[narcout]

A stir plate provides inadequate aeration if operated at a speed low enough to prevent shear stress (perform a Google search using the terms "magnetic stirrer" and "shear stress"), and if operated at a speed high enough to add significant O₂ to a culture results in shear stress being placed on the cells, which is why cultures that are stirred fast enough to create a vortex smell foul.

Is there any evidence that whatever shear stress might be placed on the yeast has detrimental effects on fermentation performance or the flavor of the resultant beer?

Physics prevents a culture in an Erlenmeyer flask from receiving O₂ after it starts outgassing because gas pressure is highest at the mouth of the flask.

Whether or not stir plates provide any appreciable level of aeration (either before or during active fermentation in the starter) is a topic I have seen debated almost endlessly.  Are you aware of any studies on this issue?  I'd love to get a definitive answer; all I've ever seen is conjecture.

It doesn't seem like it would be a difficult experiment for someone with the proper equipment.

[denny]

Whether or not stir plates provide any appreciable level of aeration (either before or during active fermentation in the starter) is a topic I have seen debated almost endlessly.  Are you aware of any studies on this issue?  I'd love to get a definitive answer; all I've ever seen is conjecture.

It doesn't seem like it would be a difficult experiment for someone with the proper equipment.

I have access to a very high quality DO meter at the brewery, but it will be a while before I'd have time to test this.

[mabrungard]

I agree that it would be difficult for oxygen to diffuse into an Erhlenmyer once fermentation and CO2 evolution is underway. That is why I pump filtered ambient air into the flask headspace. It should be able to supply O2 and keep the headspace from becoming CO2 saturated. A large surface area of the wort is possible when the flask is only partially filled. I use either 4 or 6 Liter flasks for starters and stir fairly gently (like 60 to 100 rpm) to keep the yeast suspended.

I filter through a 0.45 micron filter and use an aquarium air pump. There is no air stone since I'm not trying to get air into the wort. The large surface area on the wort is how the oxygen is transferred. I found out through many hair-pulling events that its very difficult to manage air inputs through an air stone while avoiding foam overflows. Just pumping filtered air into the headspace solved that.

[S. cerevisiae]

Is there any evidence that whatever shear stress might be placed on the yeast has detrimental effects on fermentation performance or the flavor of the resultant beer?

Anything that affects yeast health affects fermentation performance. I did not start out as a doubting Thomas.  It was the difference in performance that made me question the claims that stir plate starters were superior to shaken starters.  I never had to decant supernatant before switching to a stirred starter.  Lag time increased, and high krausen was not as active.  Lag time was increased and fermentation vigor was further reduced when I slowed the stir speed down enough to get rid of the foul smell.

Part of the problem can be attributed to stirring 1L of wort in a 2L flask, which is what most home brewers do.  Increasing the size of the flask to 4 or 5 liters should reduce the need to stir as aggressively due the large increase in head space and surface area.  One thing that I encountered while studying the physics behind shaker tables is that the culture volume should be between 10% and 25% of the flask volume, which makes a lot of sense with a conical-shaped flask.  An orbital shaker further increases surface area by turning the surface of the medium into an inclined ellipse.


With that said, I do not see how a shaker or a stirrer can get around the maximum cell density problem.  While there may be more actual biomass at the end of the process, is there a major increase in viable biomass?  By major, I mean at least a two-fold increase because anything less than two-fold is insignificant.  A viable count difference between two cultures of less two-fold results in both cultures needing to undergo the same number of replication periods to reach high krausen due to fact that yeast biomass grows exponentially at a rate of 2ⁿ. Without a two-fold increase in viable biomass, the variable that matters is yeast health.  Yeast health is dependent on the amount the amount of ergosterol and unsaturated fatty acids (UFA) that the cells hold in reserve when they are pitched as well as any stressors that the cells encountered during propagation.  Given equal amounts of dissolved O₂ at the start of fermentative reproduction (all growth in starters and wort is fermentative due to the Crabtree effect) while holding all other variables constant, the culture that is pitched at high krausen will have higher ergosterol and UFA reserves, resulting lower initial O₂ demand and a shorter lag time upon pitching.

Whether or not stir plates provide any appreciable level of aeration (either before or during active fermentation in the starter) is a topic I have seen debated almost endlessly.  Are you aware of any studies on this issue?  I'd love to get a definitive answer; all I've ever seen is conjecture.

I have seen no peer-reviewed publications on stir plates because shakers and rollers are the preferred cell culturing devices in laboratories.  It's almost a given.  Magnetic stirrers are primarily used for mixing.  They are less complex and cheaper than hermetically-sealed stirrers.

[johnnyb]

A stir plate is an unnecessary expense.  Your money is better spent on other gear.  Perform an advanced search using my user name as the poster and "stir plate" as the search term, and you will discover why a stir plate is little more than home brewing snake oil.

Do you have a sort of a single manifesto post on the subject describing why you think (maybe experimental results?) stir plates are unnecessary (potentially harmful?) and what your recommended method is without the benefit of owning an orbital shaker?

I did the search as recommended but mostly found a bunch of posts that seem to be referring to other posts that I haven't located yet.

I wish I knew about this shaker thing a few months ago. My old biotech company finally bit the dust and I'm sure I could have bought a shaker for peanuts.

See page 7 of this thread https://www.homebrewersassociation.org/forum/index.php?topic=21705.msg276016#msg276016
The takeaway is that Mark sees stressed, continually aerated wort as a bad thing and that a healthy pitch of the entire starter which was aerated thoroughly at the beginning and pitched at high kreusen is better.

