Pre-Fermentation Treatments
Sulfur Dioxide Addition
Sulfur dioxide has been used in conjunction with winemaking for centuries, and the practice of burning sulfur candles inside wine vessels to preserve the wine dates back to ancient Roman times. Today, the use of sulfur dioxide is ubiquitous throughout the world of winemaking, and many consider it a non-negotiable additive, acting as both an antioxidant and an antimicrobial agent (Section 18.1). The role of sulfur dioxide as an antioxidant in juice and wine lies in its competition with oxygen. As a reducing agent, sulfur dioxide can, to a degree, inhibit oxidation caused by molecular oxygen.
Sulfur Dioxide Rates
Sulfur dioxide has been traditionally added to harvesting bins, to crushed grapes, or to freshly pressed juice to inhibit growth of certain non-Saccharomyces yeasts and bacteria. When choosing the sulfur dioxide concentration to add to the grapes or the must, grape maturity, fruit integrity, sanitary conditions, pH, temperature, and eventual contamination risks must all be considered. The choice can sometimes be difficult. Generally, 25 to 75 mg/L sulfur dioxide added to must/juice will keep microbial load and enzymatic browning in check (Boulton et al., 1999).
Potassium Metabisulfite
The simplest way to add sulfur dioxide is to add potassium metabisulfite (KMBS) as a solid directly to the harvesting bins or dissolved in a small amount of water and added. In the vineyard, KMBS can be sprinkled over the base of the harvesting bin just before it is filled. Juice from machineharvested fruit (or damaged grapes) will drain to the bottom of the bin where it dissolves the KMBS protecting the grapes from spoilage. The great advantage of KMBS is that the sulfur dioxide is not released until it comes in contact with juice in the harvesting bin (i.e., when spoilage could begin to occur).
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