Wine aging is a crucial element that deepens the wine's flavor profile. While wines generally evolve through oxidative aging, an alternative method called "biological aging" is sometimes employed during the winemaking process.
Biological aging is a maturation technique that utilizes film-forming yeasts. These yeasts create a film on the surface of wine stored in barrels or tanks, imparting distinctive flavors and giving wines new characteristics.
Wines with distinctive flavor profiles, such as Spanish Sherry and French Vin Jaune, acquire their unique characteristics through the use of these film-forming yeasts. Among the film-forming yeasts used in these wines, "flor" (flor yeast or flor velum yeast) plays a central role in biological aging.
This article explores the ecology of flor yeast and its impact on wine.
Note: Some film-forming yeasts can negatively affect wine quality. These will be covered in detail in a separate article.
Understanding Film-Forming Yeasts: Velum and Flor
"Film-forming yeasts" refer to yeasts that form white films on the surface of wine stored in wooden barrels or tanks. This film is called "flor" or "velum." In this article, we will refer to the film-forming yeasts used in Sherry and Vin Jaune production as "flor yeast" and the film itself as "velum."
Many yeasts actually possess the characteristic of forming velum at the gas-liquid interface. This includes Saccharomyces cerevisiae, widely used in alcoholic fermentation of fermented beverages, as well as strains found in non-Saccharomyces yeasts.
Among these, all strains of flor yeast used in Sherry and Vin Jaune production belong to Saccharomyces cerevisiae. In other words, flor yeast is a collective term for Saccharomyces cerevisiae strains that possess velum-forming characteristics.
Winemaking techniques using film-forming yeasts are not particularly common. Therefore, flor yeast is often perceived as some special type of yeast. However, flor yeast is essentially the same species as Saccharomyces cerevisiae, the yeast essential for converting grape juice into wine, albeit with some differences.
Genetic Analysis Reveals the True Nature of Flor Yeast
While flor yeast is essential for producing Sherry and Vin Jaune, the exact details of how it enters the winemaking process are not yet fully understood.
Since flor yeast is the same species as Saccharomyces cerevisiae used in alcoholic fermentation, it is sometimes explained that yeasts surviving after alcoholic fermentation become flor yeasts. However, this possibility is currently considered unlikely.
Identification Through the FLO11 Gene
Flor yeast and regular Saccharomyces yeasts used in alcoholic fermentation can be easily distinguished by their genotype. While flor yeast comprises multiple yeast strains, what groups them together as flor is a gene called FLO11. Essentially, Saccharomyces cerevisiae strains possessing this unique gene are collectively called flor.
Flor yeast always carries FLO11 in its genotype. Therefore, at any point during wine production, genetic testing of the yeast can determine whether flor yeast is present.
Previous verification results show that yeasts carrying FLO11 in their genotype have not been detected during alcoholic fermentation or immediately after fermentation. Flor yeast presence has only been confirmed after wine is transferred to barrels for aging.
This suggests that flor yeast is not a continuous transition from Saccharomyces cerevisiae involved in regular alcoholic fermentation, but rather likely enters the wine through external factors, including the barrels used for aging.
Complex Factors in Gene Expression
Conversely, FLO11 expression involves complex interacting elements. For instance, fermentable sugars like glucose are among the factors that suppress this gene's expression. FLO11 expression requires multiple other conditions, and the possibility that this gene is expressed following the diauxic shift—the transition from glycolytic to respiratory energy acquisition—has not been completely ruled out.
Even if yeasts involved in alcoholic fermentation transition to flor through diauxic shift, not all Saccharomyces cerevisiae strains carry FLO11. One verification case reported that approximately 50% of strains carried this gene.
Velum Formation and Flor Yeast Buoyancy
Why does velum form at the gas-liquid interface, namely the liquid surface? The answer is simple: flor yeast floats. However, the mechanism behind why flor yeast floats is somewhat complex.
