Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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Because of sulfur dioxide’s ability to bind with the precursors and the products of oxidation, it can be used as both a preventative and a treatment. For example, an oxidized white wine with a brown tint and a nutty smell can be improved with an addition of sulfur dioxide that bleaches out some of the dark color and binds with the acetaldehyde to reduce the nutty smell.
Antimicrobial role of sulfur dioxide
Sulfur dioxide is a broad-spectrum antimicrobial agent that has an inhibitory affect on a wide variety of microorganisms. The level of sulfur dioxide at which the microbe, either yeast or bacteria, is affected varies widely by species. This variation allows winemakers to use sulfur dioxide to treat microbes in the wine or must selectively and inhibit or kill undesirable microbes without having an affect on the desirable ones.
It has been understood since the early 1900s that only the free forms of sulfur dioxide (and not the bound) have an antimicrobial effect. It was further discovered in the 1960s that molecular SO2 was several hundred times more effective than bisulfite.
The mechanism for sulfur dioxide’s antimicrobial affect works by the sulfur dioxide entering the microbe and disrupting the activity of the enzymes and proteins of the cell. Since only the molecular form of sulfur dioxide can enter through the cell membrane, it is the concentration of molecular sulfur dioxide that controlsmicrobial growth.
Although there is some evidence that a high concentration of acetaldehyde- bound bisulfite inhibits the growth of some species of malolactic bacteria, this effect is eclipsed by the role of molecular SO2. Since the percentage of free SO2 that is in the molecular form is dependant on pH, the importance of pH in the effectiveness of sulfur dioxide and the microbial stability of wine in general cannot be overstated.
Affect of sulfur dioxide on yeast and malolactic bacteria
Sulfur dioxide has some degree of inhibitory affect on all yeast; however, the Saccharomyces yeast strains that are used by winemakers for alcoholic fermentation aremuchmore resistant to it than “wild” yeasts are. Wild yeast is the term used for a number of non- Saccharomyces species of ambient yeast that are present on grapes and in the winery cellar. Wild yeast are sensitive to both sulfur dioxide and alcohol.
Wild yeast can begin a spontaneous fermentation in juice but soon are killed by the alcohol that they produce. At this point, the Saccharomyces that is present, either indigenous to the grapes or added by the winemaker, which ismore resistant to alcohol takes over to complete the fermentation.
Wild yeast can sometimes be the source of off-flavors therefore most winemakers elect to control them with a small dose of sulfur dioxide and then inoculate the must with a commercial wine strain of Saccharomyces. A molecular SO2 level of 0.4 ppm (equivalent to a free SO2 level of 20 ppm @ 3.50 pH) will kill wild yeast without adversely affecting Saccharomyces.
The inhibitory effect of sulfur dioxide on malolactic fermentation is much greater than it is for the alcoholic fermentation that is performed by Saccharomyces yeast. Malolactic fermentation (often abbreviated to ML fermentation or MLF), is the secondary fermentation by bacteria that converts malic acid found in grape juice to lactic acid, which is less acidic.
There are several species of bacteria that are capable of MLF in wine, some more desirable than others. Oenococcus oeni, the most commonly used species of malolactic bacteria, is very sensitive to sulfur dioxide and has difficulty growing at levels above 25 ppm total SO2.
The sensitivity of malolactic bacteria to sulfur dioxide can be used by winemakers to influence the flavor of a wine. MLF has several effects on the flavor of a wine, it lowers the acidity and it can produce a “buttery” aroma, particularly in white wines. In addition to these sensory qualities, it makes a wine more microbiologically-stable.
If the MLF is completed in the cellar prior to bottling, you do not have to worry about it occuring after bottling and spoiling the wine. If you wish to make a more tart, fruit forward wine such as a Riesling or Sauvignon Blanc, MLF would not be as appropriate and MLF can be prevented by an early dose of sulfur dioxide.
In a wine that will be aged for a longer period in barrels, or if you desire a softer less acidic character, MLF is more appropriate. Chardonnay and red wines often are put through MLF either prior to or during ageing.
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