Same lesson, with the figures in motion.
Viability vs Vitality
Alive is not the same as lively
1. Are they alive?
Viability is the share of cells that are alive. You measure it with methylene blue, as in Pro Cell Counting: dead cells turn blue, living ones stay clear. A pack in the fridge loses roughly a fifth of its living cells a month, so after half a year about a quarter are still alive. It is the same curve the pitch calculator uses.
2. Are they up for it?
Vitality is how lively the living cells are. Two samples can have exactly the same number of living cells. In one they bud, and the buds grow into new cells that bud in turn. In the other they lie still. Methylene blue can't tell them apart: both count as alive.
3. It shows in the fermenter
Pitch the same number of living cells into two identical worts. The lively yeast gets going within a few hours; the tired yeast can take a day or more, and in the meantime off-flavours and wild microbes get a head start. You measure viability, but you build vitality: fresh yeast, few generations, cold storage, oxygenated wort and a starter that wakes the cells before you pitch.