The Science of Flor Yeast

Best known for its role in sherry and vin jaune, flor imparts unique structure and flavor to wine. What conditions create a veil of flor—and how, exactly, does it impact wine style?

View from inside a wine barrel
When conditions are right for a veil of flor to spontaneously emerge on the surface of wine after fermentation has completed, it imparts a lasting influence on wine character. Photo courtesy of Bodegas Alvear.

Maturation under a veil of flor yeast wields significant influence over the ultimate character of multiple wines produced across different regions worldwide. Indeed, while traditionally associated with the Sherry region, flor’s influence extends well beyond Spain. It is used in the production of Vernaccia di Oristano in Italy’s Sardinia, vin jaune in France’s Jura, and Tokaji Szamorodni in Hungary. Spain itself also employs it in Montilla-Moriles, and several Australian wineries use flor for the production of Apera (formerly known as “Australian sherry” and renamed following an agreement with Spain over a decade ago).

Although they originate from distinct regions, the common presence of flor yeast imparts similar characteristics to these wines, affecting both their flavor profiles and textures. But what exactly is flor yeast, how is it utilized in the winemaking process, and what precise influence does it exert on the liquid?

What Is Flor, Exactly?

Flor is a unique strain of Saccharomyces cerevisiae—the most commonly used yeast in winemaking—that may emerge spontaneously on the surface of wines once all the sugars have been converted into alcohol.

“When the alcoholic fermentation has finished … [the yeast] rises to the surface because the cell membrane has become hydrophobic and tends to float due to the difference in density,” explains Roberto Puggioni, the enologist of Cantina della Vernaccia in Sardinia. “Its metabolism involves obtaining carbon from ethanol, a process that generates acetaldehyde, acetic acid, and other compounds as byproducts.”

The majority of flor yeast strains found in Spain, Italy, France, and Hungary share a common genetic group involving four main variants of Saccharomyces cerevisiae: beticus, cheresiensis, rouxii, and montuliensis. “These four major families exhibit varying behaviors and levels of activity, not only between different bodegas but also within different sections of a single bodega,” says Antonio Flores, the winemaker and master blender at Spain’s Gonzalez Byass. Montuliensis, for instance, is associated with acetaldehyde production, a very volatile molecule that gives flor wine characteristic appley aromas.

In addition to these four Saccharomyces variants, a recent study revealed that certain flor yeast populations may also encompass additional strains belonging to the pichia, candida, and trichosporon species.

The Specific Growing Conditions Required for Flor

To enable the development and thriving of flor yeast, the cellar environment should maintain high humidity levels, typically exceeding 70 percent, while optimal temperatures range between 64.4 to 75.2 degrees Fahrenheit. Furthermore, while conventional Saccharomyces cerevisiae develops ethanol, flor feeds on it and therefore requires a certain amount of alcohol dissolved in the liquid to survive. According to Victoria Frutos, the winemaker at Grupo Estevez in Jerez, this ranges between 15% and 15.5% ABV, yet slightly lower levels may be required in other regions. Puggioni in Sardinia claims that 14% ABV is sufficient for flor development, while Antoine Zbyrko, the enologist at the Laboratoire Départemental d’Analyses du Jura, suggests a minimum of 13% ABV.

For sherry, the necessary alcohol content is reached by adding grape spirit to the base wine. Other flor styles, however—including Vernaccia di Oristano and vin jaune—are not fortified, a practice that’s gradually being embraced in the Sherry region too.

Victoria Frutos takes a sample
Grupo Estevez winemaker Victoria Frutos, pictured above, pours a sample of sherry aged under flor. Photo courtesy of Grupo Estevez.

By turning ethanol into acetaldehyde, flor yeast can reduce the amount of alcohol in the wine. As a result, some sherry producers ensure that finos and manzanillas maintain the minimum alcohol levels of 15% ABV by calculating the potential alcohol loss at the outset of the maturation process. “We first fortify the wine to 15.4%,” Flores explains. “We know that it will lose 0.1% ABV every year [under flor], so after four years of aging, our Tio Pepe will be exactly 15% ABV.”

