SO2 and Wine
Dennis Henry
Corkscrew Society Meeting
Feb. 20, 2007
Feb 20, 2007
SO2 and Wine
1
Information Sources
●
Sulphur Dioxide by Ben Rotter

●
The Use of Sulphur Dioxide (SO2) in Winemaking by
Charles Plant

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http://www.bcawa.ca/winemaking/so2use.htm
The Wine Lab Catalog

●
http://www.brsquared.org/wine/Articles/SO2/SO2.htm
http://www.thewinelab.com/_fileCabinet/TECH_INFO/Winecatalog2006.pdf
Winetalk
Feb 20, 2007
SO2 and Wine
2
What is SO2
●
●
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Sulphur Dioxide is a compound of sulphur and oxygen.
Also commonly referred to as sulphite due to the other
forms it takes both as an additive and in the wine.
Sulphite is a natural by-product of yeast and as much
as 41 ppm has been recorded in fermentations where
no SO2 has been added.
Feb 20, 2007
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Why Do We Use SO2?
●
Antioxidant

High levels of aldehydes give wine a flat and stale
aroma and flavour, an oxidised (or maderized) aroma.

Acetaldehyde is oxidised ethanol, and gives sherry its
characteristic aroma.

Most acetaldhyde will be bound by the abundant
bisulphite form, so we don't notice the effects of
oxidation allowing the wine to retain "freshness" of
aroma.

When there is oxygen around, SO2 itself becomes
oxidized before phenol compounds in the wine do, and
so acts as an oxygen scavenger.
Feb 20, 2007
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Why Do We Use SO2?
●
Anti-Enzymatic



SO2 destroys oxidases (enzymatic catalysts of
oxidation).
It inhibits polyphenoloxidases (PPO) which catalyze
oxidative reactions in juice (total addition of 50mg/L
SO2 can reduce PPO activity by over 90%).
This will increase the oxygen available to yeast in their
growth phase when added pre-fermentation.
Feb 20, 2007
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Why Do We Use SO2?
●
Anti-yeast

●
Yeast selective

●
Dissolved SO2 gas, and to a lesser extent the bisulphite
form, inhibit yeast.
Promotes yeast selection by hindering the multiplication
of non-alcohol producing yeasts such as apiculates,
torulopsis, and candida.
Antibacteria

Lactic bacteria are sensitive to free and, to a lesser
extent, bound SO2.
Feb 20, 2007
SO2 and Wine
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Why Do We Use SO2?
●
Colour

Reduces enzymatic browning by obstructing polyphenol
oxidases (the enzymatic catalysts which cause
oxidative browning of juice).

It binds with brown quinones or reduces them back to
phenols, reducing browning in wines.

Also causes an increase in the extraction/solvency of
anthocyanins and polyphenols from fruit tissues but at
normal doses the colour increase is aesthetically
insignificant.
Feb 20, 2007
SO2 and Wine
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Why Do We Use SO2?
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Fermentation

At low levels of 5-10 mg/l SO2 delays the onset of
fermentation but later speeds up the multiplication of
yeasts and their transformation of sugars.
Feb 20, 2007
SO2 and Wine
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How Much to Use
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John Tummon says:

By ignoring the role that pH plays we commonly over
sulphite our white wines and under sulphite our red
wines.

When judging amateur wines it is not uncommon to
detect high levels of SO2 in white wines; however we
rarely encounter this problem with red wines. The
reason is that red wines typically have a higher pH than
white wines.

Surface molds, browning and wines that do not age well
are more common with reds. This can be due to
inadequate SO2.
Feb 20, 2007
SO2 and Wine
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How Much to Use
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John Tummon says:


About .4 ppm to .8 ppm molecular SO2 is needed.
When judging wines, levels over .8 ppm are commonly
detectable depending on the individual judge's
threshold.
Feb 20, 2007
SO2 and Wine
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How Much to Use
●
●
When you add SO2 some becomes “bound”, the
remainder remains “free”.
The bound portion consists of two parts.

