well, If I remember correctly from Biology class, there is a part of the Digestive system specifically made for soul devouring. Humans,due to our discontinued use of it, have lost this ability over time, but some Organisms still retain the ability.
Because souls carry with them a certain personality of their own, they will most likely be treated as an unwanted foreign object and be dealt with much like a germ. This could cause temporary effects like vomiting or headaches, but after the soul has been thoroughly beaten by the immune system, one would be able to enjoy it's fluffy, Marshmallow-like center.