If you have heard anything about stirred yeast starters you've probably heard that stir plates produce more yeast than starters that are not agitated. This seems to be a misconception that has been unfortunately widely propagated.
I'm sure Jamil Zainasheff had the best intentions when creating his Mr. Malty Starter Calculator, however it seems that brewers have taken the results out of context and drawn conclusions from the calculator that I doubt Jamil intend. The data behind the vastly popular Mr. Malty equation are from growth that had no aeration and no agitation. (Details of this experiment can be found in the widely acclaimed book: Yeast by Dr. Chris White and Jamil Zainasheff.  If you haven't read this book I highly recommend you get a copy.) There is an anecdotal reference to increased yield from agitation in the book Yeast, however without more detail on how the yield was compared we can only guess as to what the author observed. It is also common in scientific papers to use "specific growth rate" (a measure of the speed of propagation) interchangeable with yield.  Without further explanation I can only assume this is what Dr. Chris White may be refereeing to.
The lack of aeration make Chris White's experiments questionable when used to predict growth on a stir plate. Oxygen is critical for sterol production in yeast. Without aeration an the onset of fermentation the yeast membranes will lack the pliability they need to bud new cells. Given proper aeration at inoculation yeast can double four times, growing to sixteen times the initial population.  Without aeration, as was done by Chris White for the experiments that Jamil used for his calculator, the data show that cell budding was extremely limited. This limitation follows what may be expected given the volume of dissolved oxygen.
Stir plates produce yeast faster, but do not produce more yeast.
In scientific papers on yeast propagation you will generally see that the time frame used is 48 hours.  Propagation occurs much more rapidly with a stir plate so at the 48 hour mark there is often substantially more yeast produced with some form of agitation. However, given enough time, without agitation a similar amount of yeast is produced. 
Yeast can metabolize sugar much more efficiently in the presence of oxygen aerobically than without anaerobically.  Using a stir plate oxygen is constantly introduced. The common conclusion from these two facts is that with a stir plate considerably more sugar will be metabolized aerobically than without. This is, however, not the case due to the Crab Tree Effect. When the sugar concentration is typically above 0.5% w/w (0.5°P or 1.002 SG) yeast will metabolize sugar using anaerobic fermentation. The balance between aerobic respiration and anaerobic fermentation is similar independent of agitation.
 J. Novak, G. Basarova, J. A. Teixeira and A. A. Vicente,
Monitoring of Brewing Yeast Propagation Under Aerobic and Anaerobic Conditions Employing Flow Cytometry. J. Inst. Brew. 113(3), 249–255, 2007
 Greg Doss, Yeast Propagation A Practical Approach
 L. Camacho-Ruiz, N. Pérez-Guerra, R. Pérez Roses, FACTORS AFFECTING THE GROWTH OF SACCHAROMYCES CEREVISIAE IN BATCH CULTURE AND IN SOLID SATE FERMENTATION
 C. R. Murray, T. Barich and D. Taylor, The Effect of Yeast Storage Conditions on Subsequent Fermentations,
 Crab Tree Effect
 R. H. DE DEKEN, The Crabtree Effect: A Regulatory System in Yeast
Some references you should read, kind of the origin of oxygenated/stirred starters:ReplyDelete
BOULTON, C.A. and QUAIN, D.E. (1999), ‘A novel system for propagation of brewing yeast’, Proc. 27th Congr. Eur. Conv., Cannes, 647–654.
BOULTON, C.A. and QUAIN, D.E. (1995), ‘Propagation of brewing yeast under aerobic conditions’, UK patent GB2 298 651.
BOULTON, C. and MIELENENSKI, A. (2006), ‘Aerobic yeast propagation’, in Smart, K. (ed.), Brewing Yeast Fermentation Performance, 3rd edn, Oxford, Blackwell Science, in press.
BOULTON, C. and QUAIN, D. (2001), Brewing Yeast and Fermentation, Oxford, Blackwell Science.
BAMFORTH ed. Brewing new technolgies, Chapters 7 & 8
One of the findings by others using this process is that the biomass of both methods is similar, but the cell count is different. Cells from an oxygenated starter are a bit smaller.
