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Sulphur Dioxide
©Copyright Ben Rotter 2001-2002

Contents

Introduction
The Properties of SO2
Sodium and Potassium Salts
Forms and Functions of Sulphur Dioxide in Wine
Free SO2 and pH
Free SO2 and Temperature
SO2 Binding
Accounting for SO2 Binding
SO2 and Oxidation
Accounting for Oxygen Binding
Complete Example: Accounting for Oxygen and Binding
Testing for SO2 (Ripper and AO methods)
Removing Free SO2
Adding SO2
Typical SO2 Additions
Stock Solutions
Campden Tablets
References


Introduction

Sulphur dioxide, often abbreviated to sulphite or SO2, is used extensively in modern winemaking. It's use is predominantly for its suppression of yeast and bacterial action and its anti-oxidant properties. It is possible to make good wine without using sulphites, but this results in less control, consistency or biological stability. 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. [Abrahamson]
This article outlines the properties, forms, and uses of sulphite. Attention is given to the issues of pH, temperature, SO2 binding generally and with oxygen, removal, testing, stock solutions and Campden tablets. (Compounds such as SO2 are written as SO2 in the text of this document to provide better line spacing and ease of reading.)

The Properties of SO2

Antiyeast dissolved SO2 gas, and to a lesser extent bisulphite, inhibit yeast
Yeast selective at certain doses sulphur dioxide promotes yeast selection by hindering the multiplication of non-alcohol producing yeasts such as apiculates, torulopsis, and candida more than that of elliptic yeasts; bacteria are also more susceptible to its affects than yeasts
Antibacteria lactic bacteria are sensitive to free and, to a lesser extent, bound SO2 [Fornachon]
Antioxidant free SO2 reacts with dissolved oxygen reducing wine oxidation
Antienzymatic sulphur dioxide destroys oxidases (enzymatic catalysts of oxidation), it inhibits polyphenoloxidases which catalyze oxidative reactions in juice (it's inclusion in must will therefore increase the amount of oxygen available to yeast in their growth phase)
Taste sulphur dioxide reacts with acetaldehyde essentially removing its volatile presence and thus the wine retains "freshness" of aroma
Colour sulphur dioxide reduces enzymatic browning by obstructing polyphenol oxidases (the enzymatic catalysts which cause oxidative browning of juice); it also causes an increase in the extraction/solvency of anthocyanins and polyphenols from fruit tissues (at normal doses the colour increase is aesthetically insignificant)
Fermentation at low levels of 5-10 mg/l sulphur dioxide delays the onset of fermentation but later speeds up the multiplication of yeasts and their transformation of sugars [Peynaud, p.161]


Sodium and Potassium Salts

Two salt forms of sulphite are generally used for winemaking: potassium metabisulphite (K2S2O5) and sodium metabisulphite (Na2S2O5).
The molecular weight of sodium metabisulphite is 190.2 and that of potassium metabisulphite is 222.4, whereas that of sulphur dioxide (SO2) is 64.1. The salts dissociate giving two moles of SO2 for each mole of the salt. Thus, the SO2 content of sodium metabisulphite is 2 x 64.1/190.2 = 67.4 % and that of potassium metabisulphite is 2 x 64.1/222.4 = 57.6%.
SaltSO2 content
Sodium metabisulphite67.4 %
Potassium metabisulphite57.6 %
Winemakers generally prefer to use the potassium form for sulphite additions since this increases the level of potassium in the wine which later helps to precipitate tartrates when cold stabilising. Others claim that the sodium form can contribute a `salty' flavour to wine.


Forms and Functions of Sulphur Dioxide in Wine

Potassium metabisulphite dissociates in water to potassium ions (K+) and singly ionised bisulphite, [HSO3]-. (Sodium metabisulphite dissociates in the same way.)
K2S2O5 + H2O ---> 2K+ + 2[HSO3]-
Since wine is acidic, hydrogen ions are present (H+) and the [HSO3]- can then transform into sulphur dioxide:

[HSO3]-+ H+ <===> H2O + SO2
singly ionized bisulphite + hydrogen ionwater+ unionized (molecular) sulphur dioxide

Additionally,

[HSO3]- + H2O <===> H+ + SO3--
singly ionized bisulphite + waterhydrogen ion+ doubly ionized sulphite

The pKa's of SO2 in water are 1.77 and 7.20. However, in the presence of ethanol ions and the conditions found in wine a value closer to 2.0 is more appropriate. In the calculations below, a value of 1.81 is adopted.

