’s the same approach (for the same reasons – maintaining the emulsion). The liquid in the eggs doesn’t naturally want to mix with the fat in the butter, but if you go slow, it will be fine.
Tip 4: Add milk and flour alternately. Don’t dump in all the milk, mix and then add all the flour.
Once the eggs are safely in, beat for a minute or two and the mixture will become even more smooth and fluffy, full of lovely, tiny bubbles ready to expand in the oven and make your cake light and delicious.
If all this fails, take heart from Delia, who says ‘the cake won’t be as light but it’s not a disaster’. And as the next section explains, all is not lost – it can be remedied.
What can I do if my mixture has curdled?
Add a little flour and beat again. This can help bind the mixture back together. Or, once you begin to add the flour and milk, it will sort itself out naturally.