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


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Contents:
  Re: Yeast performance (Kai Troester)
  yeast performance (Fred Scheer)
  re: Yeast performance (steve alexander)


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Date: Tue, 10 Feb 2009 09:27:25 -0500
From: Kai Troester <kai at braukaiser.com>
Subject: Re: Yeast performance

> I have been growing my yeast <.2% glucose to keep the yeast in  the
> aerobic stage. Before pitching I feed the yeast > .2% glucose to force
> the yeast to the anaerobic stage. What if any problems with fermentation
> can be expected from the above? The yeast  is constantly  fed  on a stir
> plate at room temp with access to air at atmospheric pressure. It is
> assumed that extract  from a mash  at 1.020 has less than .2% glucose.

It is my understanding that when it comes to crabtree effect, which  
you are trying to avoid with the low concentration wort, all sugars  
fermentable by the yeast matter. a 1.020 wort as about 5% extract of  
which 60% are fermentable. Once the glucose and sucrose are consumed  
the yeast produces an enzyme that splits the maltose into glucose and  
as a result the yeast cell itself sees further glucose.

I started growing my yeast in about 12l of 2 %Plato wort. This is 2 l  
of 12 %Plato wort added to 10l of RO water. I do this in a carboy and  
constantly aerate with an aquarium stone which also keeps the yeast in  
suspension. A blow-off tube is necessary to capture the foam. This has  
worked very well for me so far and gives me the most active yeast I  
have ever had in brewing.

> Question Two:
> How does DME compare with extract  from an all grain mash with regard to
> nutrients for yeast?

It is the same aside from differences in fermentability

Kai






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Date: Tue, 10 Feb 2009 18:56:58 +0000 (UTC)
From: Fred Scheer <fredscheer07 at comcast.net>
Subject: yeast performance

Even you keep your yeast under glucose, yeast will 
slowly ferment and will develop CO2. Why do you want 
your yeast to be in anaerobic stage before pitching?
What you do is shocking your yeast, i.e., your wort 
to be pitched is aerated with DO, and your yeast is
 anaerobic. You will have a longer lag phase through
 your procedure.
Oxygen is rapidly absorbed by yeast during the 
lag phase. Your yeast needs all the Oxygen to
 produce important cell wall constituents.

Hope this helps

Fred Scheer


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Date: Tue, 10 Feb 2009 15:24:37 -0500
From: steve alexander <steve-alexander at roadrunner.com>
Subject: re: Yeast performance

Bobz writes ....
>
> I have been growing my yeast <.2% glucose to keep the yeast in  the 
> aerobic stage. Before pitching I feed the yeast > .2% glucose to force 
> the yeast to the anaerobic stage. What if any problems with fermentation 
> can be expected from the above? The yeast  is constantly  fed  on a stir 
> plate at room temp with access to air at atmospheric pressure. It is 
> assumed that extract  from a mash  at 1.020 has less than .2% glucose.
> Question Two:
> How does DME compare with extract  from an all grain mash with regard to 
> nutrients for yeast?

The main problem with this method is ... it doesn't work.

To the minutia - fresh wort & DME have comparable nutrients except the
latter may have somewhat lower FAN.

You are trying to use the crabtree effect (aka catabolite repression)
to force aerobic fermentation and repress the anaerobic fermentation.

This a is a laudable goal.  I *strongly* suspect this is what
dried-yeast manufacturers do as the finished stage before drying.  The
yeast have large sterol & UFA stores and probably high levels storage
carbos.  So it's a great way to propagate yeast.

The problem is it's difficult to maintain the yeast in the aerobic
phase.

If you'd like to read a good discussion look back about 10 yrs in the
HBD arcive for "Tracey Aquilla" and crabtree.

So first off - your 1.020 ~5P wort has 5P (5%) extract which is mostly
(~4.7%) carbohydrates.  Of that 4.7P carbs we roughly have

0.05P fructose
0.4P  glucose
0.2P  sucrose
2.8P  maltose
rest - polysaccharides
 ....
About 20 minutes after the yeast see the sucrose it is hydrolyzed to
equal past fructose and glucose so then we have

0.15P fructose
0.50P glucose.
2.8P maltose 
+....

You are reading the crabtree effect backwards.  Crabtree says that
even in the presence of oxygen, when you increase glucose >0.4P this
represses aerobic metabolism.  It says too much glucose represses
aerobesis.  BUT what Crabtree didn't bother to explicitly mention is
that 0.4 of fructose or ~0.8P of maltose also represses aerobic and
the effect of these sugars is additive. You can get catabolite
repression with almost any sugar including non-fermentables.  Glucose
and fructose are about equal and maltose, mannose are about half as
effective, galactose about 1/4th as effective.

To prevent the Crabtree effect you need to keep the wort
concentration down around 1P (SG=1.004) roughly.  So you'll need to
incrementally feed.  Your 5P wort forces catabolite repression and
anaerobic fermentation.

Another approach to aerobic fermentation is to use an aerobic carbon
substrate, like ethanol. Yeast will consume ethanol and convert it to
CO2 and/or acetic acid (depends on their genetics).  You need to
supply complete nutrients and O2 of course.

The other problem is the concentration of nutrients.  We all know tha
anaerobic fermentation of 12P wort provides enough nutrients and
energy to increase yeast by a factor of 10 roughly, but at that point
FAN is a limiting factor.  Also much more energy is liberated by
aerobic fermentation of wort, but no more nutrieant is present per
unit extract.

Put another way:
ANaerobic fermentations  produces 5gm yeast/100 gm sugars
Aerobic fermentation produces ~50gm yeast/100gm sugar.

SO you need to boost the nutrients in wort about 10fold (may as well
use sugar at that point - the nutrients of wort are completely
inadequare) for aerobic fermentation.  Adding common yeast nutrients
and ammonia salts at ~5gm/100gm sugar is a start.

The other point you are likely underestimating is the amount of free
oxygen necessary. To catabolize 100gm of sugar aerobically requires
about 51 grams of oxygen.

For example you have a 1 liter culture on a stirrer and you add the
crabtree limit of ~1P of wort (~10 grams of sugar) then you'll need 5
grams of oxygen to aerobically ferment this small amount. At
saturation ~16ppm O2 you're flask has ~24 micrograms dissolved O2.
You'll need to turnover the dissolved oxygen about 200K times and do
that in a few hours.  ((the rate should increase exponentially w/
yeast but ...))  Then repeat 5 times to use up all 5P of starter.
 
I really think you should bubble air thru a sintered stone which is
similar to the method use for bread yeast propagation on a molasses
and urea substrate.

Yes - it's hard to get yeast to respire.  Hopefully Fred Johnson will
comment on his attempts. 

 -S




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