I’ve made many batches of swiss meringue buttercream but this time I added lemon juice at the end for a bright flavor addition per the recipe directions.    I’ve been able to beat past the curdled stage when adding butter before however no amount of mixing was able to salvage the broken buttercream after adding the lemon juice (1 T).  Did I add it too quickly or should I expect that adding acidity at this point will cause it to curdle no matter what?  The cake was wonderful even with the curdled buttercream (covered it with chopped pistachio brittle)  and when I called the bakery that published the cookbook, they wouldn’t answer my question.  I’m wondering if next time I should just add some lemon curd at the end instead. 
Would sincerely appreciate any advice as I plan to make the cake again soon.

Thanks

I don’t know enough about the recipe and temperatures, etc. to know if your bc curdled because of acidity or because the emulsion broke.  Either way, lemon curd would be a safe bet. 

Perhaps someone with more experience making swiss meringue buttercream will chime in!

I don’t have experience with Swiss meringue.  How big was the batch size that a tablespoon of lemon juice wrecked it (or that a tablespoon of juice was enough to “brighten” the flavor)?  I wonder if they use extract in the bakery and wrote the recipe with juice (just speculation)...... but I would definitely agree that for a lemony zing, curd would be wonderful.

Thank you for your prompt reply. I am never disappointed when I post a question and truly appreciate the collegial spirit.  I too wondered if they meant to print lemon extract.  I made the buttercream recipe twice just to make sure I hadn’t goofed in some way.  I thought streaming it into the buttercream while beating might make a difference but once again it didn’t. I don’t have the recipe with me but will look at it tonight and send further information.  There’s a second lemon buttercream recipe in the cookbook that is just a slight bit different. I made that one too and while it looks better than the first doesn’t taste as “lemony.”  The nummy pistachio brittle hid a multitude of sins with the first buttercream recipe.  I’m going to remake the cake over the next two weeks.  What does everyone think about a drop of lemon oil?  Maybe I’ll stick with folding in some lemon curd. 
Thanks again

If your bc is an egg white buttercream ( and I think most swiss recipes are), then I think the curd would give a lovely flavor and richness due to the yolks.

If you happen to have boyajian lemon oil, you can add a tiny amount and taste it once it’s beaten in, in addition to any other kind of lemon flavor.

I expect that lemon extract or Limoncello would be less likely to curdle, as in my experience with egg white-hot syrup bc alcohol atcually helps, rather than hinders, the emulsion.

Lemons curd would defintely work.  I think lemon oil also.  not remembering how Swiss Meringue, I wonder if adding the lemon juice to the egg white and sugar mixture would work.  just thinking out loud

I don’t think 1 tbsp of lemon juice is the cause of curdling.  How cold was the buttercream prior adding the lemon juice?

Thank you all so much for the assistance.  I think I’ll try the recipe with the curd and some lemon extract.  I had checked about the temp of the butter before adding it.  I usually put the butter in my old mixer and cream it until it is 65 degrees before adding it by tablespoon to the buttercream.    I’ll let you all know how it works out next week.  Maybe I’ll get daring and actually post a picture of how it turns out.  Keep your fingers crossed for me.

If this helps, I don’t cream the butter at all.  I cube it in large chunks and leave it out till 70-75 oF, 65 is too cold in my opinion.  The meringue should also be at 70-75 but honestly I usually can’t wait that long and start when the meringue is at 90!

I agree with Hector, 65 is too cool and can exacerbate/cause curdling.  When I make mousseline, after beating the hot meringue for five minutes or so, I scrape it into another bowl and cover it, letting it cool to 70-75F.  Then, without bothering to clean the mixer bowl, I put in the butter and whip it until light and pale, then cover that, too and let it also sit until 70F.  Once both bowls are at about 70F or a little higher, I dump all the meringue onto the whipped butter and let the mixer have at it- it usually only takes a few minutes to come together into a gloriously fluffy buttercream.  This is with Italian Meringue, but I imagine that swiss may work similarly.

Do you keep your lemons in the fridge?