10 gal brew in two fermentors: Different fermentation rates.

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10 gal brew in two fermentors: Different fermentation rates.

Postby Charlie » Sun Mar 12, 2017 7:59 pm

I brew ten gallon batches split into two fermentation vessels, and at some point I noticed that the fermentors had different activity rates. Not wildly different, but different, and I was at a loss to explain why.

Originally I used a 2L starter in a 4L flask, but a couple of years ago I got a 23 qt pressure canner and went to 1L starters in 2L flasks because I can sterilize them in the larger pressure canner. It was at that point that I began to notice a difference in fermentation rates.

My procedure is:
1. Prepare 2L of starter wort (200g of light DME, dissolve and QS 2L).
2. Split it into two 1L aliquots in 2L flasks.
3. Sterilize the starters at 15 PSI for 20 minutes.
4. Cool the starters, decant the previous starter stage, add the contents of one flask to the decanted starter and stir until homogeneous.
5. Add the stirred starter to the other starter and stir until homogenious.
6. Pour half of the mixed starters back into the other 2L flask, and put both on the stir plate until they are done. The starters are as identical as they can be. Right? So why do I get different fermentation rates?

Image

If you screw around with something long enough you begin to notice things: Starter 1. on the left has about 2X more krausen than starter 2. on the right. It is not apparent in the photo, but starter 2. is quite a bit lighter than starter 1. indicating more suspended yeast. Here's the deal:

1. After the previous starter stage was crashed I decanted it and dumped the contents of starter 1 into its flask and stuck it on the stir plate.
2. When the yeast was re-suspended I dumped that into the sterilized wort in starter 2 and stuck it on the stir plate.
3. When the solution was homogeneous I dumped half of it back into starter 1's flask and stuck them both on stir plates.

Is that clear as mud?

Long story short, the wort in starter 1 was dumped into another flask twice where the wort in starter 2 was dumped into another flask only once. I think I'm seeing an oxygenation effect!

Charlie
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Re: 10 gal brew in two fermentors: Different fermentation ra

Postby Jimmy » Mon Mar 13, 2017 9:01 pm

That is significant. I always skimp on the oxygen when I put the wort in the primary fermenter. I'll have to rethink on my procedures now.
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Re: 10 gal brew in two fermentors: Different fermentation ra

Postby Charlie » Mon Mar 13, 2017 9:25 pm

Jimmy wrote:That is significant. I always skimp on the oxygen when I put the wort in the primary fermenter. I'll have to rethink on my procedures now.

I oxygenate the fresh cooled wort in the fermentors, but not the starters. It's a bit of a head scratcher for me too.

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Re: 10 gal brew in two fermentors: Different fermentation ra

Postby jballen » Tue Mar 14, 2017 5:48 pm

One would think that constantly stirring a starter would provide plenty of oxygen.

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Re: 10 gal brew in two fermentors: Different fermentation ra

Postby Jimmy » Tue Mar 14, 2017 7:33 pm

Oh man. We need a full test on whether the more violent mixing puts more oxygen in the mix than simple stirring. Maybe we need to toss our the stir bars and get one of these to do starters in.

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Re: 10 gal brew in two fermentors: Different fermentation ra

Postby Charlie » Tue Mar 14, 2017 8:45 pm

jballen wrote:One would think that constantly stirring a starter would provide plenty of oxygen.


I don't think that it does, and that whatever oxygen was in these starters got there when they were poured from one flask into another (or by diffusion). Unless the starter is stirred so violently that the vortex cavitates it won't become oxygenated, and once the starter becomes active CO2 will blanket the flask pretty much assuring that no more oxygen gets into it.

I always figured that allowing the starters to finish would produce pretty much the same number of cells in each flask, but right now I'm not sure. Guess I need to dig out the hemocytometer and take a look.

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