[brewinhard]

I agree.  I don't understand why people say they are having stirred starters taste/smell bad/foul.  Mine pretty much always smell like beer and usually a fairly clean version of the batch I plan on making since it just has extra light DME in it.  If it smells off, then I don't use it (dry yeast to the rescue), but I have only had that happen once in my 10+ years of brewing. 

[Stevie]

Mine for sure smell stale. Think back to college when you had to clean up the fallen soldiers.

[S. cerevisiae]

If your culture does not smell off, then you are not spinning it fast enough to cause much in the way of turbulence, which means that you are depending on the tiny amount of surface area in the flask for O₂ pickup.

[Wort-H.O.G.]

If your culture does not smell off, then you are not spinning it fast enough to cause much in the way of turbulence, which means that you are depending on the tiny amount of surface area in the flask for O₂ pickup.

Like said, not sure as reasons why. I use pure o2 before pitching and turning stir plate on.my flask is 4L.


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[S. cerevisiae]

Like said, not sure as reasons why. I use pure o2 before pitching and turning stir plate on.my flask is 4L.

Therein lies the difference between the way that you make a stirred starter versus 99.9% of the home brewers that use a stir plate.  You would achieve equal or better results without stirring.

[Wort-H.O.G.]

Like said, not sure as reasons why. I use pure o2 before pitching and turning stir plate on.my flask is 4L.

Therein lies the difference between the way that you make a stirred starter versus 99.9% of the home brewers that use a stir plate.  You would achieve equal or better results without stirring.

Will be doing exactly that for this weekends IPA with wlp090. Using a vial with 1 liter 1.040 wort, using O2 and then just let it reach high krausen and pitch into my IPA. looking forward to trying it out.


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[Whiskers]

I'd guess though that the increased O2 pickup with a stirred starter is to do with increasing the concentration gradient much more than it does with increasing surface area.  I'd also guess that a gentle stir would be enough to get you most of the way there.  Movement in a solid beer fermentation seems nearly on par with what I get with stir plates with a gentle dimple. 

And by my calcs there is way more O2 in the headspace of a starter than is needed to re-saturate the wort many, many times over.  The only reason I use foil is because it keeps the neck sanitary/clean. 

[S. cerevisiae]

I'd guess though that the increased O2 pickup with a stirred starter is to do with increasing the concentration gradient much more than it does with increasing surface area.  I'd also guess that a gentle stir would be enough to get you most of the way there.  Movement in a solid beer fermentation seems nearly on par with what I get with stir plates with a gentle dimple. 

And by my calcs there is way more O2 in the headspace of a starter than is needed to re-saturate the wort many, many times over.  The only reason I use foil is because it keeps the neck sanitary/clean.

You are not getting anywhere near the level of O₂ pickup that would you obtain in a shaken, not stirred culture or direct O₂ injected culture of equal size if you are only gently stirring the culture.  Not to mention that it is taking much longer to pick up O₂ than in a shaken, not stirred starter or a directly injected starter.  I have already shown that surface area to volume ratio trumps laminar or turbulent flow.

[Whiskers]

I don't recall that being demonstrated, either experimentally or theoretically.  I'm no yeast expert but I have done a lot of diffusion research.  And I'm not talking about initially saturating the wort, but maintaining saturation as the yeast consumes the O2. 

[S. cerevisiae]

I don't recall that being demonstrated, either experimentally or theoretically.  I'm no yeast expert but I have done a lot of diffusion research.  And I'm not talking about initially saturating the wort, but maintaining saturation as the yeast consumes the O2.

If you scroll down to Example 18-1.1 Aeration of a fermentation broth, you will discover that my assertion that increasing the surface area per unit volume trumps increasing the mass transfer coefficient (i.e. increasing laminar or turbulent flow) is correct.  Gas-liquid foam, with its high specific surface area, is one of the reasons, if not the sole reason why the Shaken, not stirred works as well as it does for being such a trivial technique. 

https://books.google.com/books?id=dq6LdJyN8ScC&pg=PA513&lpg=PA513&dq=Gas+diffusion+across+different+pressures&source=bl&ots=vKUx5Cngnm&sig=60l3GRMJLKC9mixHloplXHIgrRQ&hl=en&sa=X&ved=0CE4Q6AEwB2oVChMIidDjq6LbyAIVQ1Y-Ch3iggtN#v=onepage&q=Gas%20diffusion%20across%20different%20pressures&f=


The average culture that is shipped with 100B cells contains at least 50B cells when pitched.  That's at most two replication periods when pitched into 1L of wort.  Brewing yeast cells are Crabtree positive, which means that they do not respire in wort.  How much O₂ does a culture need post lag phase?  Additionally, having fully saturated wort from the time zero reduces the lag period, which allows the exponential phase to start earlier.

Finally, there is no compelling evidence that supports the notion that a covered, gently stirred culture is continuously aerated.   What I found interesting is that Neva Parker stated that a stir plate does not aerate culture when asked last week.  Here is a scientist who runs the analytical side of White Labs claiming that a stir plate does not aerate a culture.  If anyone has the equipment to test such an assertion, it is Neva.

[Whiskers]

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. 

[atodd]

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. 

[charles1968]

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.

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.

[narcout]

What I found interesting is that Neva Parker stated that a stir plate does not aerate culture when asked last week.

I honestly don't care very much about the answer to this question anymore, but I didn't interpret her response in the same way.

[S. cerevisiae]

It's generally the O2 availability that limits the cell density (for a starter) in most cases, right?

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.

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?

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."

http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1974.tb03614.x/pdf

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 reviewed^12,30.  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 flocculence^87,58 and an oxygen atmosphere in the head space above a culture medium has been found to inhibit yeast growth."