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


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Contents:
  Should I repitch? (Tom Puskar)
  RE: yeast propagation (Joshua Wilkins)


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Date: Mon, 16 Feb 2009 09:54:54 -0500
From: Tom Puskar <tpuskar at optonline.net>
Subject: Should I repitch?

For a number of reasons ranging from illness, work and family issues, I have 
an IPA in the secondary for about 10 weeks.  It looks clear and tastes 
decent upon sampling.

I'm hoping to bottle it (FINALLY) this coming week and wondered if I should 
add some additional yeast for carbonation.  I keep a few envelopes of dry 
yeast around and could proof one up and add it to the bottling bucket.  I 
haven't checked the gravity in a few weeks but last check it was around 
1.015 and hadn't changed for a while.

Any thoughts?

Thanks,

Tom in Howell, NJ 




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Date: Mon, 16 Feb 2009 21:27:27 +0000
From: Joshua Wilkins <jowilki3 at hotmail.com>
Subject: RE: yeast propagation


Hey all,
 
I forgot to mention in my last post on air saturation that the times I gave
were based on sterile filtered Air not O2. O2 will be a  lot faster, but O2
also has the downside of creating oxygen free radicals which are very 
toxic to cells (same reason we get wrinkles). I used to use O2 in the past
, but switched to sterile filtered air for that reason and because filtered 
air is cheaper than O2, but to be honest I have made good beer with 
both techniques.
 
Fred asked:
 
1)Do you happen to know how Wyeast and White labs propagate the 
pitchable quantities of yeast they send to breweries intended to be 
pitched directly into their production batches.
 
I quickly answered this before and the answer is no. I assume they do 
use aerobic methods because the biomass yield is much higher. I do 
know that one of them ships slurry in 2 liter bottles to be directly 
pitched into a 10 BBL batch (about 300 gallons) of beer. Of which, this
slurry is in the dormant state (obviously because the bottle/bags would
explode otherwise), but without activating this domant yeast (making a
starter)with media of some kind, this will lead to a longer lag phase 
where the yeast adjust to the new environment as I have described 
before. This result WILL lead to longer fermentation times and in some 
cases lead to "off flavors". It all depends on what you are looking for in the
beer. In some cases, these "off flavors" are desired, sometimes they are 
not they are simply process dependent. The smack packs that Wyeast uses
contains nutrients and the sachet that you pop on the inside is the yeast 
cells. Once that sachet is broken they then begin to grow. So if timed right,
the smack pack will be in the middle or end of the exponential growth phase
when pitched. I say this is about 8 hours from when the sachet inside is 
broken.
 
2) I'd venture to say that the propagation is not customized based on the
water profile and nutrients in the batch of wort at the brewery.
 
Correct, they are based off many factors I am sure and probably deal a 
lot with what is cost effective, easily shipped, easily and quickly 
manufactured, "shelf stable" etc, etc. This is why a lot of breweries will 
simply order slants and then propagate up from there as follows (slant ->
1 Liter starter -> 30 L starter -> production tank (~3000 liters). 
 
3) I also doubt that hops are even included in the propagation. To what 
part(s) of "hops" must the yeast produce "resistance"?
 
Yea, I have no idea if they are or not. The problem comes in that if the 
yeast faces ideal conditions all the time, they get accustomed to that and
will eventually drop the resistence gene/genes. The antimicrobial action 
comes from the bittering acids, (alpha acids being the largest portion), I am
not going to name all of them here as there are many and the quantity all 
depends on the hop variety. As for the gene responsible for this in yeast, I 
am not sure of its name and a quick look on google scholar did not yield 
anything, but I did find an article on the development of hop resistence by 
beer spoilage bacteria. The gene family they have is call the Hor gene family
and it acts by encoding "an ATP depen-dent multidrug transporter that 
extrudes hop bitter acids out of bacterial cells." I would imagine the yeast 
gene/genes does something similar, but I dont have anything to back that up.
 
My main two points on yeast starters was this 1) Homebrewers dont have 
tools necissary to propagate yeast through exponential phase and keep them
in aerobic growth. Yeast simply metabolize O2 too fast for this and equipment
homebrewers use cant pump O2 fast enough into solution during this growth
phase. Will aerating still result in higher biomass than other means, probably.
Which brings me to point 2) Starters should get yeast cells into prime 
production shape with no surprises waiting for them in the fermentor
which means whatever challenges will be faced in the fermentor should be
seen in the starter, otherwise this will result in a longer lag phase and 
possible off flavors.This is also the reason the second batch usually tastes
better than the first because the yeast had adapted during the first batch,
yet genetic mutations due to yeast age (#of times replicated) are kept to
a minimum, which is why yeast get changed out every 5-10 batches in 
commercial breweries.
 
As for the Crabtree effect....Well my memory failed me (Doh). I was thinking 
it was a specialized case with Glycolysis-TCA cycle being preferred during 
aerobic growth, but I had them reversed. So Yes Crabtree effect keeps
yeast producing alcohol whether its in an aerobic or anaerobic environment
unless glucose is less than 0.2%w/w and Glycolysis-TCA cycle is reserved for
specific cases.
 
 
Last thing I promise; regardless of how people brew, if you are producing
good beer that you are happy with, then by all means continue doing
what you are doing.
 
Joshua Wilkins


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