Mitigation of Fermentation Off-Characters
If an off-character is found particularly after active fermentation has ended it may appear in the finished wine at the time of bottling. Some off-characters that may have appeared to have dissipated may return in the wine during aging or post-bottling. Thus effective mitigation practices need to be employed at the winery.
Esters: Esters are volatile and will hydrolyze under wine making conditions, depending upon the level formed, temperature of the wine and conditions of storage. For the Saccharomyces fermentation esters under barrel aging conditions they have mostly hydrolyzed by six months. Saccharomyces ester taints are rarely a problem in red wine production. Esters may be problematic in white wines due to the lower temperature of fermentation and aging of these wines. Less of the ester will be lost due to volatilization and less will be lost due to hydrolysis. The grape solids level during fermentation will also impact ester retention as the solids contain esterases that catalyze the hydrolysis of esters. The problem with reduction of unwanted esters is not the efficacy of treatments but their impact on other aroma characters. Grape esters and other volatile compounds important to wine aroma may be lost in the efforts to eliminate esters. Some success has been reported with respect to using solids as a source of esterases, but these enzymes will not discriminate between desired and undesired esters. Yeast lees also contain esterases and incubation on the yeast lees will lead to the loss of wine esters, but such treatments are not neutral and other yeast-derived components will be synthesized at the same time that may or may not be desired.
Volatiles stripping technologies can be used to remove esters from wine but again these techniques are not that discriminatory. However given the high thresholds of detection of esters, reduction of the ester content in a fraction of the wine and blending back may be sufficient to reduce the ester content to a desired level.
Ethyl acetate can be problematic as it can be formed in the wine post-fermentation by the action of the acetic acid bacteria. Under these conditions active fermentation has ceased and volatile compounds will not be driven off of the fermentation. Ethyl acetate can also be a problem if formed pre-fermentation by Hanseniaspora. Hanseniaspora can produce very high concentrations of ethyl acetate that do not dissipate during typical fermentation and storage conditions simply because too much of the compound is present and it has a relatively low threshold of detection. In this case, volatiles stripping may be the only option to salvage the wine.
Sulfur compounds: Sulfur compounds are also volatile and can be removed by fermentation gasses if formed early enough in the fermentation and in low enough concentrations that stripping will be effective. H2S produced transiently during cell growth and the early stages of fermentation is frequently dissipated by carbon dioxide during the fermentation. However this does not always happen. Again whether the H2S is lost or not depends upon the temperature, vigor of the fermentation and total amount made. If the H2S or S-volatile is made after the most vigorous phase of fermentation it will not be dissipated. Winemakers will use splashing or increased exposure of the wine surface to air to allow enhanced volatilization of the S-volatiles. This works as long as the presence of air does not lead to the formation of less volatile neutral derivatives of the S-compound. The oxidized versions may be retained in the wine and when the wine becomes chemically re-reduced during aging the S-compounds can be reduced back to their aromatic form. Using an inert gas like carbon dioxide, nitrogen or conducting the volatile stripping process under a modified atmosphere should limit the formation of oxidized forms of S-volatiles.
Addition of nitrogen may be effective in reducing further H2S and complex sulfide formation. In synthetic grape juice media with differing nitrogen content only a few commercial strains responded to nitrogen addition by reducing the level of H2S formation. The observed impact of nitrogen addition in production conditions may be indirect and due to the stimulation of fermentation rate and not greater incorporation of reduced sulfide into amino acids. The signaling role of H2S and its potential to inhibit other microbes in the environment offers an explanation of why formation of this compound has been selected for inSaccharomyces and why there would be genetic diversity in the genetic path chosen to enhance sulfide production. It is possible that the nitrogen limitation leads to a deliberate release of H2S in order to inhibit other microorganisms in the environment such that the nitrogen is available for Saccharomyces. In this case nitrogen supplementation again has a secondary impact on sulfide release.
Hydrogen sulfide can also be removed by copper treatment and the formation of insoluble copper sulfide. Excess copper following the treatment may need to be removed. Copper can also remove methanethiol and ethanethiol. However the disulfide form of these compounds is not precipitable by copper and over time can lead to the re-formation of the thiols. Sulfur compounds can be removed by charcoal fining but since charcoal has low specificity other wine components may be removed as well. There are some reports that yeast cell ghosts or spent yeast lees can remove sulfides, but these claims have not been substantiated.