Curdled butter and sugar mixtures

Q: Why do creamed butter and sugar mixtures often curdle when eggs are added?

A: A badly curdled cake or pudding base can result in a heavy, coarse texture, but it can be avoided if you understand why it is happening. Curdling is directly linked with how the butter and eggs interact when they come together. When you are beating eggs into a butter mixture you are basically creating an emulsion – ultimately the fat in the butter is brought together with the water in the eggs with the help of the emulsifying ability of egg yolks (it's kind of like making mayonnaise but in reverse). When a mixture appears to have curdled, this emulsion has split.

There are three main reasons why a creamed mixture will curdle when the eggs are added:

1. The eggs are added too quickly. An emulsion is most successfully formed when you introduce the two opposing ingredients gradually. In this instance, if the water (in the eggs) is added too quickly to the fat (the butter) the emulsion doesn’t have time to form and stabilise before more water is added and therefore it will split. To help prevent this from happening, only add eggs one at a time and make sure you beat well after each addition until the emulsion is well established and the mixture is creamy before adding the next one.
2. The eggs are too cold. If eggs are colder that the temperature of the butter mixture when added, they can cool the butter down and cause it to become more solid. This will have the same effect on the emulsion as if the eggs are added too quickly and will also cause the emulsion to split. So always make sure your eggs are at room temperature when you add them to your cake batters. Beating the batter at this stage for a little longer will also help the mixture to warm slightly and the emulsion to re-form.
3. Too many eggs are added. The amount of water (from the eggs) being added to the butter is too much for the quantity of butter being used to retain the emulsion. The butter is therefore unable to ‘take on’ any more water causing the mixture to split. This one is hard to avoid as some recipes are naturally high in eggs as they contribute to the final character of the bake. However, a little amount of splitting at this stage is quite common and, generally speaking, if the balance is only slightly out you’ll find that the mixture will come back together when adding the dry ingredients and the effect on the final texture will be minimal.