Thus, the relationships of the forms of SO2 in wine are shown completely by:

H2O + SO2 <===> H+ + [HSO3]- <===> H+ + SO3--
water + molecular sulphur dioxide hydrogen ion + bisulphite hydrogen ion + sulphite

Sulphur dioxide binds with some compounds in wine to form other organic compounds. This is called "bound/combined/fixed" SO2. The remainder is called "free" SO2 (FSO2). "Total SO2" (TSO2) is the sum of free and bound SO2.


Free SO2
Bisulphite (HSO3-) is the predominant form of free SO2 at wine and juice pHs. It causes the inactivation of polyphenol oxidase enzymes and the binding and/or reduction of brown quinones in juice (50 ppm TSO2 reduces PPO activity by more than 90%). It is a successful anthocyanin (the predominant colouring matter in red fruits) extractive, yet it does bleach colour and slows anthocyanin polymerisation reactions with other phenols.

Molecular (or active) SO2 is the principal form responsible for antimicrobial activity. It also reacts rapidly with hydrogen peroxide (H2O2) formed as a byproduct of the chemical oxidation of phenolic compounds, and can therefore act as an antioxidant.

Sulphite (SO3--) is the only form which reacts with oxygen directly. At typical wine pHs the quantity of this form of sulphur dioxide is minute and it's reaction with oxygen very slow.

The amount of each of the bisulphite, sulphite, and molecular SO2 fractions of the total quantity of free sulphur dioxide is a function of pH.

Bound SO2
Stable bound SO2, also often termed "bisulphite addition products", is due to the binding of bisulphite with other compounds in wine. Acetaldehyde is the predominant component of the bisulphite addition products. These products may have some anti-bacterial activity towards certain lactic acid bacteria (especially those that metabolise acetaldehyde, thereby releasing molecular SO2). Bisulphite also reduces browning in wines by either binding with brown quinones or else reducing them back to phenols.

Unstable bound SO2 consists of SO2 bound with such substances as glucose, arabinose, galacturonic acid, polysaccharides, polyphenols, diketogluconic acid, ketofructose, pyruvic acid, and alpha-ketoglutaric acid. It provides a reserve that feeds free SO2 when it subsides through oxidation.



Free SO2 and pH

As previously mentioned, molecular SO2 is the principal form of sulphur dioxide responsible for anti-microbial activity. The quantity of molecular sulphur dioxide required to inhibit specific micro-organisms depends on their individual environment and history. However, levels used to control biological stability have generally been achieved through 0.5-1.5 ppm of molecular SO2.
Currently, winemakers generally feel that 0.8 ppm molecular SO2 provides sufficient protection for dry wines. (Some feel that a concentration of 0.6 ppm is suitable for red must or wine, while 0.8 ppm is suitable for white must or wine, and 2 ppm in sweet white wine provides good microbial stability.) The maximum before the sensory threshold is reached is generally considered to be 0.8-2 ppm.
(0.825 ppm molecular is required to suppress growth of Brettanomyces/Dekkera sp. and Saccharomyces cerevisiae according to one study. [Beech et al]). The amount of molecular free SO2 available is a function of pH. Thus, SO2 additions should be calculated with reference to pH.

The equations for these quantities are:
Molecular SO2 = Free SO2 / (10(pH - 1.81) + 1)
Free SO2 = Molecular SO2 * (10(pH - 1.81) + 1)
(SO2 levels quoted ppm ("ppm" means "parts per million"). Note that ppm is equivalent to mg/l.)
(See above under "Forms of Sulphur Dioxide in Wine" for the reasoning behind 1.81 as the chosen pKa value.)

