Q: My buttercream curdled after I added some milk to soften it. Why did this happen?
A: Buttercream curdling or ‘splitting’ after adding milk is a common problem, especially in winter. The reason for this is that butter is essentially an emulsion and when you add milk the additional water (full cream milk is almost 90% water) causes butter emulsion to split.
The best way to rectify this is to continue to beat the buttercream until it becomes smooth again. The friction created by the beating will warm the butter slightly and soften it helping to reform the emulsion and absorb the extra moisture. Alternatively, you can beat the original buttercream for longer and the warmth generated by the friction will soften the buttercream to a more spreadable consistency so there is potentially no need to add milk to help soften it.
As I mentioned, this 'splitting' is more likely to occur in winter when 'room temperature' butter is colder than it will be in summer. Also, the beater, bowl and air around the buttercream mixture will be cooler which will also lower its temperature, making it more prone to splitting when milk is added.