In a way, it's probably a little of both.I was wondering where the yeast for the successive mashes came from, ie. was it collected from previous ferments (before the boils) or was it made anew for each batch from a pure culture? I think Mike and you answered that, it either is propagated in jugs from previous brews
I baked sourdough bread for about a year. The original culture was given to me by a friend, and had to be 'fed' once a week to keep it alive. I would divide the culture, using half to bake bread (or flush down the toilet...we were living on a farm/septic system... ). I would 'feed' the other half with flour. It would show signs of fermentation, just like a mashtub.
What I learned, after a year of doing that, is how much I could change the taste of the bread, by what I 'fed' the yeast.
I'm sure the same thing applies here.