Alternatively, in table form:

pH

Free SO2 (ppm) for given molecular SO2 level

0.6ppm

0.8ppm

2ppm

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

Free SO2 and Temperature

Free SO2 increases as temperature increases.
For example, a wine containing 68 mg of free SO2 at 0 C (30 F) will contain 85 mg at 15 C (57 F) and 100 mg at 30C (84 F).


SO2 Binding

As previously stated, a portion of the SO2 added to a wine will become bound with oxygen and other compounds in wine (such as aldehydes, sugars and ketonic acids), and is therefore no longer "free."
The relationship between the amount of added SO2 and the amount of SO2 remaining free is complex. It is clear, however, that it is largely governed by the total SO2 content of the wine. [Margalit (1996), p.268]
Molecular SO2 provides the protective qualities in wine. Since the molecular SO2 is a portion of free SO2, taking account of SO2 binding is important in achieving the correct molecular SO2 level.


Accounting for SO2 Binding

Many winemakers assume that about 50% of their SO2 addition to a wine becomes bound below 30-60 mg/l total SO2. There is varying opinion on what figure should be used here, and it most likely varies from set-up to set-up and wine to wine. Peynaud quotes 100 mg/l, Margalit 50-60 mg/l [Peynaud, p.250; Margalit (1990), p.26]. It would be safe to assume a level of around 50-60 mg/l given this data. After this level, added SO2 is generally considered not to bind, providing free SO2 almost exclusively. (Though some winemakers assume that thereafter some SO2 does become bound (usually about 30%).)

Example
Using the rule that 50% of any SO2 addition becomes bound whilst the total SO2 content of the wine is under 50 mg/l, and that 10% of any addition becomes bound thereafter, the following example illustrates the additions a winemaker might make.
35 mg/l of SO2 is added to a white juice must at crush. Following fermentation, the wine has a pH of 3.1 and it is (safely assumed or) assessed that the free SO2 content is negligible and all SO2 is bound. It is desired to take the molecular SO2 level to 0.6 mg/l. 12 mg/l free SO2 is required for 0.6 mg/l molecular at pH 3.1 (see table/graph/equation above). If all SO2 added became free, 12 mg/l would be added to obtain this level. However, we have assumed (from the above rule) that 50% (half) of the SO2 addition will become bound so for 12 mg/l to remain after binding, we must add 24 mg/l (12*2 or 12*100/50).
Some time later when the wine is bulk ageing, the total amount of SO2 that has been added to the wine is larger than 50 mg/l (the original 35 + the 24 addition = 59). The pH is still 3.1 and the free SO2 has depleted to 10 mg/l.
Again, 12 mg/l free SO2 is required for 0.6 mg/l molecular at pH 3.1 (see table/graph above). Since 10 mg/l is already present, 2 mg/l (12-10) free SO2 is therefore the required addition assuming no binding occurs.
We may instead assume, however, that binding still occurs at the lower rate of 10%. 10% of all SO2 added at this stage therefore becomes bound. 2.2 mg/l (2 / 90% which is also 2 / (90/100)) SO2 is required to be added to the wine to obtain the 12 mg/l free SO2 level for 0.6 mg/l molecular SO2.


SO2 and Oxidation

A primary reason for the use of SO2 is to prevent excessive oxidation. Some oxidation contributes to wine development, softening tannins and oxidising higher alcohols into acids which can further combine with ethanol to form esters (which contribute to the aroma and flavour of the wine). Current thinking is that this favourable oxidation condition occurs when small amounts of oxygen are present (Zoecklein writes that more than 25 mg/l can hinder phenol polymerisation, in turn effecting tannin evolution, suppleness and stability [Zoecklein, 2000, under "Aromas, flavours and color"]). The 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 by wine in excess or too quickly, however, the metallic ions cannot carry the oxygen and it combines directly with ethanol and higher alcohols to form aldehydes. High concentrations of aldehydes give wine a flat and stale aroma and flavour, an "oxidised (or maderized) aroma". A common fault due to excessive oxidation is the presence of high concentrations of acetaldehyde. Acetaldehyde is oxidised ethanol, and gives sherry its characteristic aroma.

