Either ¼ cup of white vinegar diluted in 2 gallons of hot water or non-soap-based PH neutral cleaning products. Whatever you use, make sure you thoroughly rinse the cleaning product away.  Never use ammonia or bleach-based products on porcelain as it can discolor the tile, damage the coating, and alter the grout color. You should also never use solutions that contain oil-based detergents, wax, or sealant cleaners on your porcelain tiles.