Flavor in cherry tart/pie
Last night, a friend brought over a rustic cherry pie (the kind where the crust is just folded over to make a sort of freeform tart, and baked flat on a baking sheet). It was delicious, but the filling had a flavor that I just could not identify. It was very pungent, savory rather than sweet, and with a vaguely acidic undertone (though no bite).
My friend is from PA, so the potential Pennsylvania Dutch influence made me think of vinegar, but the filling didn't have the heavy acidic bite I associate with vinegar. Then I thought mustard since something about the smell made me think of yellow ballpark mustard, but when I tasted some mustard powder today, that wasn't right either. The closest I've come is sumac, but I just can't imagine that a small-town bakery would use something so unusual.
Does anyone have any ideas as to what that flavor might be in the cherry pie filling?