In the presence of SO2, however, the latter rapid/excessive reaction of oxygen with wine compounds is prevented. Oxygen still reacts with dihydroxy-phenols to form quinones. Hydrogen peroxide (H2O2), a strong oxidising substance, is also produced as a byproduct. But SO2 reacts with the hydrogen peroxide forming sulphate (SO4--) before it can react with alcohol (CH3CH2OH) to produce acetaldehyde (CH3CHO) and water. This appears to be the only significant antioxidant contribution SO2 makes to wines.

Oxygen and dihydroxy-phenols combine to form quinones and H2O2
Without SO2: H2O2+ ethanol ---> acetaldehyde+ 2H2O
With SO2: H2O2+ molecular SO2 ---> sulphate [SO4--]


SO2 depletion
Over time, free SO2 combines slowly with oxygen in wine and subsequently the level of free SO2 decreases.
Average estimates indicate that SO2 depletion may be 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. [Eisenman] (SO2 in bottle exhibits a depletion of no more than a few milligrams per year.) Because of this decrease, SO2 levels must be continually maintained.

SO2 depletion increases with an increase in temperature, headspace, and oxygen exposed surface area to volume ratio. Since it is dependant on many variables, SO2 depletion varies from set-up to set-up and wine to wine. Safe assumptions can be made based on the past experience witnessed with each set up. For this, free SO2 levels must have been measured to determine the level of decrease over a given time and situation. The most common method of measurement is the Ripper test.

Oxygen absorption
Since rapid or excessive oxidation disadvantages wine quality, 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). To account for this binding it is important to know the amount of oxygen absorbed. This can be measured with a dissolved oxygen (DO) meter. For those that do not have access to this equipment, SO2 uptake can be estimated based on the data below or by measuring the SO2 depletion rate when no SO2 is used.

Note that the figures quoted outside of parentheses below are in milligrams per litre and those inside parentheses are in millilitres per litre.

Saturation level
Air is approximately 21% oxygen. Juice saturated with oxygen contains about 10 mg/l of oxygen. [Eisenman]
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 (68 F) 8 mg/l (6 ml/l) is the saturation level, whereas at 0 C (32 F) it is 11 mg/l (8 ml/l). [Peynaud, p.248] Thus, the oxygen saturation range in wine is generally 7-11 mg/l (5-8 ml/l). [Supported by Rankine, p.187-188; Jackisch, p.115]

Rackings
Gentle rackings often cause an oxygen uptake of 1-3 mg/l (0.8-2.3 ml/l), whereas those with more turbulence and air exposure might absorb 3-8 mg/l (2.3-6 ml/l) during each racking. [3-4 ml/l in Peynaud, p.249; 5-6 ml/l in Jackisch, p.117]

Barrels
Penetration through oak wood itself is insignificant at 3-7 mg/l per year (2-5 ml/l/yr). This increases with less close-grained wood and smaller cask sizes - in tuns of 5cm thickness it is practically nil. [Peynaud, p.248]

However, when considering that barrels are often opened for testing/tasting, oxygen absorption may be around 40-53 mg/l per year (30-40 ml/l/yr) [Jackisch, p.117]. Peynaud quotes that absorption through surface exposure is about 20-27 mg/l per year (15-20 ml/l/yr), whether at the bung-on-top position with regular toppings or the bung-on-side position. [Peynaud, p.248].
A partially filled container of wine with a surface area of 100 cm2 will absorb oxygen at 2 mg/l per hour (1.5 ml/l/hr). [Peynaud, p.248]

(The conversions from oxygen's volumetric measures to oxygen's by weight measures are calculated at 1 atm and 20 degrees C (68 F). Under these conditions, 1 ml/l of oxygen weighs 1.33 mg/l. At 0 degrees C (32 F) and 1 atm it's 1.43 mg/l, a difference of only 7% which is considered negligible.)

Accounting for Oxygen Binding

Sulphite reacts with oxygen to form sulphate given the relationship
2SO3-- + O2 ===> 2SO4--
2x128 32 2x192
Since this is in an acid solution (wine), H+ ions are present and the sulphate (2SO4--) forms sulphuric acid [HSO4]-.
Considering the molecular weights of 128 (SO3) reacting with 32 (O2), 4 mg of SO2 is required to react with 1 mg of oxygen (128/32=4).

