I just made the pineapple neoclassic today and it looks slightly curdled like cottage cheese. More beating did not help. I was able to make a perfectly smooth and thick neoclassic, and the buttercream was still fine after beating in one cup pineapple puree. Then I tasted it and thought it needed more intensity. Instead of adding in 2 Tb rum or kirsch like Rose suggests, I added 2 Tb lemon juice. That curdled it. I don’t understand why. It’s the same amount of liquid. Was it the acidity? The lemon classic buttercream uses 1/4 c. lemon juice though, twice the amount I used.
The buttercream still looks useable to me. It’s thick enough to spread onto cake—just not smooth. However, I will be freezing the buttercream and defrosting it at the end of the month to complete a cake. I am worried that, in its current slightly curdled state, it will only get worse when I defrost. Is there some way to fix the buttercream?