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SO2 Addition as Liquid Solution (H2SO3)

SO2 is used as a preservative to protect grape juice, must or wines from oxidation and microbial spoilage. When SO2 is added, part of it reacts with different components present in the grape juice/must or wine to bound and become inactive as an antioxidant and antimicrobial agent. What remains is the Free SO2 and is in two parts "bisulfite" (HSO3_) and molecular SO2. The molecular SO2 is the active form against microorganisms and unwanted bacteria. SO2 additions are made at different stages of the wine-making process. The most practical usage of sulfur dioxide addition is by adding liquid SO2 solution, sodium or potassium metabisulfite.

SO2 liquid solution can be prepared by bubbling gaseous SO2 into water. The amount of SOadded as a liquid solution depends on the strength of that solution, and can be calculated from its concentration, normally expressed as % weight/volume.

 

 
SO2 Addition as Potassium metabisulfite (K2S2O5)

The potassium metabisulfite (K2S2O5) is a white crystalline salt, which contains 57.6 % sulfur dioxide (SO2). Potassium metabisulfite is dissolved in warm water before being used. When is added into juice/must or must it reacts with natural acids to release sulfur dioxide, which protects wines from unwanted microorganisms and oxidation.

 

 
SO2 Reduction

Hydrogen Peroxide (H2O2) is a powerful oxidizer and can be used to reduce SO2 level in wine.The H2O2 should be added slowly to the wine in a dilute form to avoid oxidation. After the H2O2 addition,wine should be allowed to equilibrate for several hours before any confirmation analysis of the concentration of remaining SO2 in the wine.

To avoid over adjustment and oxidation, lab trial should be performed.

 
Potassium Sorbate

Potassium Sorbate (K-sorbate) is a wine additive, used primarily as a preservative to help prevent re-fermentation of sweet or semi-sweet wines. Potassium sorbate releases 74% sorbic acid, whose anti-fungal properties inhibit the spread of yeast and molds. It's action against bacteria appears to be selective. At concentrations used in wine it does not seem to prevent spoilage from either acetic or lactic acid bacteria.

Potassium Sorbate must always be used in conjunction with proper SO2 addition. The taste threshold for experienced wine tasters has been reported to be about 130mg/L. Addition of sorbic acid often results in the formation of ethyl sorbate, which is said to impart an unpleasant odor when present in a significant level.

*The BATF limits sorbic acid addition to wines to 300 mg/L and the European Union regulations limit its addition to 200 mg/L.

 
Molecular SO2

Molecular sulphur dioxide is the most affective form involved in both the reduction of oxidation reactions and microbial activity in wine.

The recommended level of molecular SO2 for red wines is 0.5 mg/L (ppm); for white wines is 0.8 mg/L (ppm); and for dessert wines is up to 1.5 mg/L (ppm). You can use this calculator to determine how much Free SO2 is required to protect your wine, based on its pH.  Free SO2 over 50 mg/L (ppm) can be tasted and detect in nose of the wine.

 
Connection between the pH and effectiveness of SO2

Molecular SO2 is the active form against microorganisms and unwanted bacteria. The recommended level of molecular SO2 for red wines is 0.5ppm (mg/L), for white wines is 0.8ppm (mg/L) and desert wines is up to 1.5ppm (mg/L) . You can use this table to determine how much Free SO2 is required to protect your wine, based on its pH. Free SO2 over 50ppm (mg/L) can be tasted and detect in nose of the wine.

 
Strength of SO2 solution

SO2 liquid solution can be prepared by bubbling gaseous SO2 into water. The concentration of SO2 solution can be calculated from its specific gravity. You can use this table to determine the concentration of SO2 solution based on its specific gravity at different temperature.