In Jerez, where the maturation of finos occurs over a minimum of four years, flor yeast requires a continuous supply of nutrients and oxygen, achieved through regular additions of young wine to the solera system. This ongoing supply of nutrients keeps the flor alive, eliminating the necessity for frequent yeast reinoculation.

“We effectively store the flor yeast in the casks themselves, across different buildings,” says Barbadillo international ambassador Tim Holt. “But in the Barossa Valley, for instance, Seppeltsfield cultivates the yeast they use on their fino in a lab, and keep inoculating.”

In regions where the use of flor yeast is less traditional, some winemakers rely on the natural mutation of regular Saccharomyces cerevisiae into flor with each new vintage, as in the case of South African winemakers John Bouwer of Gedeelte Wines and Chris Groenewald of Smiley. “We rely on the flor to grow naturally (without inoculating it with yeast) so there will always be the risk that we don’t have it in a specific year,” says Groenewald.

Even in the Sherry region, there are occasions when it becomes necessary to deliberately inoculate the yeast, despite its natural presence in the winery environment. “Whenever a barrel [bota] presents an issue, for instance a Brettanomyces contamination, we empty it, sterilize it, and inoculate the flor,” says Flores.

The inoculation involves harvesting some flor from the closest bota—“that’s best adapted to that environment,” he says—mixing it with fino, and spraying the mixture over the surface of the wine. While Flores sources flor from neighboring casks, Bernardo Lucena, the enologist at Bodegas Alvear in Montilla-Moriles, relies on selected barrels instead.

“We have a selection of the ‘finer’ casks that house the most valuable yeast,” he says. “These are used to propagate it. There are botas that deviate from the style [of fino we like to retain]: these butts are set aside and dedicated to other types of non-fine wines.”

Barrels at Bodegas Alvear
Not only does flor impact the flavor of wines, but it also impacts the texture and structure. Photo courtesy of Bodegas Alvear.

How Flor Aging Impacts Wine Style

The aging of wine under a veil of flor has a profound influence on the wine’s character. The yeast operates within an anaerobic environment, simultaneously consuming oxygen and preventing it from entering the wine, which maintains a pale yellow color even after several years of maturation. The wine’s bouquet is often characterized by distinctive green apple notes resulting from acetaldehyde production, complemented by further aromas ranging from almonds and other nuts, to chamomile and spices, bread crust and herbs, as well as saline and briny nuances.

The texture of the wine is also notably affected. Flor’s metabolic activity often includes the consumption of glycerol. At Gonzalez Byass, for instance, flor reduces Tio Pepe’s glycerol levels from six grams per liter to less than one gram per liter.

This transformation enhances the wine’s dry sensation, yet it also results in a significant reduction in its body. To counter this, winemakers employ the process of autolysis, which benefits the wine with volume and texture. “If the wine has undergone a long biological aging process,” says Frutos, “some of the yeast cells settle to the bottom of the cask. As they die and their cell walls open, they generate a buttery sensation and volume on the palate.”

With such a wide-ranging impact on wines’ character, from texture to flavor, some winemakers understand flor maturation as an additional facet of terroir expression in their wines. As Fermin Hidalgo Garcia, the owner of sherry bodega Hidalgo La Gitana, puts it: “First we have the ‘classic’ terroir we all know: the climate, the soil, etc. The ‘second terroir’ is related to the wineries … [Aging under flor] is part of our terroir.”


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Dr. Jacopo Mazzeo is a U.K.-based freelance drinks journalist, consultant, and photographer. He contributes to leading trade and consumer publications including Decanter, Wine Enthusiast, Whisky Magazine, and Good Beer Hunting. Jacopo consults on consumer trends and marketing strategies, is a former sommelier, and judges international wine, beer, and spirits competitions. Before he embraced full-time journalism, he studied musicology at the University of Bologna and took a PhD at the University of Southampton. Follow Jacopo on Instagram @jacopomazzeophoto

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