I have not brewed since 2016, but I have been delving into sourdough lately. Every sourdough culture is different due to native microflora. What I learned after several attempts to create a healthy sourdough starter from scratch is that using whole organic wheat flour works a lot better than unbleached bread flour, which makes sense seeing that the bran contains most of the wild microflora. The fact that the grain is organically grown means that no fungicides were use on it, increasing the amount of sporulated yeast in the milled flour. The next important thing that I learned is that a starter that is started with pineapple juice works better at creating a starter that contains a higher ratio of wild yeast to undesirable bacteria than one that is started with water. The reason being that pineapple juice has a pH of 3.5. When prepared as a 50% weight by volume solution (50grams of whole wheat flour to 100 grams of pineapple juice), the starter has a pH in the range of 4.3, which is below the pH at which pathogens and other bad bacteria can replicate. The starter is stepped with 25 grams of whole wheat flour on the 3rd day and another 25 grams on the 4th day before being stepped equal amounts starter, wheat, and water. Most of the yeast in a sourdough culture comes from the flour, that is, unless one lives a high native yeast area such as grain fields, vineyards, and orchards. A large percentage of the bacteria comes the local environment, which is why sourdough cultures tend to be different from culture to culture.
That being said, applying what I have learned to brewing. I would acidify the wort down to around to between ph 4.0 and 4.5 and allow it to sit uncovered for between 12 and 24 hours (start it hot like the Belgians do). From there, I would pitch a sufficient quantity of quality dry yeast for the batch at hand (dry yeast strains are grown below the Crabtree threshold under aerobic conditions, so they should be good to go in low oxygen wort). Bacteria strains multiply three times as fast as yeast strains, so giving the native yeast and bacteria a head start should be balanced out by a healthy pitch of a known yeast. If a brewer truly wants to live on the wild side, he/she should attempt to create a sourdough starter using ground malted wheat or malted rye using the process outlined above and transition the sourdough culture to a liquid culture after it starts by using the sourdough starter to inoculate acidified starter media. I used to start cultures off of using autoclaved (pressure cooked) pH 4.0 adjusted 5% weight by volume (w/v) wort (1.020 wort).
The cool thing about attempting to use a sourdough culture to start a beer starter is that the yeast in milled flour is sporulated, which is way for yeast strains to survive hard times. Most brewing strains unable to sporulate due to not being able to undergo meiosis (sexual reproduction) due to being polyploids. All of the yeast strains in flour are diploids. Diploids are able to undergo meiosis, which means that there is the possibility of hybrid strains forming and being selected via repitching. That is how all modern brewing strains were selected.
Some food for thought...