Example
The bottling of 5 litres of wine is conducted with some splashing. It was assumed that the wine would become almost saturated with oxygen after such a racking and 12 ml of headspace would remain in the bottle once corked. We require two calculations, first for the SO2 required to bind with the bottling operation oxygen uptake, and secondly for the SO2 required to bind with the airspace in the bottle. It is assumed that SO2 binding with wine compounds is negligible in this case (which is likely by the time bottling is due).

1. 7 mg/l of oxygen is assumed to be dissolved into the wine following the racking procedure. 28 mg/l of free SO2 (7*4) is required. 140 mg might be added to the bulk 5 litres (28*5), or alternatively, 21 mg to each 750 ml bottle (28*0.75).
2. Each bottle contains 12 ml of airspace. Using the fact that air is 21% oxygen, the oxygen content in the headspace is 2.5 ml (12*0.21). 2.5 ml weighs 3.3 mg (2.5 mg * 1.33 mg/ml). Therefore 13.3 mg of SO2 is required (3.3*4) to bind with the oxygen in the headspace in each bottle.

If the SO2 is added to each bottle and not to the bulk, the total amount of SO2 in each bottle should be 34.3 mg (21+13.3).


Complete Example: Accounting for Oxygen and Binding

Combining the above sections, we can calculate a typical SO2 addition accounting for both bound SO2 (due to wine components) and oxygen binding (using up the oxygen the wine is exposed to during, for example, a racking).

Example
A wine has just completed fermentation and is to be racked gently. The current free SO2 level is assumed to be zero, and the total SO2 level is under 50 mg/l which means that approximately 50% of all added SO2 will become bound. The oxygen uptake due to racking is assumed to be 3 mg/l. The pH is 3.1 and the aim is to obtain 0.8 mg/l molecular.
Accounting for the racking oxygen uptake, 12 mg/l SO2 is required (3*4). For 0.8 mg/l molecular at pH 3.1, 16 mg/l is required. Thus, a total of 28 mg/l is required assuming no wine component binding. Yet 50% of the SO2 amount added will become bound, so we need 56 mg/l (100/50*28).


Testing for SO2 (Ripper and AO methods)

The most common method for measuring free SO2 in wine is the Ripper method. This uses a iodine standard to titrate the free SO2 in a sample. Free SO2 is determined directly while total SO2 can be ascertained by treating the sample with sodium hydroxide (before titrating) to release bound SO2. The free and total SO2 analysis is based on the redox reaction:
H2SO3 + I2 --> H2SO4 + 2HI
The completion of the reaction is noted when excess idione is complexed with added starch to form a blue-black colour end point.
The Ripper method is known to be inaccurate, particularly with reds since the dark colour makes it difficult to identify the end point. Additionally, the potential volatilisation of SO2 during titration, and the reduction of the iodine titrant by non-sulphite compounds such as phenols or pigments can effect the result significantly. (Other interferences include botrytis and ascorbic acid.) Yet despite the inaccuracy, the Ripper method remains the most common method for free SO2 determination due to its simplicity.
Using a Titrets amouple to verify the SO2 content of a stock solution

A simple and cheap way to conduct a Ripper test is to use Chemetrics "Titrets" kits (www.chemetrics.com). Titrets tend to over-estimate [Margalit (1996), chapter 2] the true SO2 content by around 10-20 mg/l (some quote 10 for whites and 20 for reds, and the increase usually equates to a 20-25% over-estimation).

To counteract these inaccuracy problems, winemakers sometimes dilute the sample (which helps determine the end-point but lowers accuracy), use a light to illuminate the sample (especially with reds), and use calibrated control solutions. Value adjustments are also often made to the reading based on previous values attained from more reliable sources such as the alternative "aeration/oxidation" (AO) or Rankine method.

With the AO method, a sample is acidified and the sulphur dioxide in it distilled into a hydrogen peroxide trap, where volatised SO2 is oxidised to H2SO4 according to the reaction:
H2O2 + SO2 --> S3 + H2O --> H2SO4
The acid formed is then titrated with NaOH to an end point, and the volume of NaOH required used to calculate the SO2 levels. This method avoids the iodine-phenol binding which occurs in the Ripper method.

