use white vinegar
I have rentals. To clean up organic gunk (like food) use a citrus based cleaner. To finish the job use Dow Foaming Bathroom cleaner. Black Diamond grout cleaner is also very good because it is enzyme-based and will not destroy your grout. Harder to find and a little expensive but worth the money. I order mine on-line from Walmart. Vinegar and other acid-based cleaners work but will break down the grout over time.
I found Bar Keepers Friend, a toothbrush and clean water and sponge did the best for me and I've tried them all from vinegar/baking soda/hydrogen peroxide/Clorox....
Don't know why the after photos will not post....but it turned out bright white like the original.