Very interesting! That is a quite a subtle distinction between the two methods. I wonder what, if any, effect that has on fermentation performance and beer quality.Delete
better oxygenated, fitter yeast, quicker start of fermentation and somewhat quicker fermentation. All you want as a pro brewer.Delete
Also, the whole starter discussion and methods very much depend on your goal.
My goal is the right amount of yeast that is fit enough to quickly ferment the wort (<1080) without oxygenating the wort itself. For that I need to stir the starter, even if I'd oxygenate I'd still have to stir to dissolve the oxygen. Just bubbling O2 is not enough. Actually the same as in a bio-reactor where the O2 uptake is controlled by the stirring frequency and not the O2 throughput.
Also I add the amount of nutrients to the starter that is equal as to what is advised to go in the wort. Nothing is added to the wort.
"fitter yeast for a quicker start of fermentation". That's a good goal. Do you have evidence of that? My experimentation has not shown that.Delete
Probably in "Brewing Yeast Fermentation Performance" - Katherine Smart (ed)ReplyDelete
Greg Doss also kind of shows it, if you pitch too late it sub optimal because your yeast is getting exhausted. One thing I see often at homebrewers is that they over-oxygenate the yeast, leave it on the stirrer way too long. Pitch that yeast and it will take at least 6 hours to pick up. A top fit yeast will pick up in a few hours.
So with yeast starters, do you recommend keeping the starter on the stirplate for 48 hours? I typically do a regime like this when I need to make a stepped starter.:ReplyDelete
24hrs - On Stirplate (Step 1)
24hrs - Chill in Fridge & decant after
24hrs - On Stirplate (Step 2)
24hrs - Chill in Fridge & decant after
24hrs - On Stirplate (Step 3)
24hrs - Chill in Fridge & decant after
Pitch at the end of brew day.
Depending on the size of the starter and the yeast count 12 - 16 hours 24 max.ReplyDelete
In a stepped starter the yeast growth (doublings) get a lot less at each step. After decanting you could transfer most of the yeast to a separate vessel and continue with the bit that is left in the flask. If you have the possibility to make two starters at the same time, split after the second step in two, you'll have a bigger yield
I agree for the typical starter where 100 billion cells are added to less than 3 liters of 9°P wort on a stir plate.ReplyDelete
Rate is proportional to cell density, and sugar density. It is also affected by agitation and temperature.
Runing the stir plate for 48 hours will ensure that glycogen reserves are built which is useful if you intend to store the yeast, or cold crash.
I might need to add a bit more information. I typically pitch one pack of yeast that never has the full 100 billion cells in it. Again, I'm using Mr. Malty and the Brewer's Friend calculators to estimate the amount of healthy yeast cells. So I typically have anywhere from 75-90 billion cells depending on freshness. Then I will pitch this into a starter that is about 1.8L (2 quarts). This is on a stirplate in a fridge set to 72F for 24hrs. It's crash cooled for 24hrs, decanted, and then stepped up to typically 2.5L (3 quarts). Again 24hrs on stirplate then 24hrs to chill and decanted. Final step is 5L (6 quarts) with same regime. Does this change things at all?ReplyDelete
Try the starter calculator that I have. (link on the navigation bar of this website). It will show you growth over time. Essentially glycogen is rebuilt when growth slows and the yeast is preparing for the stationary phase. This glycogen reserve will speed lipid production that is needed for membrane permeability at the start of the next fermentation after the cold crash.Delete
Because you are cold crashing 24 hours seems rather short. If you were pitching directly I think you would be fine.
For more details try to find these papers:
Monitoring of Brewing Yeast Propagation Under Areobic and Anerobic Conditions Employing Flow Cytometry
(note the depletion and build up of Glycogen and how it maps to cell growth phase)
Factors affecting the growth of saccharomyces cerevisiaiae in batch culture and in solid state fermentation
"Stir plates produce yeast faster, but do not produce more yeast."ReplyDelete
This just seems to make sense... to me.
Thanks! Sometimes it feels like I'm the only one.Delete