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:
Is stepping up the gravity ever a good idea, rather than sticking with the standard ~1.040?
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?
You want the yeast to be as healthy as possible and the lower the gravity (to a point) the better in that regard. There is no benefit to increasing the gravity and it might even be detrimental.
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.
From what I have read here, yeast growth is limited by stress and volume of starter. Only so much can grow in a specific size starter, the only difference the gravity makes is whether or not the yeast are stressed or not
I don’t know if I agree with that. If you’re starting with healthy yeast from a fresh starter, then my understanding is that you can build up their tolerance to gravity and alcohol in successive generations. That certainly matches my experience, at least. For really big beers I have had great results by pitching from a yeast cake from a batch that was in the 1.060’s, which was previously stepped up from a normal-gravity starter or a session beer.
It is actually limited by the amount of extract (i.e., sugar) available in solution. Here’s an experiment Kai ran a few years ago. Essentially, the same amount of yeast grew in 400mL of a 5 Plato wort as it did in 200mL of a 10 Plato wort. Yeast growth and viability didn’t start to decrease until he got to 20 Plato (~1.083).
Maximum cell density is apparently ~200B cells per liter so that shouldn’t be a problem in the steps I suggested in my second question (which is what I’m considering doing).
erockrph - I had hoped to dovetail two brew days close enough together to get to ‘drink my starter’, but it’s just not going to happen
I think that’s maximum cell density GIVEN Available carbon. The per liter is a shorthand because we use 1.040 starters most of the time. As eric says, the limit is the available food
I was never after greater alcohol tolerance: I can’t see that being a problem. It’s just growth and viability I’m after.
I never knew that the 200B/liter ‘limitation’ could be changed by available extract. I assumed there must be something else restricting further growth in a given volume.
That experiment by Kai, linked above: I don’t quite understand the inoculation rate side of it enough to rule out the possibility that his samples below 20P weren’t just topping out due to a maximum cell density factor separate from the amount of extract (if there is one - opinion seems divided!). He seems to suggest that it may have been through limited nitrogen. FWIW, I’ve put WL nutrient in the starter as a matter of course.
Actually, there is quite a bit of research showing that alcohol exposure does increase alcohol tolerance in yeast. In particular, yeast tend to alter the fatty acid makeup of their membranes in response to increasing concentrations of ethanol, and this in turn enhances their tolerance to alcohol.
As we all know, what happens in a lab doesn’t necessarily translate to what happens beer. And you can’t apply what happens with one yeast strain universally, either. But it works for me, so I’ll stick with it.
Yeast growth is actually limited by volume, dissolved O2, and the amount of carbon available to the cells (for those who do not known, sugar is carbon bound to water; hence, the name carbohydrate). Maximum cell density is maximum cell density. If volume had no bearing on cell count, then a culture would remain in the exponential phase for the length of a fermentation. Conversely, if a culture runs out of carbon before it reaches maximum cell density, it will never reach maximum cell density.
The interesting thing about dissolved O2 is that the load placed upon the medium is not solely dependent on the health of the cells when they are pitched. Different strains have different O2 requirements. A scientist named of Brian H. Kirsop outlined four classes of O2 demands by yeast strains.
Class O1: Yeasts whose requirement is satisfied if wort is half saturated with air (4ppm dissolved O2)
Class O2: Yeasts whose requirement is satisfied by air-saturated wort (8ppm is the maximum dissolved O2 from air at sea level)
Class O3: Yeasts whose requirement is satisfied by oxygen-saturated wort (40ppm dissolved O2)
Class O4: Yeast whose need is not satisfied by oxygen-saturated wort (> 40ppm dissolved O2)
I am currently working with an O3/04 yeast strain. I am almost certain that the strain is the John Smith culture, which is related to the Samuel Smith culture. I am willing to bet that most of the commercial yeast strains sold by Wyeast and White Labs are class O1 and class O2 strains, with easy to use strains such as 1056/WLP001 and 1098/WLP007 being class O1 strains.
DeClerck stated that 1 gram of extract contains enough carbon to produce 1 billion cells. Other scientists have managed to collect data that cells counts as high as 1.5 billion cells per gram are possible, but 3 billion cells per gram does not pass the sniff test. The total cell count is bounded by the maximum cell density for the culture.
It seems like that would take mutation to a more tolerant strain. In my pea brain understanding, maybe what seems like building up tolerance is actually just slowly reaching tolerance limits. I could be totally wrong
