I've just set off a 750ml shaken starter of WLP550 ('Chouffe') planning to step that up x5 by volume in the next step, ready for a 20L 1.075 tripel. That volume is all I can get in a glass vessel, which is where I'd prefer to keep it.
It occurred to me that I could step up more, by increasing gravity and volume, perhaps up to the equivalent of a 10x volume step: five to ten times (volume) per step is a guideline I've read. So, two questions:
1. Is stepping up the gravity ever a good idea, rather than sticking with the standard ~1.040?
2. If I do step up gravity, can I treat the increase in SG as effective as the same increase in volume? For example, if I do a second step of 5x volume and 1.5x gravity (1.060), would that be as effective (both in growth and health) as doing a 7.5x second step at 1.040?
Yes and yes, or at least within the same ballpark. I don't know if it is needed for 1.075, but by increasing the gravity on successive starters (I just brew a full batch of beer after step one so I at least get to drink my starters, but it's the same idea), you will grow and select for yeast that is more tolerant of higher gravities.
I'm sure there is a gravity range for starters where gravity and/or alcohol becomes high enough to affect yeast growth, but I suspect that it isn't much of a factor in the 1.060 range. Generally speaking, yeast growth is based on the total amount of extract, rather than volume. So multiplying the volume by the gravity as you are doing should provide a good estimate.