The feed doesn't really go bad if you do a little maintenance on it. Stir the batch once or twice a day and feed as much or little as you want. A bittersweet smell is the normal, proper smell. Several have said it smells a bit like sourdough bread. If it starts smelling pungent and nasty, you'll know it's going bad. Add a large glob of unpasteurized apple cider vinegar to help it get back on track.