Preventing Yeast Off-flavors: Tips from the Pros

Brewer:  Russ Seideman
Brewery:  Seidemann Brewing Co.
Years of experience:  seven months
Education:  BS in chemical engineering, University of Wisconsin,
Madison; MBA in management, Northwestern University’s Kellogg Graduate
School, Evanston, Ill.
House Beers: Rio Salado Munich Export and Rio Salado Amber Altbier

In general the more common off-flavors associated with yeast are
phenolic and some of the sour flavors (Lactobacillus and acetaldehyde).
Phenolics, associated with a plastic
or Band-Aid smell, are often caused by wild yeast. To prevent wild yeast sanitize (well).

If you aerate your wort very well, your yeast will get to work
faster and stronger. When you areate, you give yeast the nutrition they
need. The healthy yeast then crowd out bacteria.

Old yeast and insufficient aeration can cause excessive esters and
soapy off-flavors through yeast autolysis. Autolysis, releases fatty
acid. Yeast do this somewhat as a normal function of their metabolism.
If you’ve aerated well you avoid excessive autolysis decay, because the
yeast have enough oxygen to consume.

If you pitch enough yeast, aerate enough, and limit lag time (time
between pitching and the start of fermentation) to no more than six
hours, you decrease your chance of getting off-flavors. Any lag beyond
48 hours is not good. Lag time ties in with cleanliness. During lag time
bacteria can spread before the yeast have a chance to kick in. If
you’ve got a good, vigorous yeast going, it will crowd out the bacteria.
During the lag phase wort is very vulnerable.

A yeasty taste is due to autolysized yeast, which comes from leaving
your beer on the yeast too long. If you leave your beer on the yeast
for a month at a temperature above 80° F, autolysis is  likely to occur.
If yeast cells run out of sugar, they will begin to decompose. Pitching
too much yeast could also be a factor, although it is not often a
problem with homebrewers, who pitch too little yeast more often than too
much. But it can happen.

Diacetyl is another off-flavor in which yeast play an important
role. Diacetyl is associated with buttery and butterscotch flavors.
Yeast form some diacetyl. Some yeast form more than others depending on
the strain and amount. If you brew correctly, later on in the process
yeast will reabsorb diacetyl. The off-flavor can be caused if you try to
force your fermentation and bottle too soon. Some yeast are slower at
reabsorbing the diacetyl. There are others that don’t create as much of

Diacetyl is permitted in small quantities in certain styles but
never in lagers. Performing a diacetyl rest helps. After primary
fermentation let the beer increase in temperature five to 10 degrees for
a couple of days. This encourages the yeast to reabsorb diacetyl at a
faster rate. That’s done commercially as well as in homebrewing. But be
careful. If the rest is too long, you get autolysis. If you raise the
temperature for a fairly short period of time right after fermentation
has ended — one to two days depending on the yeast strain and length of
fermentation — most diacetyl can be reabsorbed.

Another yeast-related off-flavor can occur when repitching. Taste
the beer made with the original yeast to get an idea of whether there is
any sourness or any other off-flavor. Sourness indicates that bacteria
is stuck in your beer.

If the beer tastes bad, don’t repitch the yeast from that batch.
Nothing will propagate bad yeast like bad yeast. Diacetyl is also
associated with certain bacteria.

Hydrogen sulfide (rotten egg smell) is very dependent on the strain
of yeast. Certain strains, especially lager yeast strains, tend to
produce hydrogen sulfide during fermentation. You may detect this smell
during fermentation, but as long as beer stays in contact with yeast
long enough and you’re fermenting at the right temperature, the hydrogen
sulfide will be reabsorbed and go away.

High ester levels are not always an off-flavor but usually are with
lagers. Esters, which taste and smell like banana or other fruit, have a
lot to do with fermentation temperatures that are too high, in the
mid-70° F range. You want temperatures that are 50° to 55° F for lagers
and 60° to 65° F for ales, depending on the yeast. Pitching rate also
plays a role. Esters are more likely when the pitching rate is too low. A
lot of times people who pitch low amounts of yeast tend to compensate
by cranking up the temperature.

Solvent-like fusel alcohols depend on the yeast strain and fermentation that is too high, anything above 75° F.

The Tips

• Aerate your wort well to avoid autolysis, which can lead to excessive esters and soapy flavors.
• Limit lag time to avoid bacteria. A vigorous fermentation will crowd
out bacteria, which can lead to sour and phenolic off-flavors.
• Make sure the yeast you use for repitching is fresh. Taste the batch it was used in initially to test it.
• Perform a diacetyl rest that is long enough to reabsorb the diacetyl but not so long that autolysis occurs

Issue: October 1997