Reading through the online version of How to Brew it says the minium SO4 is 50ppm. Is that generally accepted? If so, what is the reason?
I don’t think that there is a minimum SO4 level. Unlike Ca++ and Mg++ it doesn’t seem to do anything to the process of brewing (at least I don’t remember seeing anything written). Certain styles may strongly benefit from it being above 50ppm, which is where that suggestion may have come from.
Kai
I have brewed with water with less than 10ppm of So4 without an conversion, hot/cold break, or flocculation problems in the finished beer.