[Philbrew]

What's the right RPM for stir plate?  Does the size of the starter matter?  I don't have one yet.  Just doing my homework.  Would a small external aquarium pump plumbed to circulate and splash starter work as well?

[denny]

It doesn't matter at all.  As long as the stir bar is moving and the yeast and wort are mixing, it's fast enough.  Contrary to the myth, a bigger vortex gains you nothing.

[TMX]

It doesn't matter at all.  As long as the stir bar is moving and the yeast and wort are mixing, it's fast enough.  Contrary to the myth, a bigger vortex gains you nothing.

Big vortex = bragging rights

[duboman]

The vortex makes no difference as mentioned, as for the aquarium pump idea, not sure. While o2 is beneficial for the starter I'm not sure if it wouldn't be detrimental pumping into the starter for the 48-72 hours it would take to ferment out. Maybe someone else has a thought on that?

[Philbrew]

It doesn't matter at all.  As long as the stir bar is moving and the yeast and wort are mixing, it's fast enough.  Contrary to the myth, a bigger vortex gains you nothing.

So it's not so much the aeration as it is the mixing that gets you the good yeast growth of a stir plate vs. intermittent shaking, etc.?

[klickitat jim]

I'm more and more convinced that a stirplate only gets the yeast up into the wort, its not doing much at all for O2. So, the O2 comes from shaking or oxygenation, or whatever method. If this is so, slow stir is fine, so is fast stir.

[denny]

It doesn't matter at all.  As long as the stir bar is moving and the yeast and wort are mixing, it's fast enough.  Contrary to the myth, a bigger vortex gains you nothing.

So it's not so much the aeration as it is the mixing that gets you the good yeast growth of a stir plate vs. intermittent shaking, etc.?

Correct.

[JT]

The vortex makes no difference as mentioned, as for the aquarium pump idea, not sure. While o2 is beneficial for the starter I'm not sure if it wouldn't be detrimental pumping into the starter for the 48-72 hours it would take to ferment out. Maybe someone else has a thought on that?

A microbiologist @ Wyeast confirmed this for me a year ago. 
After reaching terminal gravity, adding oxygen only keeps the yeast active with nothing to eat so they burn through their energy reserves.  Refrigeration when the wort hits terminal gravity is the best way to go.  From my testing a 2 liter starter only takes about 18 hours to finish out around the ideal temp of 70.  I never go past 30 hours on a plate.  Usually just 24. 

[duboman]

This makes sense and I'm not sure why I typed 48-72 hours as my starters are done in the 18-24 range as well and then placed in the fridge. Perhaps its because in know some people just go longer for no apparent gain.  That was why I was ques iining using an aerator for the entire time in lieu of a stir plate in possibly achieving the same results.

[S. cerevisiae]

JT, you were given bad information.  Adding oxygen after fermentation is complete causes yeast cells to enter diauxic shift where they consume ethanol as their carbon source.   

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3133784/

"The preferred source of carbon and energy for yeast cells is glucose. When yeast cells are grown in liquid cultures, they metabolize glucose predominantly by glycolysis, releasing ethanol in the medium. When glucose becomes limiting, the cells enter diauxic shift characterized by decreased growth rate and by switching metabolism from glycolysis to aerobic utilization of ethanol. "


In the absence of post-fermentation dissolved oxygen, yeast cells enter a quiescent state after storing glycogen.  The cells also undergo morphological changes where their walls thicken in preparation for quiescence, which is why one does not want to allow a starter to ferment out before pitching it.

[Wort-H.O.G.]

JT, you were given bad information.  Adding oxygen after fermentation is complete causes yeast cells to enter diauxic shift where they consume ethanol as their carbon source.   

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3133784/

"The preferred source of carbon and energy for yeast cells is glucose. When yeast cells are grown in liquid cultures, they metabolize glucose predominantly by glycolysis, releasing ethanol in the medium. When glucose becomes limiting, the cells enter diauxic shift characterized by decreased growth rate and by switching metabolism from glycolysis to aerobic utilization of ethanol. "


In the absence of post-fermentation dissolved oxygen, yeast cells enter a quiescent state after storing glycogen.  The cells also undergo morphological changes where their walls thicken in preparation for quiescence, which is why one does not want to allow a starter to ferment out before pitching it.

i presume everything you say is factual. but the question that lingers, is that if making a starter and pitching or even attempting to significantly slow fermentation by cold crashing is the preferred or optimal way to do it, how can it be explained that good results are obtained by letting a starter finish, cold crashing, decanting and then pitching? seems like a lot of us have done it this way over the years and for me personally, I cant say its caused any issues. just curious I guess.

