- It is generally associated with negative aromas
- It has a low threshold for sensory detection
- It has high chemical reactivity
- It is difficult to mask and/or remove.
In this post I will examine the origins of sulfur in must, the creation of sulfur compounds during winemaking, and strategies for minimizing the incidence and/or removing these compounds from the medium.
Sulfur taint has its origins in either the vineyard, the cellar, or both. In the vineyard, elemental sulfur is sprayed on the vines to combat the potential effects of powdery mildew. If this spraying is conducted too close to harvest, portions of the sulfur will remain on the grapes and make its way into the fermentation process. An example of sulfur-like off odors created in the cellar is the case of hydrogen sulfide production by the yeast to synthesize the sulfur-containing amino acids methionine and cysteine. This process is facilitated by the reduction of sulfates via the sulfur-reduction pathway. A lack of intracellular nitrogen will not curtail the process and the excess hydrogen thus created cannot be incorporated into the amino acid. Rather, it is secreted into the medium (Kennedy and Reid, Yeast nutrient management in winemaking, The Australian and New Zealand Grapegrower and Winemaker, 537, October 2008).
A listing of the sources of sulfur-like off odors is presented in Table 1.
Table 1: Sources of sulfur taint in wine production.
used as fungicide
|Reduction during fermentation|
|Excess of metal ions|
Growth of yeasts such as Kloeckera
Depletion of amino acids and micronutrients
|Native Yeasts||High hydrogen sulfide production||Compete against other yeasts for dominance of fermentation|
|Excess hydrogen sulfide from sulfate reduction||Hydrogen sulfide used to synthesize||Absence of nitrogen causes produced hydrogen sulfide to be secreted into the medium|
|High levels of sulphur dioxide added to must at crush||Allows sulphur dioxide to bypass the sulfate reduction system||Sulfur dioxide enters the yeast cell directly|
|Vitamin shortage in high YAN musts|
|Nitrogen limitation||Produces sulfur-like off odors||Production begins 30 minutes after ammonia starvation initiates|
The timing of the production of sulfur-like off odors is shown in Table 2 below.
Table 2. Production timing of sulfur taint by sulfur class.
|Sulfur Class||Production Timing||Source|
|Hydrogen Sulfide||Early in fermentation (2 - 4 days)||Nitrogen/vitamin deficiency|
|Fermentation end||Degradation of sulfur-containing compounds|
|Sur lie aging||Autolysis|
|In bottle||Generally under screw cap|
|Higher Sulfides||Late in fermentation/Sur lie aging||Release of compounds by metabolically active yeasts
There are a number of precautionary steps that can be taken to minimize the potential for sulfur taint (Lansing; Kennedy and Reid):
- Minimize the use of sulfur in the vineyards and cellar
- In the vineyard, ensure adequate time spacing between application and harvest
- Press stressed fruit separately
- Provide adequate nutrition to support the yeast during alcoholic fermentation
- The less assimilable nitrogen in the must, the greater the production of hydrogen sulfide
- Keep yeast cells suspended in the tank during fermentation
- Allows an even distribution of fermentation
- Allows the yeast full access to distributed nutrients
- Manage fermentation temperatures
- Hydrogen sulfide tends to form more commonly in hot, fast fermentations
- Mix the tank contents to prevent stratification
- An especial risk in tall, narrow-diameter tanks
- Remove wine from lees at the first hint of trouble
- Smell, smell, smell.
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