Sulfur taint in wine production: Genesis and exodus

A winemaker is continuously on guard to ensure the earliest possible detection of issues that could have potentially negative effects on the quality of in-process or finished wine. One issue that is treated with the utmost respect, and attended to with some alacrity, is sulfur taint, the primarily olfactory manifestation of sulfur compounds in the wine. Sulfur taint is the bane of the winemaker because ( Lansing, Wine defects during fermentation, Wine Business Monthly, April 2011):

• 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.

The sulfur compounds associated with sulfur taint, and the population of odors associated therewith, are illustrated in the figure below.

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.

Environment	Source	Action	Impact
Vineyard	Elemental sulfur used as fungicide	Reduction during fermentation
	Sulfur-containing pesticides	do.
	Excess of metal ions
	Vine stress
	Unsound fruit
Cellar	Cold soaking	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

Source: Compiled from Lansing and Kennedy and Reid)

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 Degradation of sulfur-containing amino acids Degradation of cell compounds during autolysis

Source: Compiled from Lansing

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.

In the cases where the odors are manifested in the wine, remedies include (i) blowing it off through volatility; (ii) inert gas sparging; (iii) precipitating with copper additions; and (iv) fining.


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