Understand Post-Fermentation SLO Management Options.

The following is a list and discussion of post-fermentation winery processing options for SLO management:

• Understand oxidation-reduction potential
• Monitor SLO
• Oxygen management, aeration, microoxygenation
• Antioxidants (sulfur dioxide, ascorbic acid)
• Lees management/yeast fining
• Tannin/silica additions
• Copper additions, copper impregnated pads
• Wine closures

A. Understand Oxidation-Reduction Potential. 

Oxidation-reduction (redox) reactions describe the general principles and behavior of most wine chemistry. An understanding of redox is key to understanding SLO and their management. Some generalizations regarding redox include the following:

• Oxidation-reduction (redox) reactions are a series of interlinked reactions involving the oxidation of one compound and the reduction of another.
• Oxidation and reduction are two different chemical processes that complement each other.
• As electrons are transferred, one compound is oxidized, while the other is reduced.
• Oxidation is the loss of electrons. Reduction is the gain of electrons.
• For every oxidation reaction, there is a reduction reaction.
• Electrons are rearranging themselves into a more favorable order.
• This order is determined by the redox potential of the compounds.
• Oxygen is not required, although it is frequently the species that starts the process.
• Redox potential can be measured in the same way that pH is measured.
• Redox potential is a measure of how oxidative or reductive a system is, measured in millivolts (mV).
• The higher the mV, the less reductive, the more oxidative.
• Aerated red wine: redox potential 400-450 mV.
• Non-aerated stored red wine: 200-250 mV.
• Redox potential changes much more easily in whites.
• Tank wines have a lower redox potential than barrel wines.
• Redox potential is lowest at the bottom of a tank, hence the significance of lees stirring.
• Post-fermentation, a wine could be in a reduced state, but this does not necessarily mean that the wine is displaying SLO.

For additional information on redox potential see Enology Notes at www.vtwines.info.

B. Monitoring SLO.  

Wine will always contain sulfide precursors, because these are normal constituents of fermentation, with an almost endless array of SLO in various states of oxidation and reduction. As a function of redox potential, these may manifest themselves in various forms post-fermentation.

The nature of the SLO compound(s) must be understood before remedial steps are taken. A sensory aroma screen should be conducted on all wines prior to bottling. The specific nature of such a screen is discussed in Zoecklein et al. (1999).  It allows for the sensory separation of three general, but important, groups of SLO: hydrogen sulfide, thiols, and disulfides.

It is essential that winemakers conduct an aroma screen on all wines prior to any remedial SLO adjustments.

Table 1. Outline of a sensory aroma screen for determining the general nature of some SLO in wine.

Three glasses of the same wine are evaluated: a control, a glass containing copper sulfate, and a glass containing cadmium sulfate. The interpretation is given in Table 1. Note: This review is solely for odor evaluation:  these wines should not be tasted.

C. Wine Aeration/Oxygenation. 

One misconception is the belief that a wine with SLO can always be fixed by oxidation. This has arisen due to some sensory changes noted on some occasions. If H₂S is present, the oxidation of H₂S to elemental sulfur can occur as below:

2H₂S + O₂ <-->  2H₂O +2 S

This leaves elemental sulfur present at the bottom of a tank, which must be removed. Otherwise, the reverse reaction would occur. While this reaction can occur, wines contain a host of antioxidants or reducing agents that will compete for any oxygen added.

There is some volatilization of low boilers like H₂S that can occur. How much volatilization is possible depends upon the particular SLO compound(s). What does occur with oxygen exposure is that the form of the sulfide changes, in accordance with the shift in the redox potential.

Figure 1.

A good example of redox is the oxidation of methanethiol to dimethyl disulfide. Oxidation of one SLO compound to a slightly less stinky one is sometimes possible. The sensory thresholds for sulfides shift markedly with small changes in molecular structure, ranging from 2 ppb to 12 ppb. Note that no oxygen is involved. One compound, like methanethiol, can be oxidized to form dimethyl disulfide. This reaction is reversible.

Oxidation of methanethiol to disulfides can easily take place with wine aeration. Aeration may not remove sulfides, but simply change their form and, therefore, their sensory descriptor and thresholds. Oxidation causes a cascading set of reactions stabilizing the electron shifts. The redox potential would be readjusted to near, but not exactly the same, as the original potential. To help avoid unwanted oxidation, especially in white wines, H₂S may be blown off with inert gases, such as nitrogen. However, this may take a significant quantity of gas and requires an understanding of the specific SLO in the wine, e.g. an aroma screen.

Microoxygenation.  It has been known for some time that microoxygenation can lower the perception of veggie/herbal character in a wine. Originally, we presumed this effect was the result of changes in pyrazines. However, that was not confirmed by our analysis. The odor of thiols complements those of pyrazines and indeed some thiols contribute to “green”-type odors. Microoxygenation results in oxidation of some thiols, resulting in both a change in the perception of SLO and veggie character as noted below. This highlights the interrelationships of aroma compounds in wines.