Irrevocably bound compounds with aldehydes and
proteins

Less stable compounds that can partly turn back to the
free form when the existing amount of free is lowered,
or even if temperature is increased
Feb 20, 2007
SO2 and Wine
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How Much to Use
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The free portion consists of three parts:



Relatively inactive sulphite (SO3=)
Relatively inactive bisulphite (HSO3-)
Molecular SO2. This is the crucial active portion and its
size depends both on pH and the total amount of free
SO2.
Feb 20, 2007
SO2 and Wine
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How Much to Use
Feb 20, 2007
SO2 and Wine
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How Much to Use
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General Recommendations

Dry reds: 0.5 to 0.6 ppm molecular

Dry whites: 0.8 ppm molecular

Sweet whites: 1.5 to 2.0 ppm molecular

According to one study, 0.825 ppm molecular is
required to suppress growth of Brettanomyces/Dekkera
sp. and Saccharomyces cerevisiae.
The recommended level for sweet whites is a bit high
in my opinion. You risk having sulphite detected by a
sensitive judge.
Feb 20, 2007
SO2 and Wine
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How Much to Use
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●
The maximum before the sensory threshold is reached
is generally considered to be in the range 0.8-2 ppm.
Excessive bound SO2 may give a “chemical” taste

Total SO2 should be limited to about 150 to 200 ppm

Legal limit for total SO2 is 350 ppm in the USA and
generally lower elsewhere.
Feb 20, 2007
SO2 and Wine
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How Much to Use
●
●
●
When the total SO2
additions are low, a
significant portion of what
is added becomes bound.
As the total additions is
higher, less becomes
bound and more
contributes to the free
SO2.
You need to keep good
records of SO2 additions.
Feb 20, 2007
SO2 and Wine
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How Much to Use
●
There are various recommendations on how to predict
how much of the sulphite you add will be free.
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Actual behaviour will vary from wine to wine.
●
The numbers I use are:



For total SO2 of 0 to 50 ppm, 50% becomes free (add
twice the desired increase)
For total SO2 of 50 to 80 ppm, 70% becomes free (add
1.4 times the desired increase)
For total SO2 of >80 ppm, 90% becomes free (add 1.1
times the desired increase)
Feb 20, 2007
SO2 and Wine
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How Much to Use
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The molecular SO2 level can be calculated by using
this formula:
Molecular SO2 = free SO2 divided by (1 + 10(pH – 1.8))
●
The value of 1.8 is approximate as it is also affected
by alcohol level and temperature



Higher temperature or alcohol will increase free SO2
For example, 68 ppm free SO2 at 0°C => 85 ppm at
15°C and 100 ppm at 30°C
Wines with high SO2 levels can be served cold to hide
the sulfurous aroma
Feb 20, 2007
SO2 and Wine
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How Much to Use
Free SO2 required for given molecular SO 2 level
pH
Feb 20, 2007
Free SO2 (mg/l) for given molecular SO
level
2
0.6 mg/l
0.8 mg/l
2 mg/l
2.8
6
9
22
2.9
8
11
27
3.0
10
13
33
3.1
12
16
41
3.2
15
20
51
3.3
19
26
64
3.4
24
32
80
3.5
30
40
100
3.6
38
50
125
3.7
47
63
157
3.8
59
79
197
3.9
74
99
248
4.0
94
125
312
SO2 and Wine
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How Much to Use
Feb 20, 2007
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How Much to Use
Feb 20, 2007
SO2 and Wine
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How Much to Use
My sulphite calculator
Starting Free
10
Target Free SO2
20
SO2 pKa
1.81
Total SO2
30
Multiplier
2.0
in wine
pH
3.2
Add SO2
21
Volume of
19
Desired
0.8
Additions Required of KMetaBisulphite
mg/l
spoons 1/8
tsp
grams
36
0.94
0.12
0.7
Required Molecular SO2
Reds: 0.5 to 0.6 ppm
Dry Whites: 0.8 ppm
Sweet Whites: 1.5 to 2.0 ppm
Table of Addition Effects
Starting Total
Free SO2
Addition
0
50%
2.0
50
70%
1.4
80
90%
1.1
Feb 20, 2007
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Addition of SO2
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The common form of suphite is Potassium
Metabisulphite. It comes in:

Campden tablets
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
Powder
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These can vary in content by up to 25%
Potassium metabisulphite is about 57% SO2 by weight
The Potassium form is preferred over the Sodium

Increases the level of potassium in the wine which later
helps to precipitate tartrates when cold stabilising.

