Depends on your soil for the grout.
If you believe it's organic (something that comes from live matter like oils, petroleum and microorganisms) use an alkaline cleaner.
But if it's inorganic (like water marks, metal or chalk deposits, and mineral buildup) use acids.
Commonly, grouts collect inorganic soils. I can recommend you use an acid cleaner for this one. Be careful, however, as concentrated acids can completely consume your grout! Use light solutions or at least diluted acids for your cleaning needs.