Complex sentence: Bromelain is present in all parts of the pineapple plant ("Ananas sp."), but the stem is the most common commercial source, it is presumed because it is readily available after the fruit has been harvested.
Split into simpler sentences: Bromelain is present in all parts of the pineapple plant ("Ananas sp."). However, the stem is the most common commercial source, presumably because it is readily available after the fruit has been harvested.