Because of sulfur dioxide’s ability to
bind with the precursors and the products
of oxidation, it can be used as both
a preventative and a treatment. For
example, an oxidized white wine with
a brown tint and a nutty smell can be
improved with an addition of sulfur
dioxide that bleaches out some of the
dark color and binds with the acetaldehyde
to reduce the nutty smell.
Antimicrobial role of sulfur dioxide
Sulfur dioxide is a broad-spectrum
antimicrobial agent that has an
inhibitory affect on a wide variety of
microorganisms. The level of sulfur
dioxide at which the microbe, either
yeast or bacteria, is affected varies
widely by species. This variation
allows winemakers to use sulfur dioxide
to treat microbes in the wine or
must selectively and inhibit or kill
undesirable microbes without having
an affect on the desirable ones.
It has been understood since the
early 1900s that only the free forms of
sulfur dioxide (and not the bound)
have an antimicrobial effect. It was further
discovered in the 1960s that molecular
SO2 was several hundred times
more effective than bisulfite.
The mechanism for sulfur dioxide’s
antimicrobial affect works by the sulfur
dioxide entering the microbe and
disrupting the activity of the enzymes
and proteins of the cell. Since only the
molecular form of sulfur dioxide can
enter through the cell membrane, it is
the concentration of molecular sulfur
dioxide that controlsmicrobial growth.
Although there is some evidence
that a high concentration of acetaldehyde-
bound bisulfite inhibits the
growth of some species of malolactic
bacteria, this effect is eclipsed by the
role of molecular SO2. Since the percentage
of free SO2 that is in the molecular
form is dependant on pH, the
importance of pH in the effectiveness
of sulfur dioxide and the microbial
stability of wine in general cannot be
overstated.
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Affect of sulfur dioxide on yeast
and malolactic bacteria
Sulfur dioxide has some degree of
inhibitory affect on all yeast; however,
the Saccharomyces yeast strains that are
used by winemakers for alcoholic fermentation
aremuchmore resistant to it
than “wild” yeasts are. Wild yeast is
the term used for a number of non-
Saccharomyces species of ambient yeast
that are present on grapes and in the
winery cellar. Wild yeast are sensitive
to both sulfur dioxide and alcohol.
Wild yeast can begin a spontaneous
fermentation in juice but soon are
killed by the alcohol that they produce.
At this point, the Saccharomyces that is
present, either indigenous to the
grapes or added by the winemaker,
which ismore resistant to alcohol takes
over to complete the fermentation.
Wild yeast can sometimes be the
source of off-flavors therefore most
winemakers elect to control them with
a small dose of sulfur dioxide and then
inoculate the must with a commercial
wine strain of Saccharomyces. A molecular
SO2 level of 0.4 ppm (equivalent
to a free SO2 level of 20 ppm @ 3.50
pH) will kill wild yeast without
adversely affecting Saccharomyces.
The inhibitory effect of sulfur dioxide
on malolactic fermentation is much
greater than it is for the alcoholic
fermentation that is performed by
Saccharomyces yeast. Malolactic fermentation
(often abbreviated to ML
fermentation or MLF), is the secondary
fermentation by bacteria that converts
malic acid found in grape juice to lactic
acid, which is less acidic.
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There are several species of bacteria
that are capable of MLF in wine, some
more desirable than others. Oenococcus
oeni, the most commonly used species
of malolactic bacteria, is very sensitive
to sulfur dioxide and has difficulty
growing at levels above 25 ppm total
SO2.
The sensitivity of malolactic bacteria
to sulfur dioxide can be used by
winemakers to influence the flavor of a
wine. MLF has several effects on the
flavor of a wine, it lowers the acidity
and it can produce a “buttery” aroma,
particularly in white wines. In addition
to these sensory qualities, it makes
a wine more microbiologically-stable.
If the MLF is completed in the cellar
prior to bottling, you do not have to
worry about it occuring after bottling
and spoiling the wine. If you wish to
make a more tart, fruit forward wine
such as a Riesling or Sauvignon Blanc,
MLF would not be as appropriate and
MLF can be prevented by an early dose
of sulfur dioxide.
In a wine that will be aged for a
longer period in barrels, or if you
desire a softer less acidic character,
MLF is more appropriate. Chardonnay
and red wines often are put through
MLF either prior to or during ageing.
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