Combine a solution of 3 parts baking soda to 1 part water in a bowl. Stir the mixture with a spoon to create a paste.
Dip an old toothbrush into the baking soda mixture and scrub over the white grout.
Continue until all residue is removed from the grout.
Remove leftover baking soda residue by wiping down the white grout with a wet cleaning rag. Rinse the cleaning rag as needed, and when all of the residue is removed, wring out your cleaning rag and wipe down the grout to collect any leftover standing water.
Let the white grout air dry, and clean with the baking soda paste as needed to keep the grout white and attractive.