How much sulfur dioxide
Since it is the molecular form (SO2)
of sulfur dioxide that has the most
potent antimicrobial effect, and the
percentage of sulfite that is in the molecular
form is directly dependent on
the pH, onemust always consider both
the pH and the free SO2 when determining
how much sulfur dioxide to
add to a wine.
By knowing the pH, you can determine
the percentage of free sulfur that
is in the molecular (SO2) form by using
the table in Table I.
For white wines, a level of 0.8 ppm
molecular SO2 will slow down the
growth of yeast and will prevent the
growth of most other microbes. This
level of sulfur dioxide will bind up
most of the acetaldehyde in a wine and
reduce any oxidation aroma considerably.
Therefore, 0.8 ppm is a good target
level for molecular SO2 immediately
prior to bottling and will provide
the maximum protection for the finished
However, sensitive tasters will be
able to detect a slight burnt match
aroma at 0.8 ppm SO2. This is usually
not a problem however because few
consumers will be able to detect it.
Additionally if the wine is bottle-aged
for a few months before consumption,
the SO2 will decrease as more sulfites
react with other chemical constituents
in the wine and become bound. Thus,
a wine bottled at 0.8 ppm will decrease
to a lower level fairly quickly and there
would be no detectable sulfur dioxide
Winemakers who seal their wine
with screw caps know that the sulfur
levels diminish more slowly after bottling
than wines sealed with corks. In
this case, 0.7 ppm would be a better
target for molecular SO2 at bottling.
During storage, after all fermentations
have completed, white wines can be
adjusted to between 0.5 and 0.8 ppm
molecular. If the wine is sweet or if you
wish to prevent MLF, the wine should
be kept at the high side of this range.
Total SO2 should be kept below 110
ppm for table wines because, at higher
levels, the wine can acquire off-flavors.
For dessert and fortifiedwines, that are
very sweet, it may be necessary to
exceed this limit to obtain adequate
For red wines, a level of 0.5 ppm
molecular SO2 at bottling is an appropriate
target. You do not need to keep
the molecular SO2 as high on red
wines as you do white wines for several
First, in most cases, MLF is complete
in reds so there is no need to try
to discourage it.
Second, red wines are less sensitive
to oxidation and their flavor is less
dependant on fresh fruity aromas so
sulfur dioxide’s preservative effects
are not as critical.