Sulphur dioxide is present in two conditions in wine: free and bound. Bound sulphur dioxide is linked to aldehydes, anthocyanins, sugars, pyruvic acid. Free sulphur dioxide is present in three forms, molecular, bisulphite, and sulphite. The equilibria between these three forms is pH-dependent, and the most important form is the molecular form. Table 1 shows the percentage of free sulphur dioxide present in the molecular form as a function of pH.
SO2.H2O <=> HSO3- + H+ <=> SO3= + 2H+
| pH | Percent molecular | Free required for 0.825molecular |
|
2.7 |
10.500% | 7.85 |
| 2.8 | 8.540% | 9.66 |
| 2.9 | 6.900% | 12.0 |
| 3 | 5.560% | 14.8 |
| 3.1 | 4.470% | 18.5 |
| 3.2 | 3.580% | 23.1 |
| 3.3 | 2.870% | 28.8 |
| 3.4 | 2.290% | 36.0 |
| 3.5 | 1.830% | 45.1 |
| 3.6 | 1.460% | 56.5 |
| 3.7 | 1.160% | 71.1 |
| 3.8 | 0.924% | 89.3 |
| 3.9 | 0.736% | 112.0 |
| 4 | 0.585% | 141.0 |
The molecular form is the most important form for microbial control and protection from oxidation. This form reacts with hydrogen peroxide, quenching this compound when produced during the oxidation of phenolics and thus retarding formation of acetaldehyde and browning compounds (Boulton, et al., 1998). This form is also that which is responsible for the odour. Boulton et al quote a detection threshold of 15-40mg/L free SO2 in wine, though the actual level will be dependent on the pH of the wine and thus the level of molecular SO2., Calculations of the molecular SO2 for a range of pH values at these levels indicate that the detection threshold for molecular SO2 is probably in the order (0.7-1.0mg/L) of the level required for microbial control.
The bisulphite form forms hydroxy-sulphonates, and is therefore the form responsible for bound sulphur dioxide. Boulton et al state that the bisulphite form is generally held to be the least desirable due to this binding with colour molecules and aldehydes. However, Boulton et al also state that the bisulphite form has been linked to the inactivation and inhibition of the phenoloxidase enzyme, and that the binding reduces quinones or binds with them to form colourless addition products.
Bound SO2 is unavailable for prevention of microbial spoilage and oxidation, however this is an equilibrium reaction; over time therefore, SO2 bound to aldehydes can become available. This can result in aldehydes being left behind when SO2 is "consumed". Therefore, methods for removal of acetaldehyde should be considered before addition of SO2. Currently the only method is the ageing of red wines. However, a wine containing high levels of aldehydes can be made more acceptable (for early drinking) by adding higher levels of SO2 to bind these compounds.
The sulphite form should be the most important for protection against oxidation, however it seems that under wine conditions the levels of sulphite are so low, and reactions so slow, that this form does not provide any oxygen-consuming capability.
Note also ascorbic acid effects.