[Wort-H.O.G.]

I'm not discounting or disputing any of your scientific facts. I'm just questioning if my beer will taste any different in a blind test with a starter fully fermented and one not.


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[S. cerevisiae]

i presume everything you say is factual. but the question that lingers, is that if making a starter and pitching or even attempting to significantly slow fermentation by cold crashing is the preferred or optimal way to do it, how can it be explained that good results are obtained by letting a starter finish, cold crashing, decanting and then pitching? seems like a lot of us have done it this way over the years and for me personally, I cant say its caused any issues. just curious I guess.

There's a difference between acceptable results and optimal results.  Cold crashing and decanting at the end of the exponential phase ensures that the yeast cells are in optimal health when pitched into a batch of wort while maximizing the use of the medium.  The cells still have ergosterol and unsaturated fatty acid (UFA) reserves, and the cell membranes have not yet had to deal with increasing levels of metabolites and ethanol.  In effect, yeast cells that are pitched at the end of the exponential phase are healthy and ready go with minimal replenishment.  Cold crashing and decanting at the end of exponential phase mimics top-cropping at high krausen, which is how  top-cropping breweries crop.  Top-cropping at high krausen is why top-cropping breweries can re-pitch almost indefinitely.  Harvey's in the UK has been re-pitching the same top-cropped yeast culture for over fifty years.

One thing that I left out in my first reply is that the ergosterol and UFA reserves that are built by mother cells while O2 is still in solution are shared with all of their daughter cells.  Peak viable yeast biomass is reached at the end of the exponential phase. From that point forward, reproduction is for replacement only, which means that we are wasting ergosterol and UFA reserves while exposing the cells to substances that take their toll on cell membrane health.  While there is a slight increase in biomass by allowing the starter to ferment out, that increase is composed of non-viable cells.  The goal when propagating a yeast culture is to increase the viable cell count, not produce ethanol.   The only cell count that matters when propagating a culture is the viable cell count.

[JT]

JT, you were given bad information.  Adding oxygen after fermentation is complete causes yeast cells to enter diauxic shift where they consume ethanol as their carbon source.   

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3133784/

"The preferred source of carbon and energy for yeast cells is glucose. When yeast cells are grown in liquid cultures, they metabolize glucose predominantly by glycolysis, releasing ethanol in the medium. When glucose becomes limiting, the cells enter diauxic shift characterized by decreased growth rate and by switching metabolism from glycolysis to aerobic utilization of ethanol. "


In the absence of post-fermentation dissolved oxygen, yeast cells enter a quiescent state after storing glycogen.  The cells also undergo morphological changes where their walls thicken in preparation for quiescence, which is why one does not want to allow a starter to ferment out before pitching it.

I don’t think it was bad info, I’m likely not stating it correctly.     Keeping the yeast on the stir plate and in optimal temps allows it to chow down on the less desirable food source.   From the same article “Cells in the diauxic shift and stationary phase are stressed by the lack of nutrients and by accumulation of toxic metabolites, primarily from the oxidative metabolism…”  So they’re still eating, but nothing good for them.   

Being that beer is a mix of art and science, I think scientific studies can be misleading for brewers when they just focus one aspect: yeast health.  Case in point: it is better from a yeast health perspective to pitch an active starter, but from a tasty beer perspective it isn’t always the best option, because the starter wort has undesirable flavors. 

[S. cerevisiae]

I would like to add that cold crashing at the end of the exponential phase reduces starter lead time from a couple of days to less than 24 hours when pitching a relatively fresh White Labs vial. This reduction in lead time means that a starter can be pitched the evening before one intends to brew, popped into one's refrigerator late morning the following day in preparation for decanting, and pitched late afternoon the following day.