(Dr. Sibylle Krieger-Weber, Personal communication)
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average ethanol levels in table wines
are increasing. Thus ethanol tolerance
is an important consideration when
selecting a bacteria strain. Ethanol
affects the growth of the bacteria, but
not malic degradation. Malic degradation,
however, will not begin until the
bacteria cell population reaches 1x106
cells/mL. Inoculation rates must be
respected for a rapid start.
Addition of sulfur dioxide is an integral
part of current winemaking practices.
O. oeni is more sensitive to the
anti-microbial effects of SO2 than other
lactic acid bacteria (Lactobacillus spp.
and Pediococcus spp) or yeast. It is not
always recognized that S. cerevisiae can
produce significant amounts of SO2 on
its own. This was initially discovered
by Fornachon (1968).
The amount of SO2 produced is highly
dependent upon the strain of yeast
and composition of the medium. Yeast
strains are classed as low, medium, or
high SO2 producers. The amount of
SO2 produced can range from ~20 ppm
to in excess of 90 ppm. It is essential
to know the SO2 levels (both free and
total) in a wine.
Fatty acids can be produced by
yeast. Their production is dependent
upon the must composition, grape
cultivar, yeast strain, fermentation
temperature, and winemaking
practices. Short and medium chain
fatty acids (hexanoic, octanoic, and
decanoic acid) are reported to inhibit
bacteria and yeast. These acids can
target and alter the bacterial membrane,
resulting in limited growth
and reduced ability of the bacteria to
degrade the malic acid (Table I).
It has also been demonstrated that
fatty acids work in synergy with
ethanol and pH, resulting in even
more challenging conditions. On a
positive note, it is possible to remove
fatty acids via adsorption by yeast
ghosts (hulls).
A number of yeast-derived metabolites
can exert an inhibitory effect
on ML bacteria. Some can be measured
(ethanol and SO2), while others
are more challenging to assess
(fatty acids). Not all malolactic fermentation
inhibition can be attributed to the production
of yeastbased metabolites.
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The topic of malolactic fermentation
always leads to lengthy conversations
due to the different formulations
available (direct addition, 1-step
preparations, or standard build-up
cultures), different inoculation protocols,
and yeast strain compatibility
with ML bacteria.
The winemaker must meet the challenges
faced by O. oeni. Such challenges
include when the ethanol level
increases above 14%, pH values are
below 3.2, free sulfur dioxide levels
are above 10 ppm, total sulfur dioxide
levels are above 30 ppm, or the temperature
is below 16°C.2
The conditions for success, although
well documented and understood by
winemakers, can still result in problems.
These parameters must be viewed
in combination (such as SO2 and pH)
and not individually (SO2 or pH).
A recent focus has been on the isolation
of strains with wider environmental
limits. It is now possible to successfully
achieve MLF in wines in excess of
16% alcohol. In addition to attempting
to predict the success of MLF using the
aforementioned parameters (alcohol,
pH, SO2 concentration, temperature),
we must also consider the initial malic
acid level, the polyphenolics content,
and the organism(s) that may be present
and their metabolites.
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Enological organisms
and their relationships
There are currently approximately
150 diverse enological strains of
Saccharomyces cerevisiae that have been
isolated from the natural environment
for a winemaker to choose from, plus
30 strains of Oenococcus oeni. If we
extrapolate the number of combinations
that can exist we would have
4,500. With so many combinations of
yeast and bacteria that may be present
(excluding native organisms), how can
we predict the relationship between
organisms and the resultant outcome
of such relationships in our wines?
The relationships known to exist
between S. cerevisiae and O. oeni include
amensalism, predation, commensalism,
mutualism, and protocol-operation.2
Put more simply, the relationships can
be described as inhibitory, stimulatory,
or neutral.
Inhibition of O. oeni by
wine strains of S. cerevisiae
Ethanol was the first yeast metabolite
reported as being antagonistic toward
O. oeni. It is generally accepted that
average ethanol levels in table wines
are increasing. Thus ethanol tolerance
is an important consideration when
selecting a bacteria strain.
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