HOMEBREW Digest #5609 Wed 16 September 2009

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  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 Return to table of contents
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 Return to table of contents
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