I made Celtling some shortbread cookies the other night, and they came out with a nasty bitter aftertaste?!? The initial flavor was seriously yummy, so much so that I took a second bite after trying the first one. But this bitterness crept in and overwhelmed our mouths. It was almost acrid, very “chemically”.
I know the butter was fine, because I cooked with the other two sticks from the pound an hour before.
The flour may have been a weensy bit old, and it was technically bread flour. Could that impart a bitter aftertaste? It’s been well sealed inside it’s paper bag and then a ziplock.
I used a small amount of dark brown sugar (the way I always do them) and it tastes fine too.
I use a fair amount of madagascar vanilla. It’s in the last third of the jar and probably also on the old side. I tasted some by itself and didn’t get the same response at all.
I added a bit of corn starch because the butter got a bit too soft before I was ready to bake. The corn starch was fresh and worked fine in almond cookies last week. The bag was unsealed for about 5 days in between.
I baked them on a silicone sheet with no added fats. I did a second batch on parchment paper with the same result.
Any ideas? It’s a very strange, pure bitterness, and there’s no hint of it until after I swallow. I can’t remember anything else like it,
I have no helpful ideas, but I wanted to let you know that I now have a intense and immediate desire for short bread (and fresh strawberries). I hope you’re happy! 
:: sings :: Momma’s little baby loved shortenin’ shortenin’, Mama’s little baby loved shortenin’ bread. . .
For some reason I’m in touch with my Mother’s Soutehrn roots the last couple of days. Weird.
If those were the only ingredients, and you tasted them all separately (other than the flour it seems), I’d blame it on the flour. Flour can go rancid because it does have a tiny amt of fat. People often refrigerate or freeze flours for this reason. Taste the flour if you want to check. The fact that it’s bread flour wouldn’t matter in terms of flavor, old or not.
I will say that that’s really not a good flour to make cookies/cakes/quick breads with, though. It’s high gluten, which is great for yeast breads but not suited to leavened goods and pastry. Also, adding cornstarch because the butter was soft? No, no, no … added starch doesn’t counter having butter that’s too warm. Again, nothing to do with the bitter flavor, but …
I’d bet it was the flour. ( Whole wheat flour goes rancid ten times as fast as white) but I wonder if the intense summer heat did anything to the white flour in my pantry. I’ll have to put a note in there - ‘taste first’.
Definitely the flour. I’ve had it happen on the infrequent times I bake.