Walking the tightrope between oxidation and reduction in white wine production: Reduction

Reduction is the other side of the oxidation/reduction coin. When elemental oxygen combines with wine compounds, it can take a pair of electrons from the compound. The compound losing the electrons is said to have been "oxidized" while the oxygen, which has gained two electrons, is said to be "reduced."

Reduction is of principal importance to the winemaker as it relates to sulfur compounds. According to Jackson (Wine Science):

When present, elemental sulfur can be assimilated and used in the synthesis of sulfur-containing amino acids and coenzymes. It also may be oxidized to sulfate and sulfur dioxide or reduced to hydrogen sulfide. The reduction of sulfide to hydrogen sulfide may be a means, albeit aromatically unpleasant, of maintaining a favorable redox balance in yeast cells under anaerobic conditions.

According to Zoecklin (Enology Notes #96, 12/20/2004, vtwines.info):

Since wine is fermented by yeast through an anaerobic process (without oxygen), a number of reduced compounds are produced. Reduced sulfur and and nitrogen compounds, in the form of hydrogen sulfide and mercaptans (ammonia and amines), are known particularly for the negative characters they impart to wines. Thus, it is possible to have a wine with an unpleasant and undesirable reduced character.

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

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

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.

Oxidation and reduction are twin evils in the world of (especially) white wine production but there are aspects of both that are beneficial to the final product. Making wines which call on these qualities is called oxidative and reductive winemaking, respectively, and I will cover those styles in upcoming posts.

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