Figure 2

D. Copper Addition. 

The use of copper poses an interesting dilemma to winemakers. It can be used to treat H2S and some thios but, at the same time, will reduce the concentration of desirable VSC compounds. It does not discriminate between SLO and VSCs contributing to varietal character. Some of the considerations regarding the use of copper include the following:

• Legality/perception
• Reactivity only with certain SLOs
• Protein haze
• Timing of addition: yeast stress, redox
• Sensory impact on varietal character and intensity
• Impact on longevity

Copper can react with some SLO, while not others:

• H₂S and thiols react with Cu⁺²
• Disulfides and thioesters do not react with Cu⁺²
• Thioesters can degrade to thiols (and esters), which can react with Cu⁺² 

Copper reacts with hydrogen sulfide according to the following reaction:

H₂S + CuSO₄ → CuS + H₂SO₄

Copper also reacts with some thiols, including methyl mercaptan. However, copper does not react with disulfides (thiol oxidation product).

2 CH₃SH + ½ O₂ → CH₃SSCH₃ + H₂O
Methyl mercaptan      dimethyl disulfide

SO₂
Ascorbic Acid
←

Copper sulfide will not react with disulfides or heavy sulfur compounds. However, copper sulfide has been used with sulfur dioxide and ascorbic acid, whereby the SO₂ cleaves the disulfide, resulting in two mercaptans which can then be bound with copper.
The ascorbic acid acts as an antioxidant to keep the mercaptan from oxidizing. One of several problems is that this reaction is very slow at wine pHs. The above reaction illustrates the importance of conducting aroma screens.

In addition to only reacting with certain SLO, copper also has the disadvantage of being a strong oxidizer, possibly impacting wine longevity. The potential oxidizing effect is illustrated by the Fenton-type reaction:

H₂O₂ + Cu⁺²   →  Cu⁺³ + OH^-  + OH*

The OH*, or hydroxyl radical, is the most oxidative species. This is a potentially large problem, notably in white wines with relatively lower concentrations of oxidative buffers, such as phenols.

Addition of copper sulfate to the fermentor is a practice used by some in an attempt to limit SLO production. While the majority of the copper (about 60% or more) is bound to yeast and precipitates from solution, such additions are not benign. Copper addition, either during or post-fermentation, can have a large negative impact by lowering the varietal intensity of the aromas derived from VSC. As such, the varietal characters of Sauvignon blanc, Riesling, Gewürtztraminer, Petit Manseng, and Chenin blanc, are diminished due to copper’s ability to bind mercapto- compounds.

Copper and Glutathione. Copper also impacts wine longevity as a result of oxidation and removal of antioxidants. One of the most important antioxidants in white wines is glutathione. Glutathione is a sulfur-containing polypeptide both found in grapes and produced by yeasts. It is a strong antioxidant. As such, it helps protect labile aroma/flavor compounds from oxidative degradation. Copper additions, in the form of Bordeaux sprays and as a remedial winemaking activity, have the ability to bind and completely inactivate glutathione. Optimizing the production and management of glutathione may be the key to winemaking of low phenol whites.

E. Ascorbic Acid and SLO Management.

Understanding the mechanisms of oxidation is important. As illustrated in Figure 3, wine oxidation can involve the oxidation of a phenol to produce a quinone (oxidation product) and hydrogen peroxide. In the example below, the hydrogen peroxide generated oxidizes ethanol to acetaldehyde (coupled oxidation).

Figure 3.

It is important to note that sulfur dioxide additions do not bind the oxygen and, therefore, do not prevent the first step in this coupled oxidation.  Some winemakers use ascorbic acid, or vitamin C, as an antioxidant. Ascorbic acid sometimes protects the fruit and acts as an antioxidant, while at other times it can act as a protooxidant, or oxidative promoter.

The two roles of ascorbic acid are mainly the result of concentration and the presence of adequate sulfur dioxide. As illustrated below from Zoecklein et al, 1999, when ascorbic acid is added to wine, it binds oxygen rapidly to form two reaction products, dehydroascorbate and hydrogen peroxide. If there is not enough ascorbic acid maintained to react with the oxygen, oxidative degradation, including coupled oxidation, can occur. If there is not adequate sulfur dioxide maintained to bind with the hydrogen peroxide formed by the ascorbic acid, wine oxidation can occur.

Figure 4.

Therefore, the keys to optimizing the performance of ascorbic acid as an antioxidant are to maintain a concentration of about 50 mg/L and to have adequate sulfur dioxide.  Therefore, the use of ascorbic acid involves the following considerations:

• Reaction between ascorbic acid and oxygen much more rapid than SO₂
• SO₂ does not directly react with oxygen, but mainly with reaction products, such as H₂O₂
• Optimum levels of ascorbic acid (50 mg/L or more), and more SO₂ can prolong the antioxidant phase of ascorbic acid.
• For example:  If 100 mg/L ascorbic acid in wine reacts completely with oxygen, 62 mg/L SO₂ is required to react with the ascorbic acid oxidation product 

F. Wine Closures and SLO. 

Wine closures can impact post-bottling SLO. The following are important considerations:

• Low or no oxygen ingress screw cap-type closures/liners are more prone to cause accumulation of thiols post-bottling
• Low oxygen ingress results in a lowering of the redox potential
• Lack of oxygen to oxidize thiols to disulfides can impact SLO perception
• To deal with this potential problem, some are adding copper at bottling
• Cu⁺² bottling can impact longevity, but can bind H₂S and some thiols
• Copper addition at bottling has no impact on disulfides and thiolesters