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	FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
		Digest Janitor: pbabcock at hbd.org


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Contents:
  re: swing top and rousing yeast (SteveA)
  RE:Swing top gaskets ("Mike Patient")


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Date: Wed, 16 Sep 2009 06:54:28 -0400
From: SteveA <-s at roadrunner.com>
Subject: re: swing top and rousing yeast


> Rousing yeast... some yeast in primary/secondary can
> stall. By agitating the fermentor you can get the yeast
> back in suspension and release CO2. In the bottle you
> are getting the yeast back in suspension so they can
> work on the sugars that are there. Again, some yeast
> need to be roused, some don't.

I don't disagree with the method, but the explanation isn't quite 
right.  Brewing yeast flocculate in a controlled manner; when growth 
conditions become absent the expression of flocculation genes cause cell 
surface changes that in turn cause yeast cells to agglomerate - clump 
together, and sediment and transition to a semi-dormant state where 
fermentation proceeds at a very slow pace..  Adding back the growth 
factor will reverse the cell changes and cause yeast to 
'de-flocculate'.  When shaking or stirring a fermenter we are changing 
growth conditions, not re-suspending yeast.  For example while stirring 
an open fermenter CO2 is released and oxygen is included.   The oxygen 
products is a growth factor and dissolved CO2 is a growth inhibitor.

In a closed bottle fermenter we have an interesting problem.  We expect 
that during fermentation the dissolved CO2 level becomes higher than the 
headspace concentration and  slowly approaches equilibrium.  Shaking a 
bottle brings dissolved CO2 and the headspace CO2 concentrations into 
equilibrium.   The dissolved CO2 inhibits part of the pyruvate energy 
pathway, but head pressure increases osmotic pressure and also is a 
growth inhibitor.  So does shaking (decreasing the dissolved CO2 and 
increasing the head pressure) in a closed primed bottle help or hurt the 
fermentation progress ?

Simple head pressure has a salutory effect on lager fermentation 
byproducts around 0.5-1 bar by decreasing fusels and esters, but also 
decreasing growth rate.   At 4 bars of head pressure yeast growth is 
halted and at 8 bars all fermentation is halted.  The 1 or  1.5bar of 
gauge pressure in a bottle certainly reduces growth.   I wish I had 
comparable good information wrt the inhibitory effects of dissolved CO2.

To make the topic more complex, the presence of sterols and specific 
amino acids can dramatically improve the ability of yeast to tolerate 
osmotic pressure (due to head pressure or solutes), so the commercial 
practice of refermenting/bottle-fermenting with fresh yeast and a wort 
kreusen is probably the gold standard.  Using spent yeast and sugar 
priming is the least desirable case.    Having said that, most 12P beers 
can be sugar primed on old yeast and bottle fermented w/o problems.   
Above ~15P I think better bottle fermentation  conditions should be 
applied.

There is an old paradox related on this forum.   Overfilled primed 
bottles carbonate slowly and less completely while modestly underfiilled 
bottle carbonate more rapidly.   Attempts to eliminate headspace oxygen 
as a factor have been made w/o impact.   In these two cases, the same 
amount of CO2 (same amount of sugars fermented) would result in the same 
carbonation level and head pressure at equilibrium.  One major 
difference is that the similar surface area and dissimilar headspace 
means overfilled bottled approach equilibrium faster.  If this explains 
the paradox, then perhaps the extra headpressure is more inhibiting of 
fermentation than the dissolved CO2.  If so, then any shaking of sealed 
primed bottles may slow carbonation !

 -S



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Date: Wed, 16 Sep 2009 09:59:43 -0400
From: "Mike Patient" <mpatient at rta.biz>
Subject: RE:Swing top gaskets

Any idea on where I can find neoprene gaskets online?  
I After looking into it a lot of people have had problems
 with the cheap gaskets made out of some other rubber, 
but the neoprene ones are what Grolsch uses and is agreement that they work
better.  
The problem is no one lists what theirs are made of online.  
Anyone have a good source?

Mike




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