How you approach cleaning your grout on your granite countertops depends on if the grout is new, lightly stained, or deeply stained. The condition of your grout determines how you should approach and treat the grout.
Cleaning New Grout
When your grout is new, instead of worrying about cleaning it, you need to worry about keeping it clean. Try to avoid using any surfaces where you put grout down for two weeks. Keep the area as clean as possible, and use a wet cloth to quickly wipe up and clean any messes.
After two weeks has passed, use a grout sealer on your new grout. A grout sealer will help keep dirt from settling into the grout and will help prevent the grout from getting stained.
Lightly Stained Grout
Over time, your grout may end up getting lightly stained. It is best to deal with the grout when it is lightly stained instead of putting cleaning off until later when the stain is more serious.
With lightly stained grout, you are going to want to create a bleach solution. This should be a stronger bleach solution, with about a cup of bleach to a gallon of water. Protect yourself from the bleach with a mask, safety goggles, and gloves. Also, wear old clothing so if the bleach gets on your clothing, it doesn't ruin anything. Keep the air flowing as well.
Then, use a small scrub brush to scrub the grout. Work and scrub a small area at a time. Don't scrub too hard; you don't want to damage the grout. Just work the area, rinsing it off with clean water as you go.
Deeply Stained Grout
If you didn't stay on top of cleaning your grout and it is now deeply stained, the best course of action may be to remove the grout. You can purchase a tool from your local hardware store that will allow you to remove the grout without damaging the surrounding tile.
Once you remove the grout, clean the area where the grout was with bleach; this will help remove any remaining dirt. Then, apply new grout, let it cure, and seal it.
How you approach cleaning your grout on your granite countertops depends upon the age of the grout and how it is stained. For new grout, keep it well sealed. For lightly stained grout, use a bleach solution. For deeply stained grout, just replace it.