Hmmm…good question. I don’t really know. All I can say is that I successfully repitch yeast from a fermentation done like that.
And your experience doing that is most valuable and applicable. But the idea that I will get zero increase in the number of yeast cells I pitched from a ferment of 5.5 gallons is pretty hard to believe.
My method’s not on that list, so FWIW, this is it.
I ferment in a plastic jerrycan from no-chill and have around 15L wort in a 23L jerrycan, so headspace. When it’s cool I squirt pure O2 into the headspace after pitching and shake like it owes me money. After 18 - 24 hours fermentation I do the same again following the guidance in “Yeast” which is that this is the period at which oxygen is most beneficial for the synthesis of sterols. This practice also aligns with the principles of the shaken not stirred starter in that the availability of oxygen to wort is maximised in the oxygen rich foam from shaking.
As others have said, this may not be necessary, but my fermentations are always healthy and never any off flavours from fermenting.
cheers
steve
http://www.lallemandyeast.com/articles/crabtree-effect-and-overflow-metabolism
“When the wort has more than >0.2% sugar, the function of the O2 is to assist the yeast in producing lipids. The lipids in the cell wall act as a growth factor by keeping the cell wall fluid, allowing buds to form. The production of these lipids require trace amounts of oxygen to move the squalene to the lipid stage. With out O2 the mother cell cannot produce any lipids and must shares her lipids with her daughter cell. This can occur for about 3 - 4 cycles before the cell wall becomes leathery and will not allow a new bud to form. Active Dry Beer Yeast initially contain enough lipids in their cell wall for 3 - 4 growth cycles. This is enough to complete most beer fermentations.”
If there’s enough cells in a dry yeast pack such that there is no cell growth, you could never have an overpitch by re-using a yeast cake, right? Because the yeast cake would be the same number of cells as a dry yeast pack. Maybe I’m confused. I don’t think it’s possible that there’s no growth.
If you pitch a single packet of dry yeast into 5 gallons of wort, there will be several replication periods.
If you assume maximum cell density of 200 billion cells per liter, a 5 gallon batch size, and that a dry yeast packet contains 200 billion cells (I don’t know how many cells a dry yeast packet actually contains), you are looking at approximately 4.2 replication periods. If you assume there are 400 billion cells in a packet, you are looking at approximately 3.2 replication periods.
S.C. laid out the math here: Yeast Cultures are Like Nuclear Weapons | Experimental Brewing
I suppose you could pitch enough yeast so that there was little or no cell growth, but I believe it would negatively impact flavor (that would be an interesting experiment).
Assuming growth=replication, Denny’s statement seems incorrect. Hence my extrapolation that if this is correct, you couldn’t overpitch using a yeast cake. Except that I’m certain that using a full yeast cake results in a drastic overpitch.
My understanding is that dry yeast doesn’t need oxygen because it has sterol reserves already built up in whatever science-y way they go about making it.
Agree, and Denny has been saying the same thing WRT liquid yeast…paraphrasing, ‘if you pitch enough healthy yeast then there will be no growth and therefor no need for dissolved O2 in the wort’.
I think it’s inevitable that there will be some growth, but the need for growth is greatly reduced.
Keep in mind the viability of the age of the packet you are getting. Even with how slowly dry yeast drop-off, I can’t imagine that 100% of the cells are ready to rock especially when the packet has a bit of age on it. Not all survive the rehydration stage either (whether its in wort or water).
I need to go make an olive oil, flour, juicy ipa…
Exactly. But there is massive growth in the number of yeast cells! 4 replications (read 4 doublings) means that there will be 16 times as many cells in the yeast cake as compared to the original pitch.
What Narcout quoted from Lallemond said that there is enough lipids in dry yeast to accomplish 4 replications. Anything more will require additional O2.
3-4 replications is the commonly quoted target number.
That is correct. O2 demand is lessened if one pitches a starter at high krausen as well.
Good to see ya, Sacc!
It is nice be back, Denny.
Yes, that is why I think some aeration is a good idea even if you are pitching dry yeast.
In the last year and a half, I’ve pitched either dry yeast or slurry.
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I pump into the fermenter and I splash the stream around to create as much foam as possible if I’m pitching rehydrated dry yeast.
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If I’m pitching slurry, I get out the O2 stone.
Good idea! Denny (and Gary) will really drink it up and enjoy as long as you hop it with lots and lots of Fuggles.
;D
I’ve been slowly making my way through White & Zainasheff’s Yeast(y) tome; this topic comes up at page 75.
Long story short, adding enough O2 leads to a faster fermentation and improved attenuation. 8-10ppm is the target for most wort, and shaking only gets you in the high 2s.
Not saying anyone’s process is wrong, especially if you’re satisfied with the results, but why leave anything to chance? A healthy pitch is just the start. What about the rest of the race to FG? 