Traditionally, the yeast strain most commonly used in winemaking is Saccharomyces cerevisiae. However, with more and more desire to create a truly unique wine and to also keep up with the changing market (and changing climate), winemakers are looking for more alternatives to the traditional approaches in order to create something different than all the rest. One way to achieve this uniqueness is to utilize new strains of yeast in order to keep ahead of the game and remain competitive in todayâ€™s market.
Specifically in the review paper summarized today (see full citation of the source below), some are now looking into using non-Saccharomyces yeast strains to produce wines with more unique characteristics. Particularly, species of the genus Schizosaccharomyces are known to reduce the malic acid content in wines and have started to generate research interest.
Schizosaccharomyces yeasts are used frequently in the production of rum and cocoa liquors in Madagascar, and have been previously thought of as spoilage yeasts in wine. In the past, these yeasts have been isolated from fermentation vessels where the fermentation process was stuck or stopped, or from wines with significant â€śoffâ€ť aromas, however, up until now it hadnâ€™t actually been determined if these yeasts were the source of these issues, or if they just happened to be in the â€śwrong place at the wrong timeâ€ť.
Recent studies have shown that mixtures or use in sequential fermentations could actually improve the complexity of wines and improve the aromatic profile of the wine. Though not the yeast Schizosaccharomyces, currently, there is a commercial kit available that utilizing as sequential inoculation of the yeast strains Torulaspora delbrueckii and Saccharomyces cerevisiae.
It is thought that by using Schizosaccharomyces strains of yeasts during malolactic fermentation, the â€śgreen appleâ€ť aroma could be significantly reduced (â€śgreen appleâ€ť is often caused by malic acid, which is effectively reduced by Schizosaccharomyces). Schizosaccharomyces has also been shown to reduce gluconic acid and ethyl carbamate levels, which would increase the overall quality of the finished wine. Gluconic acid is known to be produced after grapes are attacked by various fungi, including Botrytis or Aspergillus, therefore employing Schizosaccharomyces could help â€śsaveâ€ť wines made from grapes attacked by these organisms.
Taxonomic research from the 1960s classified four species belonging to the Schizosaccharomyces genus, though more recently this number has beenreduced to three. The three species currently known to be part of the Schizosaccharomyces genus are: 1) S. japonicus; 2) S. octosporus; and 3) S. pombre. These yeasts are native to climates ranging from temperate to very hot.
In terms of fermentation, Schizosaccharomyces yeasts can produce wines with alcohol degrees ranging from 10o to 12.6o when in anaerobic conditions, and 13o to 15o when in slightly aerobic conditions. During malolactic fermentation, Schizosaccharomyces yeasts can metabolize malic acid and produce ethanol and carbon dioxide. Malic acid, along with tartaric acid, constitute between 70% and 90% of the wineâ€™s total acidity which can have a significant effect on the aromatic profile of the wine. Winemakers often aim to remove as much of the malic acid as possible, particularly from red wines in colder climates where malic acid levels are much higher than they are in warmer climates. Research has shown that Schizosaccharomyces yeast strains are probably the best yeasts for reducing malic acid content, and found that they can reduce malic acid levels by between 75% and 100% (most other strains can only reduce malic acid levels by 20-25%).
Warning: things are about to get hard core organic chemistry here, so you may want to look away if that isnâ€™t your cup of tea (or should I say glass of wine?)â€¦.
One of the things that makes Schizosaccharomyces yeasts so potentially desirable is the mechanism by which it reduces malic acid and then transitions to alcoholic fermentation almost in concert with this process. In very basic terms, for every one molecule of malic acid, one molecule of alcohol and two molecules of carbon dioxide are produced. Getting more specific, this is basically what is occurring in the wine when Schizosaccharomyces is employed:
1. Malic acid is broken down into pyruvic acid when in the presence of Manganese2+ and Manganese3+ ions.
2. The pyruvic acid then goes into the alcoholic fermentation process.
a. Pyruvic acid gets decarboxylated resulting in acetaldehyde while then gets further reduced to ethanol.
When in anaerobic conditions, breaking down 2.33g/L of malic acid by the above process yields 0.1% v/v alcohol. Currently, S. pombre is available commercially for this purpose.
Traditionally, the organisms used during malolactic fermentation (or anytime where malic acid is reduced) have been Oenococcus oeni and Lactobacillus plantarum. Both of these organisms are lactic acid bacteria, and are known to produce undesirable by-products during malolactic fermentation, including the production of biogenic amines. Employing Schizosaccharomyces yeasts gets around this issue, as they do not result in the production of the undesired biogenic amines.
Another question that comes up in terms of the use of new yeasts strains in winemaking is how will they behave when in an â€śaging on the leesâ€ť situation? In simple terms, lees are basically the dead yeasts and other molecules that have deposited or precipitated into the wine during the after fermentation. Many winemakers choose to age their wine on the lees, as even though the cells aretechnically dead, the cellular components interact with the wine to create more complexity in the aromatic profile of the wine and often times increase the quality of the finished wine. Recent research has shown that the cell wall composition of Schizosaccharomyces is complex and show great potential for improving the quality of the finished wine, including the stability of the wineâ€™s color.
Finally, in addition to reducing malic acid levels, gluconic acid levels, and ethyl carbamate levels, and also improving the stability of wine color, Schizosaccharomyces yeasts strains have been shown to produce lower levels of alcohol than traditional Saccharomyces yeasts strains. With increasing temperatures to due climate change, many vineyards are seeing changes in grape ripening, with some reaching maturity earlier and earlier, resulting in wines with higher and higher alcohol content. Using Schizosaccharomyces yeast strains may help combat this â€śby-productâ€ť of climate change, as the alcohol levels produced by the yeast are lower than their Saccharomyces counterpart when presented with the same starting material.
The authors of the study highlight some issues, with one being that the methods for isolating other Schizosaccharomyces yeast strains have yet to be developed. Media and other methods should be developed in order to test these other strains to determine their usability in the winemaking process.
I think these yeasts show great promise for use in winemaking today, as winemaking of the future molded by a changing climate. Certainly more research would need to be done on these strains to get a better sense of how exactly the aromatic profiles are changed, however, so far the research to date has shown Schizosaccharomyces may be a positive addition to the winemakerâ€™s â€śtoolboxâ€ť.
Iâ€™d love to hear what you all this of this topic! Have you any experience with this yeast? Even if you havenâ€™t, please feel free to comment!