culprits that have died after causing,
or contributed to causing, the stuck
fermentation. Non-Saccharomyces
yeast severely deplete nutrients and
vitamins that Saccharomyces need, and
they may also have some inhibitory
activities.
Lactic acid bacteria
Stuck and sluggish wines are often
attacked by Lactobacilli (as described
above). Any CO2 production in a stuck
or sluggish wine should be viewed
with suspicion, for it could be caused
by Lactobacilli (or Brettanomyces) rather
than Saccharomyces resuming fermentation.
Monitor stuck wines every day
or two under the microscope while
reinoculating with fresh yeast. In
wines over 3.6 pH, it may be advisable
to reduce pH or add lysozyme prophylactically,
to reduce the chance of
Lactobacillus spoilage.
Brettanomyces/Dekkera
Fortunately, very few musts become
contaminated with Brettanomyces, and
anyway, they tend to grow slowly compared
to Saccharomyces. Stuck wines may
become infected, especially if they are
moved to wood to complete the fermentation.
To detect Brettanomyces as soon as
possible, culture any stuck red wine once
per week, or check periodically by PCR.
Production of 4-ethyl-phenol/4-ethylguaiacol
(4-EP/ 4-EG) lags behind cell
growth so this is not the best way to discover
a new Brett infection in a stuck
wine. Certainly, if apiculate yeasts are
seen microscopically in a stuck wine, it
should be tested for Brettanomyces immediately,
but by that time the population
may be already quite high.
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Film on surface
Both Acetobacter and surface film
yeasts (mainly Candida and Pichia)
enjoy conditions that prevail in a stuck
wine during reinoculation: warm, low
SO2, not topped. Any film should be
examined microscopically. To prevent
film, the container should be brought
as full as possible, and CO2 or other
inert gas used liberally. Acetobacter can
also be discouraged by lowering pH,
but yeasts are not as sensitive to low
pH as are bacteria.
Dry wines undergoing extended maceration
are also at risk from Acetobacter,
because there is not enough
CO2 produced
to prevent their growth.
ML FERMENTATION
Microbes to watch out for: Acetobacter,
Lactobacillus, Pediococcus,
Brettanomyces, Oenococcus, surface
film yeasts
Danger signals: VArise (rise > 0.15 g/L,
which is produced by Oenococcus from
citric acid) in stuck wine, film on surface,
off-odors or flavors
Monitoring methods: Sensory cues,
microscopic exam, PCR
Commercial freeze-dried cultures
are produced following extensive
quality assurance procedures to be
sure they are free from contaminants.
Once they are put into must or wine,
the resident bacterial flora may grow
along with, or instead of, the inoculated
strain(s).
Check wines during MLF for
spoilage bacteria, both microscopically
and by PCR methods, if possible.
This is especially important if the
culture is propagated before 
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adding it, allowing unwanted lactic acid
bacteriamore opportunity to grow as
well (or instead), and also giving aerobic
yeasts and acetic bacteria a
chance to spoil the starter. Acetobacter
growth is particularly unfortunate in
ML starters and during MLF.
Microscopically, it is sometimes confused
with Oenococcus.
If Lactobacillus and/or Pediococcus
are a significant part of the wine’s bacterial
flora, it may be best to kill them
with lysozyme and reinoculate with a
resistant Oenococcus strain. However, if
the wine is dry, the winemaker may
decide to accept the presence of some
Lactobacilli during MLF, despite a tendency
to introduce “pickle” or “sauerkraut”
aromas.
Pediococci can make lactic acid from
sugar but not acetic acid. They are considered
undesirable mainly because
some can continue growing after wine
is dry and has finished MLF, and
because of biogenic amine production,
though off-flavors have been ascribed
to some strains.
In cellars heavily infected with
Brettanomyces, culturing once a week
or testing with PCR during MLF is recommended.
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