Flor yeast doesn't float because of low cell density. Again, flor yeast is essentially Saccharomyces yeast. Normally, these Saccharomyces yeasts present in large quantities during alcoholic fermentation don't float to the surface and form films. They are only lifted by vigorously rising fermentation gases. Once fermentation settles, these yeasts quickly sink. Buoyancy is a characteristic unique to flor yeast among Saccharomyces yeasts.
Buoyancy Mechanism Through Hydrophobicity and Aggregate Formation
The buoyancy characteristic of flor yeast stems from FLO11, the gene that makes flor yeast what it is. FLO11 expression increases the hydrophobicity of the yeast cell surface. Simultaneously, adhesive-like components that bind cells together are produced, promoting multicellular aggregate formation.
Carbon dioxide gas generated by alcoholic fermentation of residual sugars becomes trapped within these aggregates, providing buoyancy that allows them to float to the gas-liquid interface and further connect to form velum.
The buoyancy of flor yeast—more specifically, the underlying hydrophobicity strength—is not constant and varies according to each flor yeast's genotype and wine conditions. Flor yeasts with stronger cell surface hydrophobicity, and thus greater buoyancy, tend to form thicker velum.
Biological Aging: Wine Transformation by Flor Yeast
Biological aging using flor yeast occupies a special position as a winemaking technique. Why employ such a specialized method?
Historical origins aside, now that wine styles using film-forming yeasts are established as a genre, the goal is the distinctive flavors and aromas obtained through flor yeast metabolism.
Balancing Oxidation Prevention and Flor Yeast Metabolism
In wines where flor yeast forms velum at the gas-liquid interface, flor yeast performs oxidative metabolism while the wine itself remains protected from oxidation.
For example, browning—the change to brownish color that serves as a clear oxidation marker in wine—has been reported not to occur in wines with formed velum. Thus, while protected from the oxidation that wine most abhors, the wine deepens its aging through flor yeast metabolism.
General Effects on Wine
The degree of flor yeast impact on wine varies greatly depending on the flor yeast strain type and the condition of the wine where velum forms. However, the following changes are consistently confirmed effects of flor yeast on wine:
- Increased acetaldehyde from ethanol oxidation
- Decreased glycerol, acetic acid, and amino acids
These various effects alter wine's nuances, particularly aroma and taste, bringing distinctive characteristics to Sherry and Vin Jaune.
Wine Composition Changes Through Biological Aging: Balance of Metabolism and Production
Biological aging brings various changes to wine composition. In wines with formed velum, component changes are broadly classified into three directions:
- Components that decrease through metabolism
- Components that increase through production
- Components unaffected, with unchanged content
Components Decreased by Metabolism
Ethanol (alcohol) is one of the representative components whose content decreases through biological aging. Extended biological aging gradually reduces wine's alcohol content. In Sherry's case, the minimum alcohol content is set at 15% by denomination of origin regulations, so biological aging lengthy enough to fall below this level is generally not practiced.
Glycerol is another substance that significantly decreases in content through biological aging. It is metabolized more than ethanol. Glycerol is a viscous, sweet component produced by yeast metabolism during alcoholic fermentation in wine. Essentially, what yeast produces during fermentation is consumed by yeast during aging. Flor yeast's glycerol metabolism may bring changes to wine's taste, viscosity, and mouthfeel.
Acetic acid may be present in wine through acetic acid bacteria contamination from rotten grapes or other microbial influences. Yeast also produces small amounts of acetic acid during alcoholic fermentation.
Acetic acid is one of the negative elements for wine quality, with legal upper limits in some countries and regions. Generally, acetic acid contained in wine cannot be removed by conventional methods. If contained in quantities that violate legal regulations, entire tanks may need to be discarded.
Acetic acid wine contamination can occur during aging after alcoholic fermentation. However, since flor yeast can metabolize acetic acid, aging with velum can keep acetic acid content low.
