Wine Character Ruined! Understanding Film-Forming Yeasts and Their Control

Have you ever encountered a red wine that tasted oddly flat yet somehow spicy, with aromas reminiscent of dust, mold, or sulfur compounds? Such wines may have been affected by film-forming yeasts during their aging period.

While the term "film-forming yeast" is not commonly heard, some readers may be familiar with flor, which is utilized in sherry production. This article provides a comprehensive examination of film-forming yeasts.

Film-forming yeasts represent a fundamental winemaking issue that is relatively easy to prevent. Consequently, consumers rarely need to be concerned about them. However, there appears to be an increasing frequency of encounters with wines exhibiting such characteristics in recent years. While this knowledge is essential for winemakers, it also serves as valuable information for wine enthusiasts seeking to enhance their appreciation of wine.

What Are Film-Forming Yeasts?

Film-forming yeasts are, as the name suggests, yeast groups that create film-like structures on the surface of their proliferation targets. In wine, when these yeasts proliferate, they appear as a white film on the liquid surface.

Note: The color of the film varies depending on the yeast proliferation state. A white film indicates an early stage of proliferation and may change color over time.

However, this is not an actual film but rather a phenomenon where large numbers of yeasts proliferating on the liquid surface become entangled, creating a film-like appearance. These yeasts have an elongated oval shape and may appear thread-like in appearance. When such shaped yeasts become entangled, they create what appears to be a film on the surface.

"Film-forming yeast" does not refer to a single yeast species but is a collective term for multiple yeast groups possessing the aforementioned proliferation characteristics.

Relationship with Sherry Film-Forming Yeasts

The most famous example of film-forming yeasts is the utilization of flor in the production of sherry, a fortified wine produced in the Andalusia region of Spain.

Note: "Flor" strictly refers to the film state formed by film-forming yeasts and does not denote the yeasts themselves.

In sherry production, flor is utilized to prevent excessive oxidation of the wine while acquiring distinctive flavors. This alone might suggest that film-forming yeasts are beneficial.

However, there is a crucial pitfall here. The classification "film-forming yeast" includes multiple different yeasts, and the film-forming yeasts used in sherry production are fundamentally different species from those that typically occur in wine. More precisely, while the film-forming yeast groups that occur in wine do include the types used in sherry production, these represent only a small fraction.

Therefore, we specialists rarely use the term "film-forming yeast" to encompass all film-forming type yeasts. This is because we need to distinguish between beneficial sherry yeasts and harmful yeasts. In this article, we will specifically distinguish the yeasts used in sherry production as "sherry yeasts."

Differences Between Sherry Yeasts and Common Film-Forming Yeasts

The major film-forming yeasts that negatively affect wine include:

• Pichia membranifaciens
• Pichia fermentas
• Candida vini
• Candida zeylanoides
• Issatchenkia orientalis

In contrast, sherry yeasts belong to the Saccharomyces lineage, representing a completely different taxonomic group. In sherry production, alcohol content is increased to limit yeast proliferation conditions, selectively utilizing only beneficial yeasts. Conversely, in typical wines, multiple yeasts like those listed above proliferate chaotically, each exerting different detrimental effects on the wine.

Causes of Film-Forming Yeast Development

The cause of film-forming yeast development is straightforward and singular: the presence of oxygen.

When aging wine in barrels, evaporation and sampling cause the liquid level to drop, creating an air space (headspace) between the wine and barrel, allowing these yeasts to proliferate.

While typical microorganisms proliferate using sugars as a nutrient source, and complete fermentation of dry wines can often prevent such proliferation to some extent, the characteristic of film-forming yeast groups is their ability to proliferate even in sugar-free environments. This means that even dry wines fermented without residual sugar can suffer significant damage when the liquid surface is in contact with oxygen.

When considering microbial proliferation control, SO₂ (sulfurous acid or sulfur dioxide) addition comes to mind. However, when the liquid surface is in contact with oxygen, it means the wine is constantly in an oxidizing environment, where free SO₂ is consumed and sufficient quantities cannot be maintained. Under such circumstances, the microbial suppression effect of SO₂ cannot be adequately expected, making prevention of film-forming yeast development nearly impossible.

Furthermore, many film-forming yeasts possess resistance to sulfur dioxide.

Conversely, since film-forming yeast groups are aerobic yeasts, they cannot proliferate in oxygen-free environments regardless of sulfurous acid addition.

Beware of Misinformation on the Internet

Some online articles incorrectly state that film-forming yeasts exist as anaerobic yeasts during alcoholic fermentation and then become aerobic. This is incorrect.

Film-forming yeasts do possess alcoholic fermentation capability (though not high). Therefore, when present in juice before alcoholic fermentation, they may participate in alcohol production alongside Saccharomyces-type yeasts.

However, even in such cases, film-forming yeasts become inactive after alcoholic fermentation and no longer cause detrimental effects as aerobic yeasts. In other words, they become harmless even when present in wine.

Effects of Film-Forming Yeasts on Wine

Film-forming yeasts comprise multiple yeast groups, each exerting strictly different effects. However, the following effects are commonly observed:

• Production of ester compounds
• Decomposition of alcohol
• Decomposition of glycerol and organic acids

These factors precisely determine why film-forming yeasts are detrimental to wine.

Masking of Grape Varietal Characteristics

Film-forming yeasts produce ester compounds including ethyl acetate and amyl acetate, which possess aromas similar to banana and apple. These aromas mask the inherent grape variety-specific aromas and flavors, destroying the wine's individuality.

Additionally, they may produce sulfur odors resembling rotten eggs.

Beyond ester compounds, they also affect flavor through the production of certain higher alcohols and impart oxidative and thin nuances to wine.

