Can I substitute ap flour for bread flour?

Toast

I’m on day 8 of feeding my starter, currently using 30g of whole wheat, 70g of ap and 100g of water. I’m running out of ap flour, but I have bread flour can I use that instead of the ap?

I keep a sourdough starter made from bread flour and water. We have a small canister of bread flour in the house, but the bulk of the bread flour is in a freezer in the garage. Sometimes if I run out of bread flour in the house when I am making sourdough starter, I'll get lazy and weigh out the remaining flour from the all-purpose flour canister. It works fine, and I am sure that it will work equally well the other way. 

 

For this purpose and in the short term, AP and 'bread' flour are completely interchangeable, assuming they are fairly typical AP and 'bread' flours.  For one or a few feedings, it won't matter at all.  Over a longer term (months), perhaps, your starter microbial populations might trend into different proportions relative to one another than they would have had you stuck with the same old AP.  But that starter would be 100% fine too.  Keeping them well and routinely nourished is more important than such a tiny detail as the difference between AP and 'bread' flour, which, in the universe of flours, can be about as close to one another as any two flour types.

That said, there are no official standards for what sells as AP and 'bread' flour (in the US at least).  So there are probably AP and 'bread' flours on the market that are chemically almost indistinguishable and those that are miles apart.  Basically, 'bread' flour is just higher protein AP flour and your starter's bugs will not complain about the tiny improvement in their nutrition.

Tom