Steam clean the grout. One effective and environmentally friendly way of cleaning grout is to attack it with a steam cleaner. Outfit your steam cleaner with a thin and pointed nozzle, and a brush attachment if it’s available. Aim the nozzle at the grout lines, apply continuous steam, and follow the grout lines with the steamer.
Even if the steam cleaner doesn’t get the grout perfectly clean, it will clean it somewhat, and help loosen the rest of the dirt and debris to make it easier to clean with other products. Attack it with baking soda and vinegar. Take two tablespoons (30 g) of baking soda and add enough water to make a thick paste. Dip a clean toothbrush or grout brush into the paste, and then use the toothbrush to scrub the grout with the paste.
For the best results, use a new toothbrush with hard bristles, and scrub the grout in a circular motion, rather than back and forth. When you have finished scrubbing the grout, use a spray bottle to spray the area with a half-and-half mixture of vinegar and water. Let it bubble for about half an hour. Use the toothbrush to scrub the baking soda paste and vinegar into the grout again. Rinse the area with clean water.
Move on to baking soda and hydrogen peroxide. In case the baking soda and vinegar mixture didn’t to the trick, try hydrogen peroxide instead of vinegar. Make a thin paste with baking soda and hydrogen peroxide, and use the toothbrush to scrub the paste into the grout. Pour on some additional peroxide if necessary. Rinse with warm water when you’re finished.
You should never mix vinegar and hydrogen peroxide, so clean the area well and wait a couple days before trying this method. If the hydrogen peroxide doesn’t remove all the dirt and grime, it will at least improve the appearance of the grout by removing stains, and will kill any mold that’s present.
Give borax and lemon juice a try. In a small bowl, mix together ¼ cup (63 g) borax, ½ teaspoon (3 ml) lemon oil, and enough liquid soap (such as Castile soap) to make a paste. Use your toothbrush to scrub the paste into the grout, and then rinse with warm water.