[Harmless Teddy Bear]

Prior to 1850 all bread was whole grain sourdough.

No... it wasn't.

Flours were milled and sifted finely to make unbleached white flour and brewers yeast was used to get a fine light bread since at least the medieval period.

Whole wheat bread was lower class and sourdough was mostly found in the Americas where there was not a bunch of brewers handy.

[Mariner]

Yes they very rich had sifted white flour.

But the average Joe had only whole wheat, and that was 99%

Brewers yeast will ferment only sugars, not the starches of flour.