From the HBD Archive
From: Pete Soper <soper@maxzilla.encore.com>
Subject: stuck fermentation
Date: 1990-02-05 21:38:00 GMT

To Chuck and Ashok (The "Brews Brothers"), about their stuck
fermentation: This subject has come up several times in past months
but I never had time to put my 2 cents in. So here it is, with compounded
interest.

One thing that can cause the yeast to stop in mid-fermentation
is lack of ethanol tolerance. Low enthanol tolerance is usually due to
lack of oxygen during respiration. Yeast can multiply through a few
generations without oxygen but in this case they have to share certain
cell materials across those generations rather than building fresh
materials from oxygen and nitrogen compounds in the wort. This can leave
the cell count low and the cells themselves very weak and subject to
dropping out of the picture before fermentation is complete. So, you
need to ask yourself if enough oxygen was present in the wort at the
time the yeast was pitched. If the cool water you combined with your
wort was from the tap, then chances are it had a lot of dissolved air.
If it had been boiled and then cooled chances are you had virtually no
dissolved oxygen if you failed to rouse the resulting mixture before
pitching. (Note that commercial brewers saturate the wort with oxygen prior
to pitching; the yeast will consume all available oxygen during respiration
and any fear of other hazards, like contamination from room air, should be
overridden by concern for getting the yeast's cell count and energy reserves
built up quickly).
Another thing that will shut down a fermentation like a switch is a
sudden drop in wort temperature. Ten degrees overnight has a drastic effect
and I can testify that 30 degrees in 8 hours is entirely effective for
ruining a fermentation (see below). This subject has been covered a lot
recently so I'll move on.
It isn't clear to me whether the next set of issues could stop
a fermentation entirely, but they are worth mentioning. First, you let
the wort and cold water mixture cool, but was its temperature really
matched up to that of the rehydrated yeast? According to one source,
if the yeast is subjected to more than an 18 degree jump at one time,
its sugar uptake ability may be disabled by cell mutation. However it
seems that this would show up as a weak fermentation that took a long time
rather than the complete stop you observed. Also, although
you should be applauded for rehydrating your yeast before pitching it,
the water used for rehydration should start out much warmer than room
temperature before being matched to the temperature of the wort (just a
quibble).
Back to that cold water. Water straight out of the tap
contains chlorine and above a certain level this is toxic to yeast,
in addition to reacting with wort to create chlorophenols, etc. Again,
this is not likely to be the main problem but is worth mentioning.
Other long shots (not applicable to your case) include lack of
nitrogen and phosphates in a recipe calling for very little malt and a
lot of sugar. Some yeast nutrient (aka ammonium diphosphate) is called
for in this case, but this would probably not help at all if added
after respiration had finished. With a load of corn sugar you also run
the risk of the dreaded "Crabtree Effect" in which the yeast cells sort
of forget how to ferment maltose. Finally, old yeast or yeast stored at
high temperatures might leave so few cells that the remaining
viable ones are stressed a lot by the need to multiply across more
generations than usual.
But what should you do? I'm pretty shaky here and can only describe
what I would do if faced with this situation. Take this with a pinch
of noniodized salt and hope for advice from folks who have hands-on
experience with recovering from stuck fermentations. My only similar
experience was recovering from premature flocculation when I accidently
cooled my wort to near freezing right after pitching. I used the
procedure in the next paragraph with liquid yeast after my wort had
warmed up and all was well in the end, although the fermentation took
a few days longer than it should have.
I'd make a quart starter with malt extract and shake air into it for
a full 90 seconds, then add a fresh packet of rehydrated yeast. I'd
allow this starter a few hours to get going like gang busters and take
up all its air and then pitch it into the main wort. Oh, and I'd be
extremely careful to match the rehydrated yeast temp to the starter
temp and the starter to the wort, trying not to jump temperatures more
than a few degrees at any point.
- ----------------------------------------------------------------------
Pete Soper +1 919 481 3730
internet: soper@encore.com uucp: {bu-cs,decvax,gould}!encore!soper
Encore Computer Corp, 901 Kildaire Farm Rd, bldg D, Cary, NC 27511 USA

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