Components Increased by Production
While alcohol, glycerol, and acetic acid decrease through flor yeast metabolism, acetaldehyde, acetoin, and multiple higher alcohols including acetic esters increase through production.
Particularly, acetaldehyde production—one of the components characterizing aging aroma and oxidation aroma—tends to be very high. Typical biologically aged wines contain 3-4 times the pre-aging amount, with reports of 7-8 times in extreme cases.
Challenges in Aging Using Flor Yeast
Aging using flor yeast not only brings distinctive nuances to wine but also provides benefits for wineries, such as reducing certain wine components. However, several challenges should be recognized.
Unpredictable Acetaldehyde Production
Aging with flor yeast tends toward high acetaldehyde production. Acetaldehyde is a compound produced when alcohol breaks down. It's also produced in the body after alcohol consumption, but lacking the enzyme to break down this acetaldehyde can cause hangovers and headaches.
While acetaldehyde shows toxicity to humans, predicting how much will be produced during biological aging is difficult. This applies not only to acetaldehyde but to all substances involved in flor yeast metabolism.
Non-uniform Flor Yeast Behavior and Reproducibility Challenges
With Saccharomyces yeasts used in alcoholic fermentation, using pre-selected dried yeast allows some prediction of fermentation behavior and products. However, flor yeast typically doesn't use such pre-selected yeasts, making it unclear which strains comprise the velum-forming yeasts.
Furthermore, reports show that even the same strain often exhibits different morphological velum characteristics. This means even the same flor yeast strain may show completely different behavior.
Such high uncertainty makes achieving reproducibility in flor yeast aging difficult. Sherry's solera system functions as a means to resolve this quality instability by repeatedly blending contents from multiple barrels.
Velum Formation Instability
Flor yeast proliferation occurs under extremely harsh conditions for microbial survival. Therefore, some strains may fail to form velum.
In biological aging, wine is protected from oxidation because velum covers its surface. Without velum formation, wine loses oxidation protection and may suffer critical quality damage.
While aging with flor yeast certainly has benefits, there are also drawbacks that make it difficult to approach casually.
Expanding Use of Film-Forming Yeasts
New Application Trends
Traditionally, actively utilizing film-forming yeasts in winemaking was uncommon. Sherry and Vin Jaune, which actively use flor yeast, are exceptional cases overall.
However, recently, cases incorporating film-forming yeasts into winemaking are increasing. Even in regions that previously didn't use film-forming yeasts, some producers are adopting this technique for regular still wine aging.
Most new film-forming yeast aging initiatives proceed within the natural wine framework. The approach seeks to reduce sulfite additions by generating film-forming yeasts during wine aging to protect wine from oxidation while enhancing microbial stability. This differs somewhat from traditional flor yeast applications.
Challenges and Considerations in New Implementation
The idea of applying film-forming yeasts for wine stabilization has merit. However, there are aspects that cannot be easily accepted. The primary reason is the unclear nature of yeasts forming velum.
Even traditional regions like Sherry and Vin Jaune, with long histories of flor yeast use, don't know exactly where flor yeast enters the winemaking process. Furthermore, film-forming entities include microorganisms classified as contaminants, different from flor yeast. Velum formation doesn't guarantee flor yeast presence.
Under these circumstances, suddenly introducing film-forming yeasts where flor yeast wasn't previously used raises questions about whether it's actually flor yeast.
Even if contaminants form velum, wine might be protected from oxidation. However, if such bacteria begin undesirable metabolism, wine quality would deteriorate significantly.
Current Technology Development and Future Prospects
Research continues on technologies to reliably transfer strains with known characteristics to other locations, like dried yeast, in the flor yeast field. Flor yeast itself shows behavioral non-uniformity and unstable results. Nevertheless, using strains with known origins and characteristics is expected to at least reduce situations where contaminants form velum while allowing fatal metabolic activities.
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