Some claim that the sodium form can contribute a
`salty' flavour to wine.
Feb 20, 2007
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Addition of SO2
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Storage

The powder should be stored in a glass jar with a
plastic coated metal lid.
●
●

Plastic containers let oxygen through
The SO2 will attack metal
I find that stored like this that even year old sulphite has
good strength.
Feb 20, 2007
SO2 and Wine
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Addition of SO2
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Most people recommend adding sulphite as a 10%
solution.

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

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Mix 100g of sulphite with enough water to make 1 liter
final volume.
The solution can be stored for a while, but will start to
degrade when exposed to oxygen.
Makes it easier to measure the amounts.
Make sure to stir it into the wine, it won't mix by itself.
If you can measure the powder accurately, I find that
direct addition works well.

Just sprinkle in and stir to mix and dissolve.
Feb 20, 2007
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Oxidation and SO2 Depletion
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Oxygen first combines with certain metallic ion
catalysts (such as iron and copper). Later, these
oxidised metal ions oxidise tannins, pigments, sulphur
dioxide, and possibly acids.
When oxygen is absorbed in excess or too quickly

The metallic ions cannot carry the oxygen.

Oxygen combines directly with ethanol and higher
alcohols to form aldehydes.
SO2 can be used to bind with oxygen and prevent
aldehyde formation when rapid oxidation might take
place (such as during racking or bottling procedures).
Feb 20, 2007
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Oxidation and SO2 Depletion
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With the slow absorption of oxygen in wine, free SO2
is consumed and the level of free SO2 decreases.

Lose around 5 mg/l per month in wines stored in large
tanks in cool cellars with small headspaces.

Wines stored in warm cellars with large headspaces
often lose 10-20 mg/l per month, or more.

In bottle depletion is no more than a few milligrams per
year.
Feb 20, 2007
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Oxidation and SO2 Depletion
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SO2 depletion increases with an increase in



Temperature
Headspace
Oxygen exposed surface area to volume ratio
Feb 20, 2007
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Oxidation and SO2 Depletion
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Oxygen Saturation Level

The saturation level of dissolved oxygen in wine
depends on temperature (it increases with a decrease
in temperature) and the alcohol content of the wine (it
increases with an increase in alcoholic content).

At 20 C, 8 mg/l is the saturation level, whereas at 0 C it
is 11 mg/l.

Thus, the oxygen saturation range in wine is generally
7-11 mg/l.
Typically takes several days to a week for SO2 to
consume all the oxygen in a saturated wine.
Feb 20, 2007
SO2 and Wine
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Oxidation and SO2 Depletion
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A partially filled container of wine with a surface area
of 100 cm2 will absorb oxygen at 2 mg/l per hour.

A 19L carboy half full has a surface area of about 450
cm2 so you reach saturation in about 1 hour

Filled to the neck it is about 11 cm2 so it takes about 2
days to reach saturation
To react with 1 mg of oxygen, 4 mg of SO2 is required

●
Filled to the neck this is 22 ppm of SO2 per day
I find that with a solid bung, a carboy filled to the neck
looses SO2 very slowly (only a few ppm per month)
Feb 20, 2007
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Oxidation and SO2 Depletion
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Racking

Gentle rackings often cause an oxygen uptake of 1-3
mg/l (loss of 4-12 ppm SO2)

Those with more turbulence and air exposure might
absorb 3-8 mg/l during each racking (loss of 12-32 ppm
SO2).
Barrels

Penetration through oak wood itself is insignificant at 37 mg/l per year.

