Stir plate efficiency

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stunsm

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So how much does a stirplate really increase cell count? I've read some posts and literature that claim up to a 10 fold increase, but the Wyeast calculator seems to show only about a 30% increase. I've used a stirplate on all of my starters and have always had very vigorous fermentation, but I'm curious what the real effects are, and if the 30% figure is accurate, I've been grossly underpitching but still getting very good results.
 

kanzimonson

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That depends on what you're comparing it to. It's about 30% more than "shaking the starter every hour." It's probably twice as many as a straight starter that isn't touched after pitching.
 

Pezman1

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I was wondering this thing myself. I use the "shake when you walk by" method with great results. However, a stir plate IS a cool piece of gear...

Pez.
 

724b

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The calculator over at mrmalty.com gives you an idea of yeast count with different methods including periodic shaking.
 
OP
S

stunsm

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I've used the calculators at mrmalty and wyeast, but they seem to give slightly different results. Anyone with a hemocytometer ever do any side by side analysis?
 

724b

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It's my understanding that wyeast used some of Jamil's calculations to build their calculator. I know they are different though.
 

Pivovar_Koucky

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I too am interested in hard data regarding the effectiveness of stir plates. Specifically, how does O2 get into the starter after fermentation begins and CO2 starts billowing out?
 

724b

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Isn't O2 constantly getting absorbed because of the movement? That's one of the reasons that growth rates are so much higher with a stir plate.



I too am interested in hard data regarding the effectiveness of stir plates. Specifically, how does O2 get into the starter after fermentation begins and CO2 starts billowing out?
 

BoundForBeer

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Isn't O2 constantly getting absorbed because of the movement? That's one of the reasons that growth rates are so much higher with a stir plate.
But when it is fermenting it produces co2 which would push the o2 out of the container...the co2 blankets the wort
 

724b

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If you are using an airlock, that could be a problem.

If you're using foil or a foam stopper, which is recommended, I think that there is airflow because of the liquid movement.
 

kanzimonson

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People believe that because CO2 is denser than air it forms a distinct layer, but this is just not true. There's CO2 in air... it moves around and gets incorporated.

CO2 and air are not oil and water, they're honey and maple syrup - eventually they mix.
 

Pivovar_Koucky

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People believe that because CO2 is denser than air it forms a distinct layer, but this is just not true. There's CO2 in air... it moves around and gets incorporated.

CO2 and air are not oil and water, they're honey and maple syrup - eventually they mix.
Yes, but they mix by diffusion or by convection. If you have 1 liter of wort at 20 degrees C (68 F) and the gravity goes from 1.040 to 1.010 then you make 16.67 L of CO2. If that takes 48 hours then you're talking 347.3 cubic centimeters per hour. Unless O2 can diffuse against that flow rate, CO2 will quickly push all of the O2 out of the flask.
 

kanzimonson

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I'm curious if there's a correlation between the way liquid moves in the starter and the way gas moves in and out of the flask.

What I mean is, the whirlpool of the starter creates a lower pressure region in the center of the flask. Does that mean that more CO2 is leaving the center of the starter, in general? Or do you think this is a negligible effect? If it's true, perhaps more O2 is able to enter the liquid at the edge of the surface?

Just throwin out some pseudo-science here...
 

Pivovar_Koucky

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I'll tell you what, someone measure the internal diameter of the mouth of a 1L erlenmeyer flask and give me some ballpark numbers for their starting and ending gravities and the time it takes to ferment and I'll crunch the numbers and come back and post my results.
 

kanzimonson

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My 2L flask (which I'm guessing is the size that most people use) is 1-5/8 (1.626") in diameter at the mouth. To be clear, this is not including the thickness of the glass.
 

Pivovar_Koucky

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I'm curious if there's a correlation between the way liquid moves in the starter and the way gas moves in and out of the flask.

What I mean is, the whirlpool of the starter creates a lower pressure region in the center of the flask. Does that mean that more CO2 is leaving the center of the starter, in general? Or do you think this is a negligible effect? If it's true, perhaps more O2 is able to enter the liquid at the edge of the surface?

Just throwin out some pseudo-science here...
I don't think that the whirlpool is due to lower pressure, it's due to the fluid flow being constrained at the walls. If anything I would expect that less CO2 is leaving at the center than at the edge because the height of the fluid column is less there, so less yeast is farting out CO2.
 

kanzimonson

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Maybe I've got it wrong, but I thought I've heard it explained that the lower pressure is due to the whirlpool. Because of the constraint at the walls, the highest pressure is at the edge of the flask. The lowest pressure is in the dead center of the tornado. Fluid moves from the tornado, out to the wall, down to the bottom along the wall, towards the tornado along the bottom, and then up to the top of the flask through the tornado.
 

Pivovar_Koucky

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My 2L flask (which I'm guessing is the size that most people use) is 1-5/8 (1.626") in diameter at the mouth. To be clear, this is not including the thickness of the glass.
Would you say you usually fill the flask up all the way to the 2L mark or is it more like 1.5L?
 

724b

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Interesting. That's a lot of gas!

It is constantly degassing co2, right? When it degasses, something needs to flush back in to take it's place. Otherwise it would create a vacuum.

If a bottled beer absorbs oxygen through a crown cap (which is does). Surely oxygen flushes back into a starter through foam or aluminimum foil. Right?

I feel like there is enough movement to cause gas movement in and out of the starter.

Are feelings part of science? :)
 

Pivovar_Koucky

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Interesting. That's a lot of gas!

