I have been making my own baby cereal by grinding various grains and seeds into a powder. Many commercial baby and breakfast cereals are iron-fortified.

What can I add to this dry powder to fortify it with iron?

I have been making baby cereal powder by grinding rolled oats, flax seed, and hemp seed into a powder. I prepare this cereal by adding water and heat.

Many commercial powdered baby cereals (and general breakfast cereals) are iron-fortified.

What can I add to my homemade dry powder to fortify it with iron?

Note: Please hold off posting any opinions. I'm looking for factual answers with links to reference data.