Removing Free SO2

Sometimes excessive SO2 is (accidentally) added to a wine. SO2 will slowly deplete during an oxidative ageing process, but sometimes winemakers wish to reduce the SO2 level in a short period of time. There are two methods commonly employed for such situations.

Aerating
SO2 is often removed from wine by aerating. Usually the wine is transferred 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.
A wine saturated with oxygen will contain 5-8 mg/l oxygen (see section "SO2 and Oxidation, Saturation level" above). Assuming a complete reaction, this amount of oxygen will remove 20-32 mg/l SO2.
If the aim is to reduce SO2 by over 20-32 mg/l then this method can be used on a periodic basis more than once. If less, the aerating should be done with less violence.

Using Hydrogen Peroxide
Free SO2 can be removed by adding hydrogen peroxide (H2O2) to wine.
SO2 + H2O2 ===> SO4--+ 2H+
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.

The peroxide reacts with molecular SO2, changing the SO2 equilibrium. Since this equilibrium is continually re-establishing, the H2O2 should be added slowly. Additionally, since H2O2 is such a powerful oxidiser, the amount added should be calculated carefully. Analytically testing the SO2 content before and after H2O2 addition is advised.

Solutions of H2O2 commonly come as 3% solutions. If they are mass/mass solutions (this appears to be the usual case) they should contain about 30.3 mg/ml H2O2. If they are volume/volume solutions they should contain about 42.3 mg/ml H2O2. (See "Information on H2O2 content" below for more details.)

Example using H2O2
15 litres 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 litres 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 litres for the drop to 40 mg/l.


Information on H2O2 content
Pure (100%/weight) H2O2 has a density of about 1.41 g/ml.
Mass/mass solutions: 3 g H2O2 / (97 g H2O + 3 g H2O2) means a volume of 97 ml + (3 g / 1.41 g/ml = 2.13 ml) = 99.1 ml. This contains 3 g per 99.1 ml which is 30.3 mg H2O2/ml of the 3% solution.
Volume/volume solutions: 3 ml H2O2 / (97 ml H2O + 3 ml H2O2). 3 ml H2O2 provides (3 ml * 1.41 g/ml =) 4.23 g H2O2 per 100 ml solution, which is 42.3 mg H2O2/ml of the 3% solution.


Adding SO2

Adding SO2 generally
When adding SO2, it is important to ensure that it is evenly distributed in the wine. Addition in liquid form, and post-crush additions, are preferred since they assist in SO2 distribution and help prevent evaporation and combination with solids.

Adding SO2 to musts
Members of "the brown juice club" do not add SO2 to whites before fermentation. This allows for early enzymatic browning and most of the brown polymers precipitate during, or soon after, fermentation.
There is however another side to this in that not adding SO2 to a must can contribute to a slow or stuck fermentation [Zoecklein, 2001, under "Sulfur Dioxide in the Fermenter"]. As mentioned under "SO2 and Oxidation", phenols oxidise to form their corresponding quinones (brown products). Polyphenoloxidases are the substances in juice which catalyse this reaction. The reaction can use significant amounts of oxygen in a must, potentially leading to an oxygen content insufficient for healthy yeast growth. However, SO2 inhibits polyphenoloxidase enzymes, allowing for a higher oxygen concentration in the must, which in turn provides a healthier fermentation.


Typical SO2 Additions

Those winemakers who do add SO2 pre-fermentation add around 25-50 mg/l at crush. This is followed by a post alcoholic fermentation (or post malolactic fermentation) addition of between 120-150% of the amount required to maintain the desired molecular SO2 level. During bulk ageing, and for bottling, the wine is maintained at this same molecular SO2 level.

As mentioned previously, molecular SO2 levels are pH dependant. Many winemakers cannot assess pH in their wines and, therefore, quantities of total SO2 to add at particular times or procedures of winemaking are given by rough (personal) guidelines.
These quantities vary from winemaker to winemaker (and on wine type and set-up). Some common additions are presented in the following table.

