Why do apples sometimes taste bitter? I sometimes come across apples that taste very bitter (not sour like a granny smith, but more like tannin). This is maybe one in 10 or 15 apples. It doesn't seem to be restricted to a certain variety (I've tasted it in pink lady, opal, honeycrisp and some I've forgotten). Sometimes it's just one, and sometimes it's a whole batch. Washing doesn't remove the flavor

Google search suggestions show other people have noticed this too, but I can't find a good answer as to why. It seems too common to me to not have a good answer though; maybe it's something only some people can taste, like cilantro tasting soapy or cyanide smelling like almonds?
We have an old variety apple tree, similar to Yellow Transparent, of uncertain origin. When ripe - that is, the flesh has softened to edibility and sweetness has developed - the skins provide a highly tannic note. Removing the skins before using the apples in cooking dramatically reduces the tannins.