I have noticed the same thing, and actually, I did TEM imaging of the precipitates (see picture attached) and the bacteria tend to adhere to them. They also seem to have a particle-like morphology, and they look very similar to some iron nanoparticles that I have made before by thermal autoclaving around the same temperature 150C. I would filter the solution and add it aseptically afterwards; also some vitamins are heat-liable, so I would add those after, too.

Hope this helps.