From the HBD Archive
From: jhersh@rdrc.rpi.edu (Jay Hersh)
Subject: blowoff
Date: 1989-01-30 20:00:08 GMT

Mike Fertsch was wondering why big brewers don't use blowoff. I think he
or someone else indicated that at least one of them does. I was pondering
this question with the people at catamount a few months ago they didn't know
eihter but I think I have now realized the answer.

What do homebrewers typically do that commercial brewers typically don't.
Aerate the wort!
When wort is aerated there is lots of free oxygen in solution in it.
For those of you who have been studying yeast metabolism, something I
have been looking into a lot lately.
Aerobic fermentation, where free oxygen is utilized, occurs via a
different chemical reduction process than anaerobic fermentation.
It yields far more energy than anaerobic fermentation does, allowing
the yeast to reporduce more rapidly. Since it uses a different reduction
mechanism it produces different fermentation by-products, many of these
the "higher alcohols" like fusel alcohol.

Homebrewers aerate their wort and add typically small amounts of yeast
which then reproduces during aerobic fermentation until the free oxygen
is totally used up. The yeast then switches over to anaerobic fermentation.
This creates the foam up that gets blown off. Yeast is also quite interesting
in that many of the higher alcohols produced can be reduced at a later point
in the fermentation process. This may account for the fact that many people
who don't use the blowoff technique still produce beers with no nasty off
flavors as a result of these fermentation by-products.

The big brewers on the other hand add a quantity of yeast that is sufficient
to ferment the size batch they work with. The yeast is worked up to a
critical quantity before hand, and my guess is this is either done via
anaerobic methods, or if done aerobically it occurs in such a manner as to
force the yeast to have already metabolized the high order alcohols.
Aerobic fermentation is typically not done in the fermentation vats,
therefore the quantity of by-products and the resultant foam up is
proportionally less than that which would occur for homebrewing.

It all sounds logical huh?? Of course being a poor misguided engineer
I'll bet some biologist will come along and blow this theory straight to
hell. In any case well informed rebuttal or support of this theory is
welcome as I perfer to see misconceptions laid to rest rather than
propogated.

- jay h

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