OK I’ll go with that. I’m not doubting that shaking with a large head space brings the wort to saturation quickly. It’s just that once the yeast start picking up that initial 8ppm or whatever saturation is, with a stagnant wort, the only way it’s going to get any more is from diffusion across the interface. That’s not going to be very much once diffusion starts and the concentration gradient drops off drastically. With even a gently stirred wort, the concentration of O2 at the interface will be the same as it is everywhere else in the wort, dropping as the yeast consume the O2. This speeds diffusion as it increases the gradient. If it’s stagnant, the interface remains near saturation with respect to the headspace, and diffusion is slowed. See Fick’s 2nd law. Solutions are reasonably simple with simple, well-defined boundary conditions.
It’s generally the O2 availability that limits the cell density (for a starter) in most cases, right? If you raise the saturation by injecting pure O2, say to 12 or 15ppm, even with a stagnant wort, don’t you get more cells because of this? I thought that this was the case, and not just homebrewer heresy. If it was the case, then I was thinking that adding more during the drop from 8 to 0 would also create more cells. Not the case?
On another note, 8ppm in a litre is 8mg. 3L of 23wt% O2 air, with air at 1.23g/L, means there is about 850mg of O2 in the head space. I bring this up because it seems to me it puts to rest the idea of needing to bring any air into the head space from outside the container. You could re-saturate 100x with what is already present in the head space.
I used this method for my first lager (Wyeast 2124), I pitched the starter after ~16 hours and it was bubbling away. I did pitch the yeast when the wort was 55F which may have been a little low for an initial pitch, but the beer was done fermenting within a week and tasted great. I will be using this method again and sharing it with friends as I find this much easier to use and less time constraints opposed to using my stirrer plate.
That’s interesting. I guess the pertinent question is how quickly the yeast use up the O2 when they become active. It would also be interesting to know how much O2 they need to build up ergosterol sufficient to ferment a 5 gallon batch that hasn’t been aerated. I reckon a good starter can provide all the ergosterol needed, obviating the need to aerate the main wort.
No, that assumption is not correct. The primary limiting factor is available carbon followed by room to grow, nitrogen, and oxygen (O2). Brewing yeast strains are Crabtree positive, which means that they do not respire in wort that contains more than 0.3% glucose. All replication in wort is fermentative. O2 is used for ergosterol and unsaturated fatty acid biosynthesis. Ergosterol and UFAs are synthesized primarily during the lag phase. These compounds factor into cellular health, which means that cells will stop budding if they compounds drop too low.
It’s more of a home brewing oversimplification that is based on old and erroneous information than flat out heresy. Many older home brewing books stated that the growth phase was an aerobic phase, which is incorrect. Additionally, not all strains require the same level of O2 saturation. Dissolved O2 level can even be an inhibitor to growth. Brian H. Kirsop published a seminal paper on the subject in 1973 entitled “Oxygen in Brewery Fermentation.”
Here’s paragraph from the above linked paper that offers food for thought:
“Inhibitory effects of excessive oxygen on the growth of yeast are known and have been reviewed12,30. Such inhibition is unusual in brewery circumstances, but it has been found24 that one yeast strain with a very low oxygen requirement grows less rapidly if oxygen-saturated wort is used as the medium. Excessive aeration has been held to lead to diminished yeast crop and increased flocculence87,58 and an oxygen atmosphere in the head space above a culture medium has been found to inhibit yeast growth.”
Thanks for clarifying. So, assuming an initial saturation, whether it be 8ppm from air or more with pure O2, the difference in cell count between a stirred and stagnant starter is not significant, or is it significant and due to something other than O2? Wyeast seems to say stirring makes more (but not that much more) but doesn’t say why. The yeast book implies it makes a chunk more, and I think the MB Raines text says it can make a phenomenal difference.
It’s been a few years since I made a lager. My general approach was to make a liter or two to get the yeast going, and pitch the whole starter into another 2gal starter (was generally making a 16.5-17gal batch of 1.060 lager). The second stage was in a 3gal carboy with no way to utilize a stir bar. (I would have been using a stir plate/bar for the initial liter) I thought I could make up for the no-stir by leaving the O2 tubing and stone in the wort for the first couple of hours - under a big foil cap of course. I’d come by every 20 min and give the wort another 20s blast two or three times. Hard to measure volume by looking at a layer on the bottom of a carboy, but there seemed to be quite a bit more yeast compared to the times when I’d do the 2gal with only one initial blast of O2. I probably should have measured the solids once decanting the beer, but I never did.
Some cultures never develop much of a head on starter for some reason whereas others produce a sizable head. Bottom fermenting ale and lager yeast strains tend be part of the former group. With bottom fermenters, high krausen often looks more like a thin layer of foam or even foam patches than a typical krausen.
Doesn’t go krausen crazy by any means. Starter wort depth for 1L in my 5L flask was about 1.5 inch. Thin layer of krausen and lots of yeast clumps circles on top.
I am more interested in seeing what WLP090 looks like when fermenting a batch of beer at full tilt. There is zero doubt in my mind that WLP090 is an isolate from either BRY 96 or Whitbread “B.” My bet is that WLP090 is a Whitbread “B” isolate because it is fast fermenter. House strains that descend from Whitbread “B” appear to dominate the San Diego brewing scene.
Is there any use in trying to guess where in the progression of stages the starter is based on CO2 evolution, or are there too many contributing factors to make this tek at all useful? The reason I ask is because once I’m confident that the starter is into the CO2 phase, I used to turn off the stirplate, but I have a feeling that the drop of antifoam I used for the boil is squashing the head. Even solid top croppers don’t seem to make too much of head. However, it’s easy to put your ear near the foil cap and listen to the CO2 crackling. If one could detect the time at which the crackling slows down a bit, could you say that you are at the beginning of the stationary phase or the end of it?
Thanks for the posting the photo. Please let me know if the head gets bigger and/or if it switches over from being a foam head to being a yeast head. I am curious as to the origin of this yeast strain.
Sure thing. Will take another tomorrow a.m. And p.m. After that it will be approaching 72 hrs and winding down.
Edit: wlp090 is by far my favorite ale yeast. Love how it lets the malt and hops shine in pale ales and IPAs . Strong fast fermenter that is extremely clean and great attenuation. Flocs very well. Reminds me most of wlp007.
