Many new sourdough bakers struggle with creating sourdough starters.  The most common problem I see is new bakers trying to create a starter with 100% all-purpose flour (or 100% bread flour). 

The best flour blend for creating a new sourdough starter is 50% whole-meal flour (whole wheat or whole rye) and 50% bread flour or all-purpose flour.

I recommend a 50/50 mix of whole wheat flour and bread flour. 

Why do you need to use these two types of flour?

When establishing a new sourdough starter, the vast majority of the yeast cells and lactic acid bacteria cells come from the flour.  The microbes attach themselves to the wheat berries in the wheat field and are lying dormant in the bag of flour.  Contrary to popular belief, the microbes in your kitchen that come from the air, or your hands, or your utensils are an infinitesimally small contributor to the microbial population of a new starter.

The beneficial microbes (yeast and lactic acid bacteria) are found on the outside hull or shell of a wheat berry.  When you buy a bag of whole-meal flour (whole wheat or whole rye), it includes 100% of the wheat berry (the hull, the endosperm and the germ).  So, the microbes attached to the outside of the wheat berry are ground up in the milling process and end up in your bag of flour.

With processed flours like bread flour and all-purpose flour, much of the bran (the hull of the wheat berry), is extracted from the flour and discarded in the milling process.  Many of the microbes attached to the outside of the wheat berry are discarded with the hull and do not end up in a bag of bread flour or all-purpose flour.  Refined flours, like bread flour and all-purpose flour, are also subject to more mechanical milling which can damage the cell membranes of the microbes – reducing the viable population of microbes.  

Lastly, whole-meal flours contain unique nutrients and enzymes which are beneficial to jump-starting the microbial population in a new sourdough starter. 

Why not use 100% whole-meal flours?

You can create a starter with 100% whole-meal flour, but, in my experience, you will get best results with a blend. 

As the yeast and lactic acid bacteria populations begin growing, they need a food source (sugars and starches).  Bread flour or all-purpose flour provide a more readily available food source than whole-meal flours.  Refined flours are easily converted to sugars, which are the essential food source for your new starter. 

A simple way to think about a 50/50 flour blend is – the whole-meal flour provides the “bugs” and the refined flour provides the “food.” 

Do I need to continue feeding the 50/50 mix forever?

Once your starter is established, you can transition to feeding it 100% bread flour or all-purpose flour.  An established starter (e.g., 14-30 days after creation) has a robust microbial population, and the feedings from that point forward are primarily providing the food, not additional microbes.  

I feed my starter a 50/50 mix of bread flour and whole-wheat flour, but many bakers maintain strong starters with bread flour or all-purpose flour.