It is constantly degassing co2, right? When it degasses, something needs to flush back in to take it's place. Otherwise it would create a vacuum.

If a bottled beer absorbs oxygen through a crown cap (which is does). Surely oxygen flushes back into a starter through foam or aluminimum foil. Right?

I feel like there is enough movement to cause gas movement in and out of the starter.

Are feelings part of science? :)
Nothing needs to come back in because the CO2 is being created by the yeast from sugar, this increase in the amount of gas causes a rather small increase in pressure which drives the CO2 above the liquid out.
 

kanzimonson

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Would you say you usually fill the flask up all the way to the 2L mark or is it more like 1.5L?
Hmmm, it feels like I'm usually making a full 2L starter, usually because I'd rather use a small bit of my stored slurry and spend the money on the DME for the starter.

In reality, it's probably about 66% of the time, and the rest is a mix of 1-1.5L starters.
 

Pivovar_Koucky

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So this problem was a hell of a lot more complicated than I had anticipated. Mostly because of boundary conditions. Its a 2nd order linear PDE so I need 2 boundary conditions and an initial condition to solve.

The initial condition was fine, C=C0 for all x (atmospheric concentration at all points).
The boundary conditions were harder. All of the conditions I could think of made the problem trivial (C=0 at x=0 for all t, i.e. what I was trying to prove), stupid (C=C0 at x=L for all t, i.e. atmospheric concentration at the mouth of the flask. This results in the concentraion not being a function of time.), or a beast (C=f(t) at x=0, for all t. Basically, specifying a function to describe the concentration at the liquid-gas interface).

What I ended up doing was to compare the diffusion distance versus the distance traveled by moving gas in a semi-infinite tube. The result of this is that after about 50 min the convection distance (v*t) exceeds the diffusion distance (2*sqrt(D*t)). I interpret this to mean that after 50 min there is no O2 left at the liquid-gas interface.

I realize that this is a vast oversiimplification so I certainly wont be offended if no one takes this as an absolute refutation of the stirplate increasing O2 in solution.
 

gtheroux

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When there is oxygen present the yeast multiple, not ferment... the reason the stir plat is so effective is that when it is stirring they just have sex... no eating the sugars at this point... hump hump hump all night long... and then when you stop there are crazy amounts of yeast who only then start fermenting... To my understanding, this is why the stir plate is so effective

Who has starters fermenting with in 24hrs anyways?
 

kanzimonson

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Thanks Pivovar for the laborious calculations. Pretty interesting stuff, though I don't know what to think of the findings. In my head I was like, "So what, should I just turn off the stirplate after an hour?" But there's also got to be some benefit to keeping the yeast suspended and encouraging CO2 to blowout. Plus statistically, there's got to be some oxygen there. Science!
 
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But when it is fermenting it produces co2 which would push the o2 out of the container...the co2 blankets the wort
I would think the motion of the spinning liquid would disrupt that "blanket" of CO2. See the response to the next quote below:

Maybe I've got it wrong, but I thought I've heard it explained that the lower pressure is due to the whirlpool. Because of the constraint at the walls, the highest pressure is at the edge of the flask. The lowest pressure is in the dead center of the tornado. Fluid moves from the tornado, out to the wall, down to the bottom along the wall, towards the tornado along the bottom, and then up to the top of the flask through the tornado.
Bear with me, I know it's wikipedia, but it's pretty straight forward:
Wind is caused by differences in pressure. When a difference in pressure exists, the air is accelerated from higher to lower pressure. On a rotating planet the air will be deflected by the Coriolis effect, except exactly on the equator. Globally, the two major driving factors of large scale winds (the atmospheric circulation) are the differential heating between the equator and the poles (difference in absorption of solar energy leading to buoyancy forces) and the rotation of the planet.
The circular motion of the liquid created by the stir bar and resulting vortex, and the friction of the movement between the air and surface of the liquid should definitely create at least a "starter-sized" breeze in the flask/jug/whatever.

I have a 12 month old who always ends up with bits of food/snacks on the floor around her high chair. I picked up a couple pieces of some snack she was eating that floats in water and put them in a flask of water and put it on the stir plate to see how they moved around. They all spun on the surface until being drawn into the vortex and then were kicked out of the vortex by being batted by the stir bar or simply tossed because of the centrifugal force. Sometimes they went all the way out to the outside wall and up to the surface to repeat and other times they were drawn back into the vortex by the flow of the liquid.

I know, it's based on floating pieces of food and not yeast cells, but it demonstrated a flow even better once the pieces got a little soggy and started breaking apart.

When there is oxygen present the yeast multiple, not ferment... the reason the stir plat is so effective is that when it is stirring they just have sex... no eating the sugars at this point... hump hump hump all night long... and then when you stop there are crazy amounts of yeast who only then start fermenting... To my understanding, this is why the stir plate is so effective

Who has starters fermenting with in 24hrs anyways?
I would think the vortex spinning in a manner to draw liquid, objects, air, etc down into it would create a vacuum in the flask and thus draw air from outside the flask into it. Add onto that the potential of CO2 created by the yeast if there is fermentation or gases created by their reproduction and I would think it would cycle just fine.

All I can say is after pitching my first batch of yeast from a stir plate on Saturday, I had a layer of thin krausen covering the entire surface of the wort in under 5 hours. Not just air lock activity, but a surface of foam. I had 3-4" of krausen and vigorous fermentation in under 12 hours. I don't think I've had that even with dry yeast.
 
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