Barm and Porter musings

I have been pondering why bakers, apparently around 1800, stopped just using yeast from the brewery and switched to barms.

Everything I have read on the subject of barms suggests that hop bitterness was considered a necessary evil by bakers at the time. They didn't like it, but the hops were needed to stop the barm spoiling.

I had in my mind that there must have been some significant change in the way beer was brewed that forced bakers to switch.

I don't know whether there was a sudden shift to using more hops around that time, which would have increased the bitterness.

The only other idea is that at that time porter was the dominant brewing style, in London at least. Porter had already been popular for decades prior to 1800, but it was made with brown malt. The much more highly roasted black patent malt was invented in 1817, and the porter brewed using this might have given the yeast a burnt, toasty flavour that bakers didn't want in their bread.

Could there be something in this theory? I think it is most probably rubbish, but someone else may know something I don't.