When added Amount of total SO2 added (ppm or mg/l)
fruit/crush 25-50 (up to 100 under unfavourable conditions)
(none for brown juice club)
before MLF none / under 20
racking with MLF none / 10-15
racking reds 20-30
racking whites 30-40
racking sweet whites 80-100
bottling reds 10-20
bottling dry whites 20-30
bottling sweet whites 50-60
totals (from crush to bottling) for reds under 150
(For typical Campden tablet additions, see the Campden Tablets section below.)

Stock Solutions

Stock solutions of dissolved sodium or potassium metabisulphite salts are often made up as they provide a quick and easy way of adding sulphite to a wine, especially when a gram scale is not available and a measured volume of solution can be used instead of weighing out very small quantities of powder.
It is important to keep a stock solution in an air tight container since contact with air will decompose the sulphite. (It should also be noted that plastic is, to some extent, breathable and stock solutions stored in plastic bottles should therefore be remade relatively frequently.)

As an example of the calculations used in making and using a stock solution, a 10% stock solution can be made up by adding enough water to 100 grams of potassium metabisulphite to make up a total volume of 1 litre [100 grams / 1000 mls * 100 = 10%]. This solution contains 100 mg/ml of potassium metabisulphite. Since potassium metabisulphite is only 57.6% SO2, this solution then contains 5.76% SO2 [10% * 0.576 = 5.76%] or, alternatively stated, it contains 57.6 mg/ml of SO2 [100 mg/ml * 0.576 = 57.6 mg/ml].
10 ml of this 10% stock solution added to 20 litres gives 50 mg/l (ppm) of potassium metabisulphite [100 mg/ml * 10 ml / 20 L = 50 mg/l] which gives 28.8 mg/l (ppm) of SO2 [50 mg/l * 0.576].
Alternatively, to obtain 30 mg/l (ppm) of SO2 in 15 litres, this requires 450 mg of potassium metabisulphite [30 mg/l * 15 l = 450 mg] for which 7.8 ml of the 10% stock solution is required [450 mg / 100 mg/ml / 57.6 % SO2 = 4.5 / 0.576 = 7.81 ml].

Campden Tablets

Campden tablets are designed to have a mass of 0.44 grams. However, consistency of the tablet size in manufacturing is questionable, and many winemakers claim there is little certainty that tablets contain the amount of metabisulphite they are intended to (figures have been seen to vary by up to at least 25%).
Additionally, some winemakers claim that the "fillers" often used in Campden tablets to increase the bulk size of the tablet taint wine flavour and affect clarity.
They do, however, remain a simple way of adding a small (if rough) quantity of sulphite to a must or wine.

Rules of thumb for the use of Campden tablets are generally quoted as:
One tablet should be added per gallon (Imperial or US) initially and then one at each of the 2nd, 4th, 6th, etc rackings.
Or, if heat is used in preparing the must, none initially but one per gallon at each of the 1st, 3rd, 5th, etc rackings.

Assuming one Campden tablet contains 0.44 grams of potassium/sodium metabisulphite, the following sulphite levels are obtained by the addition of 1 tablet to the given volumes:

Salt per Imperial gallon per US gallon per litre
Sodium 65 ppm 78 ppm 297 ppm
Potassium 56 ppm 67 ppm 254 ppm

In practise, these figures tend to vary by up to 25%, possibly more.


References

Abrahamson, Lyle C., (1991), Sulfite Production in Wine Fermentations: How much sulphite is produced by yeasts in grape must with no added sulfites?. Article's author is Enologist at Hallcrest Vineyards: The Organic Wine Works, Felton, Santa Cruz County, California.
Beech, F.W., L.F. Burroughs, C.F. Timberlake, and G.C. Whiting, (1979), Progres recents sur l'aspect chimique et antimicrobienne de l'anhydride sulfureux. Bulletin OIV 52(586):1001-1022.
Eisenman, Lum, (?), Oxygen Uptake in Wine, article at The Home Winemakers Manual.
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Zoecklein, Bruce, (2001), Enology Notes #26, August 29, 2001.
Zoecklein, Bruce, (2000), Harvest Memo #4, September 27, 2000.



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