Diagnostic Key Types of Fermentation-Driven Off-Characters
Types of Fermentation-Driven Off-Characters
The types of off-characters that can form during fermentation fall generally into one of three main categories: Sulfur-containing volatiles, esters, and fusel compounds. Aldehydes, volatile acids and free aromatic fatty acids can also be formed under some conditions and may mar the aroma profile of the wine. Hydrogen sulfide is typically generated from the reduction of sulfate during amino acid biosynthesis. Esters and fusel compounds also come from amino acid biosynthesis or degradation. The more complex S-volatiles can be generated via degradation of the S-containing amino acids, cysteine or methionine, or their metabolic derivatives within the cell. S-containing volatiles can also come from degradation of S-containing vitamins, from S-containing plant metabolites or from S-containing pesticide/fungicide residues. In the vast majority of cases, the role of pesticide/fungicide residues can be ignored as it is rare that these components would have been used close to harvest.
Off-character formation may occur prior to the establishment of Saccharomyces, during the early stages of fermentation, during the active phase of fermentation, at the end of fermentation or during aging on the yeast lees. Pre-fermentation aromas often come from the activity of non-Saccharomyces yeasts and bacteria. The most common early off-characters are acetic acid (a pungent note) and ethyl acetate (glue, solvent).
During active biomass accumulation, H2S and ester production may occur. In this case ester formation is stimulated by the presence of nitrogen indicating that biosynthetic reactions are the source of these compounds. Once active growth has diminished and ethanol is accumulating amino acid degradation occurs and at this time additional esters and fusel compounds may be produced. The fusel compounds come from the degradation of amino acids as nitrogen sources via the Ehrlich pathway.
As fermentation ends the cells enter a resting phase. At this time there is a release of accumulated intermediates to the environment. The cells are entering a non-growth phase and are filling cellular storage pools to capacity. Excess nutrients or components that would be energetically costly to convert to storage forms are excreted. Some components may be degraded in the conversion process to storage. Fatty acid esters may form at this time as well as some complex S-volatiles. This stage is characterized by a release of amino acids and other components to the environment and the availability of these components is stimulatory to the growth of the ML bacteria. Free fatty acids may also be released at this time. Generally aromatic fatty acids are not produced at levels above their thresholds of detection and at low levels can add to complexity, but at higher levels can impart a rancid or cheese note to the wine. These characters can also be produced by bacterial activity so the role played by Saccharomyces when these types of defects appear is unclear.
Aldehydes derived from sugar and carbon catabolism, such as acetaldehyde (apple, bruised apple and nutty; depending upon the matrix and concentration), can be found in wine but generally not released in high enough levels during fermentation to be problematic. Grape alcohols, such as the C6 hexanols can be oxidized to aldehydes by yeast activity. The C6 aldehydes can contribute stronger green or unripe characters to the juice and wine than their progenitor alcohols. Oxidation of the C8 alcohols to aldehydes leads to mushroom and earthy characters, described by some as a charred wood note.
If the wine remains in contact with the yeast lees additional aroma compounds may form. Yeast cell autolysis, the breakdown of internal cellular structures and the release of hydrolytic enzymes, occurs. Fatty acids and other compounds may be released that impact mouth feel in addition to impacts on aroma. S-volatile formation may also occur depending upon the composition of the yeast and the chemical conditions of the wine.