What’s the Best Tool to Clean Grout? 

Cleaning grout is a necessary part of any tile maintenance- whether its in your kitchen or bathroom.
While you may want to use a sponge or scouring pad, the fact of the matter is that they often do not apply enough agitation to release the small pieces of dirt and grit from the grout. You can scrub for hours and see minimal results.
The best "tool" for cleaning grout is a nylon brush with stiff bristles. They should be stiffer than a toothbrush, but still retain a small amount of flexibility as you move the brush across the grout. A brush that is too stiff will not conform to the ridges in the grout to properly agitate the dirt.
When cleaning, brush parallel to the grout. This allows the bristles to penetrate the grout's surface with enough pressure to clean.
Unlike coarse scrubber brushes, such as steel wool, nylon brushes won’t damage or remove the grout when cleaning. Nylon brushes are also not abrasive enough to damage the tile, either.
Before you begin cleaning, you should always verify your cleaning chemicals are safe for use on your tile and grout. Otherwise, you may still damage your tile and grout, even when using a nylon brush. An alkaline chemical is the best starting point.
Whether you are cleaning tile on walls or on floors, a nylon brush is the most effective and safe tool to ensure that your grout is thoroughly cleaned.