Starter with water, flour, grated apple

I successfully obtained a working sourdough starter using rye and wheat flour, water, and grated apple. The dough made with it rose in a banetton and was inverted onto a hot baking stone. The bread had a good crumb, was thoroughly baked and moist. That to say that the starter worked. I am puzzled by the starter's aroma. It is not the smell of a starter going bad, more like a slightly bitter undertone, like beer. The starter is fed regularly and is still bubbling away, but the undertone is persisting. It has not affected the quality or flavor of the bread. Any ideas?