Whole wheat and rye flour help sourdough become more active and ready for use faster because they contain more microorganisms that are good for sourdough, more so than white flour. The whole grains and higher protein content can help speed up the fermentation process of the bacteria and yeast.   *Keep this in mind for an older starter that may be struggling after being dormant in the fridge for a while. You can use whole wheat or rye to feed and boost a tired and struggling starter, especially if regular flour does not seem to be helping at the rate you would like. *White flour is better for the majority of feedings because it helps produce a more consistent rise and a lighter-textured sourdough. Whole wheat is great for the microorganisms at first, but it will result in a denser-textured sourdough if used for a longer period.