Sourdough Starter

I'm now on day 5 of my sourdough starter. I have bubbles, I have separation, but I don't have froth. Tonight I made mashed potatoes and thus saved some of the water to feed my starter with, good thing too, as I accidentally threw out more then half! I'm hoping this trick of using the "starch" water will be the turning point. I'd like to try sourdough this weekend!