While sherry yeasts are expected to protect wine from oxidation, film-forming yeasts in typical wines provide no such beneficial effects.

This series of effects dramatically alters the overall impression of the wine.

Alcohol Decomposition and Increased By-products

One major detriment caused by film-forming yeasts is alcohol decomposition.

While alcohol decomposition by film-forming yeasts occurs primarily in the area directly beneath the film where yeasts contact the wine, which might appear to have limited overall impact, the effects should never be underestimated.

Research has reported that Pichia ferinosa-type yeasts decomposed 50g/L of alcohol over three months, while Candida-type yeasts decomposed an astonishing 70g/L of alcohol over the same period.

Additionally, alcohol decomposition produces acetaldehyde, acetic acid, and ester compounds.

Acetaldehyde possesses strong toxicity and is a substance that consumes large amounts of free SO₂. Increased content of this substance directly adversely affects wine quality.

Acetic acid also becomes a target for decomposition by film-forming yeasts following alcohol.

Wine Flattening Due to Reduced Extract Content

The effects of film-forming yeasts extend to the extract content in wine.

Using the same research example, wines with Candida-type yeast films showed approximately 10g/L reduction in extract content over three months, while Pichia-type and Hansenula-type yeasts showed approximately 10g/L reduction over nine months.

The reduced components included glycerol as well as organic acids such as malic acid, citric acid, and lactic acid.

This reduction in extract content renders wine flavor and impression flat. Combined with the previously mentioned strong aromas of ester compounds, the overall impression of the wine changes dramatically.

Film-Forming Yeast Control Methods

Prevention Measures

Film-forming yeasts can be easily prevented by constantly keeping wine storage containers full of wine, avoiding air space creation.

Since film-forming yeasts are aerobic yeasts, their existence absolutely requires oxygen. Therefore, they rarely exist in liquid except at the surface, meaning that the presence or absence of sulfurous acid addition does not carry significant importance in this context.

Note: This does not mean SO₂ lacks film-forming yeast development suppression effects; therefore, adding sulfur dioxide as insurance is advisable.

Simply checking wine condition regularly and periodically replenishing amounts removed for sampling or lost to evaporation can avoid most damage from these yeasts.

Other prevention methods include:

• Maintaining high alcohol content in wine
• Storing wine in cool, dark places

While there are no specific temperature standards, when stored in cool locations, alcohol content of approximately 10% vol. can considerably suppress proliferation of these yeasts, and 12% vol. or higher can completely prevent proliferation.

Conversely, in warm environments, proliferation has been confirmed even at 14% vol. alcohol content.

Therefore, when aging wine in barrels, it is crucial to place wine in the lowest possible temperature environment. This also helps suppress natural wine evaporation to some extent, ensuring safety in a dual sense.

Attention should be paid when storing barrels stacked in multiple tiers. Barrels closer to the ceiling will have higher ambient temperatures than those near the floor. Therefore, even when managing wine aging cellars at low temperatures, barrels stacked in upper positions may be stored under higher temperature conditions than anticipated, potentially resulting in wine surfaces covered with white films before detection.

Treatment When Film Formation Occurs

If film formation is discovered on wine surfaces, rather than adding additional sulfurous acid, first replenish wine in that container.

When replenishing wine, do not stop at filling to the container's mouth but allow overflow until all formed film flows out. Since it is better not to leave portions near the liquid surface affected by yeasts, while losses may be greater, it is safer from a wine quality perspective.

After removing yeasts mixed into wine in this manner, replenish free SO₂ that is presumed to have decreased due to yeast effects or oxidation. Failure to do this may result in effects from microorganisms other than film-forming yeasts, requiring careful attention.

Finally, securely cork the barrel, wash the surface with water, and clean away overflowed wine.

Conclusion: Protecting Wine Quality

Film-forming yeasts represent relatively easily prevented and treated issues, but when neglected, they cause serious effects on wine. While not highly frequent, wines that appear to have been affected by these yeasts are occasionally encountered.

Since preventing film-forming yeasts does not require essential use of sulfur dioxide, with proper knowledge, damage can be avoided even when producing wines that avoid SO₂ addition.

Despite this, the appearance of wines in the market showing signs of damage from these yeasts represents nothing other than simple negligence by winemakers. Moreover, equating them with sherry yeasts and tolerating contamination of wine with these yeasts is absolutely inexcusable.

Consumers should also question the reliability of producers when encountering wines that appear to show effects of these yeasts.

Film-forming yeasts do not suddenly form films across entire wine surfaces at once. They begin as small ring-like formations that gradually enlarge and eventually become films. Moreover, this progression occurs in a visually observable state.

Therefore, reaching a state of significant effects from these yeasts means nothing other than leaving that condition unattended for a considerable period. This is not a category of accidental mistake.

Recently, many winemakers pursuing uniqueness in wine have moved in directions that could be called eccentric. Among such trends, examples of incorporating Brettanomyces or film-forming yeasts as winemaking techniques have emerged.

Ultimately, if consumers affirm the flavors derived from such practices, whatever they may be could be considered correct. However, it is essential to properly understand what "that" actually is.

Supplement: Aerobic Fermentation Techniques

Some Pichia-type yeasts included in the film-forming yeasts discussed in this article are also being explored for use in aerobic fermentation methods.

Since conventional alcoholic fermentation is conducted anaerobically with Saccharomyces-type yeasts, this aerobic fermentation is said to yield different flavors and nuances from conventional methods.

However, this applies only to fermentation and not to aging. In other words, this represents a completely separate endeavor with entirely different purposes and applications from the content explained thus far.

While this demonstrates that film-forming yeasts may not be entirely without utility value, it is extremely important to clearly distinguish and understand the timing and purposes for which they are intended.