I don't know what nutritional yeast is actually in as they only get Purina duckling starter. They are only 4 and 5 weeks old but also get a bit of regular duck food as I am weaning them off of the starter. By the time I got home from work their mouths were just fine so I don't know what I was actually seeing. Thanks for your help.I would think so, actually. And maybe lay off of the nutritional yeast for a bit at the same time.