In George Watkins' classic 1760 work The compleat brewer, in the description of filling beer into casks, I came across this:
Do I understand this right? The yeasty sludge is to be reserved, and mixed with small beer to improve it.
And William McEwan's letter almost a hundred years later confirms: "This extract of the yeast and also the dregs of the Butts are mixed along with an inferior beer which is sold in the neighbourhood."
I guess this must have been the small beer which history tells us was drunk as an alternative to water before clean municipal tap water became widespread. But how would it be improved by adding yeasty goop to it?