2. SO2 Production by Yeast
3. Sodium and Potassium Salts
4. Forms and Functions of Sulphur Dioxide in Wine
5. SO2 Binding
6. The Properties of SO2
7. Free SO2 and pH
8. SO2 and Temperature
9. Sensory Threshold
10. SO2 Loss
11. SO2 and Oxidation
13. Accounting for SO2 Binding
14. Testing for SO2 (Ripper and AO methods)
15. Removing Free SO2
16. Adding SO2: Practical Considerations
17. Typical SO2 Additions
18. Storage and Purity
19. Stock Solutions
20. Campden Tablets
21. Sulphur Wicks and Rings
|Sodium metabisulphite||67.4 %|
|Potassium metabisulphite||57.6 %|
|HSO3-||+ H+||<===>||H2O||+ SO2|
|singly ionized bisulphite||+ hydrogen ion||water||+ unionized (molecular) sulphur dioxide|
|HSO3-||+ H2O||<===>||H+||+ SO32-|
|singly ionized bisulphite||+ water||hydrogen ion||+ doubly ionized sulphite|
|H2O +||SO2||<===>||H+||+ (HSO3)-||<===>||2H+||+ SO32-|
|water +||molecular sulphur dioxide||hydrogen ion||+ bisulphite||hydrogen ion||+ sulphite|
|Substance||Source 1||Source 2||Source 3||Source 4||Source 5||Source 6|
|Formaldehyde||1.2 × 10-7|
|Acetaldehyde||2.5 × 10-6||5 × 10-4||1.5 × 10-6||1.5 × 10-6||1.5 × 10-6|
|alpha-ketoglutaric acid||8.8 × 10-4||8.8 × 10-4||5 × 10-4|
|Benzaldehyde||1 × 10-4|
|Acetone||3.5-4.0 × 10-3||3.8 × 10-3|
|Furfural||7.2 × 10-4|
|Chloral||3.5 × 10-2|
|Arabinose||3.5 × 10-2|
|Glucose||2.2 × 10-1|
|Pyruvic acid||0.3 × 10-3||1.4 × 10-4||4.0 × 10-4||4.0 × 10-4||3 × 10-4|
|Glucose||9 × 10-1||6.4 × 10-1||6.4 × 10-1||6.4 × 10-1|
|[HSO3-] / [SO2]||=||K / [H+]|
|[HSO3-] / [SO2]||=||10-pKa||/ 10-pH|
|[HSO3-] / [SO2]||=||10pH - pKa|
|Free SO2||=||[HSO3-] + [SO2]|
|( Free SO2 - [SO2] ) / [SO2]||=||10pH-pKa|
|( Free SO2 / [SO2] ) - 1||=||10pH - pKa|
|Free SO2 / Molecular SO2||=||10pH - pKa + 1|
|Free SO2||=||Molecular SO2 * ( 10pH - pKa + 1 )|
|pH||Free SO2 (mg/l) for given molecular SO2 level|
|0.6 mg/l||0.8 mg/l||2 mg/l|
14.1. Ripper method
The Ripper method for SO2 uses an iodine standard to titrate the SO2 in a sample. Free SO2 is determined directly while total SO2 can be ascertained by treating the sample with sodium hydroxide before the titration to release bound SO2.
Figure 10. Using a Titrets ampoule to verify the SO2 content of a stock solution.
|SO2||+ H2O2||===>||SO4--||+ 2H+|
Figure 11. Localised discolouration of pomace indicating high point-concentration of SO2.
Due to the rapid enzymatic oxidation reactions in musts, SO2 ought to be in contact with the juice as soon as possible after crushing the fruit. This is the principal which should be followed in any SO2 additions to musts. Exactly how this is practised may vary from set-up to set-up. In the case where fruit is partially crushed upon harvesting, SO2 should be added to the fruit with the aim to take action within the juice resulting from partial crushing. |
Addition of SO2 to the uncrushed fruit in the case of reds, or crushed and unpressed fruit in the case of whites, will result in SO2 binding with fruit solids. Such binding should be accounted for, and higher SO2 additions may be required in such situations.
Oxidation of draining press juice will be significant in the absence of SO2 and SO2 might therefore be added to the marc of the post-free run press fraction. Delteil  argues that this practise results in increased aromatics and varietal expression, greater palate volume and decreased sensations of palate dryness.
As mentioned previously, molecular SO2 levels are pH dependent. However, many winemakers cannot assess pH in their wines and, therefore, quantities of total SO2 to add at particular times or procedures of winemaking are made based on rough guidelines. Exact quantities vary from winemaker to winemaker (and on wine type/style and set-up). However, dosages can be amplified or reduced depending on the circumstances surrounding the quality of the fruit, juice, and wine. For fruit and musts, the following situations will require increased SO2 dosages: high suspended solids, ruptured/diseased fruit, violent or prolonged fruit transport, increased handling, higher temperatures, or a longer duration between crush and fermentation. For wines, increased temperatures and increased exposure to air tend to call for increased SO2 dosages. At bottling, wine style and the intended duration of ageing dominate dosage decisions.|
It is worth noting that, because of the differences in environmental conditions and typical practises in different countries, typical additions vary among different countries and regions. The additions in France, for example, are often much higher than what is considered necessary or normal in California. Likewise, hotter climates tend to receive higher doses (e.g. Languedoc Roussillon vs Burgundy). Common SO2 levels for addition to must are presented in Table 4 and levels for addition to wines are presented in Table 5. Note that these values are not the dosage additions themselves, but are the quantity of free SO2 that should exist in the must/wine after addition (binding should be taken into account upon addition to ensure that these levels are met). Table 6 shows recommended maximum values of total SO2. On an international scale, these values are relatively conservative.
(For typical Campden tablet additions, see the Campden Tablets section below.)
Figure 12. Weighing metabisulphite powder.
|Circumstance||Free SO2 (mg/l)|
|white, healthy fruit, low pH|| 25-50 |
|white, healthy fruit, high pH|| 60-80 |
|white, fruit with some rot|| 80-100 |
|red, healthy fruit, low pH|| 50 |
|red, healthy fruit, high pH|| 50-80 |
|red, fruit with some rot|| 80-100 |
|Circumstance||Free SO2 (mg/l)|
|before MLF||none / under 20|
|dry white, maintenance||30-40|
|sweet white, maintenance||40-80|
|dry white, bottling||20-30|
|sweet white, bottling||30-50|
|white (conservative)||under 150|
|red (conservative)||under 150|
|white, dry (liberal)||under 200|
|white, white (liberal)||under 400|
|red (liberal)||under 300|
Figure 14. Stock solution and syringe.
Stock solutions of dissolved sodium or potassium metabisulphite salts provide a fast and simple way of adding sulphite to a wine. This is especially the case when a gram scale is not available and measuring a volume of stock solution is preferential to 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 breathable to some extent, 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 of potassium metabisulphite (100 mg/ml * 10 ml / 20 L = 50 mg/l) which gives 28.8 mg/l of SO2 (50 mg/l * 0.576).
Alternatively, to obtain 30 mg/l of SO2 in 15 litres, this requires 781 mg of potassium metabisulphite (30 mg/l * 15 l / 0.576 = 781 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 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 (expected concentrations have been seen to deviate by up to 25%). Additionally, some winemakers claim that the "fillers" used in Campden tablets to increase the bulk size of the tablet, taint wine flavour and affect clarity.
Nevertheless, Campden tablets 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:
In practice, these figures may vary by up to 25%, possibly more.
Figure 15. Campden tablets - no need to weigh.
|S + O2||===>||SO2|