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


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Contents:
  Re: Aerobic Yeast Propagation (Kai Troester)
  Re: Yeast Performance / Aerobic propagation (Joshua Wilkins)


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Date: Thu, 12 Feb 2009 10:10:58 -0500
From: Kai Troester <kai at braukaiser.com>
Subject: Re: Aerobic Yeast Propagation

Ever since I started propagating yeast in a low gravity aerobic  
environment I have been paying close attention to propagation  
practices used in the industry in order to get an idea of the  
drawbacks that such an approach may bring. I know that Danstar  
produces their dry yeast aerobically with incremental feeding and I'm  
not sure if the other 2 major yeast companies (WYeast and Whitelabs)  
do the same. One would imagine that they do b/c their objective is to  
get as much yeast as possible out of their raw materials.

But when it comes to yeast propagation in the brewing industry I don't  
find references to aerobic yeast propagation. While in the initial  
stages the yeast is propagated in an aerobic environment the stength  
of the worts uses is strong enough for the crabtree effect to be in  
effect which causes fermentation. But that isn't really a problem if  
there are enough sugars in the wort as the main objective for the air  
is to provide oxygen for the sterol synthesis in the yeast. If O2 is  
the limiting nutrient the yeast will have limited sterol reserves and  
only weak cell walls. It might be that low gravity propagation isn't  
good either b/c the yeast may get uses to the lower alcohol/sugar  
concentration both of which are stress factors.

After the initial aerobic stages yeast is commonly propagated with  
Drauflassen where only the added wort is aerated and no aeration is  
done in between.

In Brewing and Malting Technology Kunze makes a note that freshly  
propagated yeast is very healthy. To healthy in fact as it becomes a  
poor SO2 producer and that some brewers mix the new yeast with some  
old yeast to increase the SO2 production during fermentation. I'm not  
sure how relevant this is for home brewing but I have heard home  
brewers say that the beer from the 2nd use of a yeast tastes better  
than the batch made with freshly propagated yeast.

When I started propagating yeast in an aerobic environment I was  
looking for a large vessel to control the foaming (I don't like foam  
control, so don't suggest that as a solution ;) ) and turned to a  
carboy for that. this also meant that I suddenly had much more volume  
to fill and thought it would be much better for the yeast to live in a  
low gravity environment anyway.

The fermentations with that yeast have been the fastest I had up to  
that point but I still have to see if there is a difference between  
propagating in 10l of 2 Plato wort or 2 L of 10P wort when it comes to  
fermentation performance and most important taste.

Even if you don't use a pump to aerate your starter making a starter  
in a carboy may help you. Simply add the 2 L of starter that you would  
use to a sanitized carboy, top off with sterile water (I actually  
trust my RO tap) until you have 12-14 l (3-3.5 gal) in there. Add  
yeast and shake to aerate the wort. The large head space contains  
enough O2 to saturate the wort with it and the large wort volume holds  
much more O2 than a 2 L starter would have if it was only aerated once.

Fred,

In your experiments, did you ever do a side-by-side with the same  
yeast from different propagation techniques.





Return to table of contents


Date: Thu, 12 Feb 2009 18:30:53 +0000
From: Joshua Wilkins <jowilki3 at hotmail.com>
Subject: Re: Yeast Performance / Aerobic propagation


I just wanted to throw in a few points to an already extensive conversation.

I do have experience at a professional level with aerobic fermentation and
the biggest problem that I see with this method of propagation for homebrewers
is that the equipment to control such a fermentation is not cheap nor readily
available (we are talking tens of thousands even for a used fermentor and
controller and this is even before the probes are bought). Yes, stir plates,
sinter stones, and even Dissolved oxygen meters are available, but they will
not be sufficient enough to maintain aerobic phase of growth once the yeast
cells (biomass) gets into exponential growth without additional/ more
specialized equipment. I'd say 90% of the air and/or o2 released in this
kind of set up simply exits the top of the vessel due to the scale of
equipment used in the average yeast starter. But the only thing that you
are gaining from aerobic propagation is greater biomass, and yes its
important, but is not the only thing a brewer should want.

The creation of a starter should accomplish the following goals: 1)Expand
the yeast cells to the point where the exponential phase of growth in the
starter can be carried over into the exponential phase of the production
vessel (minimize lag phase in the production vessel). This also means that
the yeast in a starter ideally should have enough nutrients/fermentable sugar
such that they do not enter stationary phase nor become dormant. 2) Have the
yeast cells tread the fine line of being healthy yet also be challenged, This
is mainly referring to a resistance to hops and alcohol. Most people forget
that it takes time for this resistance to develop. The cells have to activate
the genes necessary for these resistances, build the proteins necessary, and
then finally implement the resistance activity and this process can take
several hours. If one gives a completely ideal condition to the yeast, then
the yeast will have to condition themselves in the production fermentor,
which WILL lead to flavor constituents that may not be ideal. 3) You want
to match the nutrients that will be in the wort to  those in the starter.
This is due to the fact that yeast have a hierarchy for various sugars and
nutrient pathways. So you want to again expose them early to this so that
genes are activated to create the enzymes necessary for these pathways. Yes,
once they get into the production fermentor they will start at the top of
this hierarchy, but the enzymes will already be made for the more complex
nutrients and can already begin breaking them down.

These to me are the more important points but I can sum up the rest with a
single statement. Match the conditions of your production fermentor to that
of the starter. This does not mean if your doing a high gravity beer then
to do a high gravity starter, it simply means if the production is going to
be a batch fermentation then the starter should be a batch fermentation. If
you are using wort in the fermentor then use wort for the starter. If you are
using a certain water profile with specialized salts then do that in the
starter as well. etc etc.

As for aeration, I say aerate the wort until it is 100% saturated then pitch
the yeast. No more is needed for both the starter and the production
fermentor. But I do support the use of stir plates for starters mainly to
keep the yeast from settling.

I tried to keep this basic, but if anyone wants to get into the details
feel free to ask and I can explain from there.

Josh Wilkins




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