Thanks! Read the entire thread. Very interesting.

[dkfick]

See page 7 of this thread https://www.homebrewersassociation.org/forum/index.php?topic=21705.msg276016#msg276016
The takeaway is that Mark sees stressed, continually aerated wort as a bad thing and that a healthy pitch of the entire starter which was aerated thoroughly at the beginning and pitched at high kreusen is better.

Actually, that's an incorrect takeaway.  A stir plate provides inadequate aeration if operated at a speed low enough to prevent shear stress (perform a Google search using the terms "magnetic stirrer" and "shear stress"), and if operated at a speed high enough to add significant O₂ to a culture results in shear stress being placed on the cells, which is why cultures that are stirred fast enough to create a vortex smell foul.  Physics prevents a culture in an Erlenmeyer flask from receiving O₂ after it starts outgassing because gas pressure is highest at the mouth of the flask.   

With that said, stir plates and orbital shakers are completely unnecessary in a home brewery (I purchased my orbital shaker for experimental reasons).  A better investment is an O₂ diffusion stone and a source of O₂.   Brewing yeast cells will grow to fit their environment if given enough O₂ and carbon (sugar is carbon bound to water; hence, the term carbohydrate).   Most brewing yeast cultures do not need to be stirred because viable cells naturally remain in suspension due to something known as NewFlo flocculation.  NewFlo strains do not aggregate until glucose, mannose, maltose, sucrose, and maltotriose have reached genetically set levels; hence, most brewing cultures do not need to be stirred  to keep the cells in suspension.

As I have mentioned many times before, my method is a low cost, low-tech way to produce a healthy yeast culture. I did not set out to create a low cost, low-tech method for making healthy starters.  It was a case of serendipity.  I was preparing starters using English measurements at that point in time. I made one quart starters in a 48oz glass Ocean Spray Cranberry juice bottle.  I went to make a starter and noticed that the bottle was cracked, so I decided to use a 1-gallon glass jug that I used to make mead for my starter.  Shaking until the culture was almost completely foam was the result of being strong at that point in my time due to spending my teenage and my twentysomething years in the gym.  I used the method for several years before it dawned on me why starters made in the 1-gallon jug worked better than those made in a 48oz container.  The reason is foam.  It is easier to make 1 quart of wort expand into foam in a 1 gallon container than it is in a 48oz container, and wort in gas-liquid foam form has a much higher specific area surface than wort in liquid form, which leads to increased O₂ pickup.  In essence, my method is a poor man's O₂ injection system.

I agree that it would be difficult for oxygen to diffuse into an Erhlenmyer once fermentation and CO2 evolution is underway. That is why I pump filtered ambient air into the flask headspace. It should be able to supply O2 and keep the headspace from becoming CO2 saturated. A large surface area of the wort is possible when the flask is only partially filled. I use either 4 or 6 Liter flasks for starters and stir fairly gently (like 60 to 100 rpm) to keep the yeast suspended.

I filter through a 0.45 micron filter and use an aquarium air pump. There is no air stone since I'm not trying to get air into the wort. The large surface area on the wort is how the oxygen is transferred. I found out through many hair-pulling events that its very difficult to manage air inputs through an air stone while avoiding foam overflows. Just pumping filtered air into the headspace solved that.

This is the same procedure I use. Never any foul smells etc.

[narvin]

There have been a few people who have measured the results of closed vs open starters and found that there was increased yeast growth. Ex:

http://braukaiser.com/blog/blog/2013/03/19/access-to-air-and-its-effect-on-yeast-growth-in-starters/

http://braukaiser.com/blog/blog/2013/05/19/yeast-growth-and-the-question-of-quality-vs-quantity/

[a10t2]

There have been a few people who have measured the results of closed vs open starters and found that there was increased yeast growth.

My own data support that as well. An open, stirred starter results in much more yeast than an airlocked, stirred starter.

[bboy9000]

Whether or not stir plates provide any appreciable level of aeration (either before or during active fermentation in the starter) is a topic I have seen debated almost endlessly.  Are you aware of any studies on this issue?  I'd love to get a definitive answer; all I've ever seen is conjecture.

It doesn't seem like it would be a difficult experiment for someone with the proper equipment.

I found this and posted it on the forum quite some time ago.  There's probably more information out there somewhere.

http://www.brewangels.com/Beerformation/AerationMethods.pdf

[bboy9000]

I also found this after 15 minutes of Googling.  I haven't read all of it but there's probably more information out there.

https://www.wyeastlab.com/hb_oxygenation.cfm

http://www.mbaa.com/districts/Ontario/Events/Documents/2014%20MBAA%20Chris%20White%20Yeast%20Management.pdf

https://08071324485917489618.googlegroups.com/attach/5b81da2ae746b03b/Influence%20of%20Aeration%20During%20Propagation%20of%20Pitching%20Yeast%20on%20Fermentation%20and%20Beer%20Flavor.pdf?part=14&vt=ANaJVrGtazyRjAY9cgaJgsEvvYNk_6_-LOBJ1DVhbO6t1AIwc-y6HfdFId1NYJ_B3KMozgXqFbvn2ZJDfa26brwfqj9mWuRyY2EPCG_at2DsTUx4yLlQrTE

http://www.pivarstvo.info/forum/files/yeast_propagation_and_maintenance_607.pdf