What is better in this case: starting with a small starter (e.g. half a litre) and stepping that up to one and then two litres, or simply making a two litre yeast starter right off the bat? With "better" I mean what will give me the largest population of healthy cells? And, more to the point, why?
Just re-read your OP: Step starter or single step?
[EDITS]
It largely depends on how many cells you think you have and limits on inoculation rate.
- Inoculation rate too low (<25 million/ml), e.g., large starter volume and only a small amount of viable cells, she won't grow well.
- Inoculation rate too high (>100 million/ml), e.g., the starter is more like a vitality starter, there's not as much growth, she's being prepped for the pitch, waking up so to speak, building up sterols and her strength so she can bud.
These are general guidelines of course, target numbers.
If you harvested from a previous fermentation and you now have a pint to a quart of dense settled slurry (mixed with trub) in the fridge, estimate her age, use one or more of those yeast calcs, to see how many cells you may have, let that be your guide.
1) If it's less than 2-3 months old, you could pitch a cup or so directly. Or make a 1/2 - 1 liter
vitality starter from that cup 4-6 hours ahead of pitching to wake her up, preparing her for the big job. Pitch the active starter as is, no cold crashing and decanting.
2) If it's older, say up to 6-12 months old, it may still be fine to pitch as is, just use more. Either make a
vitality starter, pitch as is, or if time allows, a
traditional starter, a few days to a week ahead, and overbuild it to save some out to make a starter from for a next batch.
3) If it's much older, say well over a year, and you indeed may only have a small amount of viable cells left in your harvested slurry. Making a
step starter from some of it (e.g., 2 Tbsp) would be the better option, due to limits on inoculation rate. Then crash, decant and do a next step.
Before your first step you could rinse ("wash") the whole or half the slurry to help separate the live yeast from the boat load of trub and dead cells. Live cells remain in suspension while everything else settles out.
In this case use the supernatant for propagation, of course, including its volume (relatively large) in your yeast starter (total volume / gravity) calculation.
4) If it smells really bad, like marmite/vegemite, burnt rubber, or worse, don't use it, it has autolyzed. Ready for composting.