When barrels are often opened for testing/tasting,
oxygen absorption may be around 40-53 mg/l per year.
Feb 20, 2007
SO2 and Wine
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SO2 Measurement
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●
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Titrets are the most common and are based on the
ripper measurement method
Aeration-Oxidation is more complicated and
expensive, but more accurate (used by many smallmid sized wineries)
Spectrometers would be nice but are too expensive for
even small-mid sized wineries.
Feb 20, 2007
SO2 and Wine
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SO2 Measurement
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Aeration-Oxidation Test Kit

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$425 US
http://morewinemaking.com
Reagents:

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Hydrogen Peroxide (H2O2),
30%, dilute to 1% for analysis
A/O Indicator solution
0.01 N NaOH
0.01 N HCl to standardize NaOH
Phosphoric acid (25%)
Feb 20, 2007
SO2 and Wine
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SO2 Measurement
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Ripper Method Limitations:






Reacts with phenols, resulting in false-high reading.
As free SO2 is reduced, more is released from bound
SO2, again giving a false-high, particularly in reds.
Rapid test execution will minimize these reactions.
The dark color of red wines makes it difficult to identify
the end point of the titration.
Potential volatilisation of SO2 during titration, test needs
to be done rapidly.
Botrytis & ascorbic acid can also give false-high results.
Titrets tend to over-estimate SO2 content by around 1020 mg/l (some quote 10 for whites and 20 for reds).
Feb 20, 2007
SO2 and Wine
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SO2 Measurement
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Titret Manufacturer's Instructions


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

Snap tip
Put tip in sample and squeeze to draw some in
Wait 30 seconds
Rock to mix
Continue to add and rock until it turns colourless
Feb 20, 2007
SO2 and Wine
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SO2 Measurement
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My Tips





Draw the sample from the carboy and test immediately
Put the tip in the sample before breaking it
Keep the tip in the sample the whole time
Have a strong light to observe the colour
Work quickly through the test
Feb 20, 2007
SO2 and Wine
36
SO2 Measurement
●
Measuring Reds






You can get a reasonable indication of free SO2 by
doing two measurements, but accuracy is limited
Do the first measurement as for a white wine
The end point is when the blue hue disappears
Add a few drops of hydrogen peroxide to the sample
and test again
The end point is when the colour matches the end point
of the first test (hold them side by side)
Subtract the two readings to get the free SO2
Feb 20, 2007
SO2 and Wine
37
SO2 Reduction
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SO2 is often removed from wine by aerating.

Wine is racked from one vessel to another in a violent
manner (with turbulence) to encourage oxygen contact.

This method can be traumatic for a wine, potentially
over oxidising and "damaging" its delicacy. However, it
remains a simple solution to reducing excessive SO2.

This will remove between 12 and 32 ppm SO2.
If the aim is to reduce SO2 by more, then this method
can be used on a periodic basis more than once.
If less, the aerating should be done with less violence.
Feb 20, 2007
SO2 and Wine
38
SO2 Reduction
●
Hydrogen Peroxide (H2O2) can be used to reduce
SO2

●
I consider this method more damaging than racking as
the addition of oxygen is very concentrated and would
highly oxidize a small portion of the wine before it is
mixed through.
Amount of reduction

The molecular weight of SO2 is 64.1 and that of H2O2
is 34.

Therefore, 0.5304 g (1/64.1*34) of H2O2 is required to
react with 1 g of SO2
Feb 20, 2007
SO2 and Wine
39
SO2 Reduction
●
Example using H2O2




15 liters of wine has a free SO2 level of 70 mg/l. It is
desired to reduce this to 40 mg/l.
The reduction of 30 mg/l (70-40) requires an H2O2
addition of 16 mg/l (0.5304*30).
Thus, the 15 liters requires an addition of 240 mg
(15*16) of H2O2.
Using a 3% mass/mass solution of H2O2, 7.9 ml
(240/30.3) of the solution needs to be added to the 15
liters for the drop to 40 mg/l.
Feb 20, 2007
SO2 and Wine
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