Sherry Fortification and Flor Yeast: The Biological Aging System

Sherry fortification is the addition of grape-derived wine spirit to finished dry white wine, raising its alcohol to one of two target levels that determine its entire aging destiny. The Consejo Regulador describes wine alcohol, produced by distilling wine, being added to the base wine once alcoholic fermentation has finished. Towards the end of December, after racking, tasters classify each batch of new wine: pale, fine wines, generally from must obtained with little or no pressure, are marked with a single slash, the palo, for ageing as finos or manzanillas, while batches with more structure, or gordura, are marked with a circle for oloroso. The first group is fortified to 15 per cent by volume, a strength the Consejo says allows the yeast cells to form the flor while being intolerable for other micro-organisms; the second is fortified to at least 17%. Above 17 per cent by volume, the Consejo says, biological activity becomes impossible to sustain, and the wine, losing its flor and its protection, begins a slow oxidation seen in its gradually darkening colour. Fortification began, the Consejo notes, as a way to stabilise wines for long overseas voyages, and the DO's pliego de condiciones adds that the practice played a decisive role in the appearance, by natural selection, of the flor yeasts behind biological ageing.

• The pliego de condiciones (consolidated 2026) requires fortification with alcohol of wine origin: neutral alcohol of wine origin of at least 96% vol, or wine or raisin alcohol of 95% to 96% vol
• César Saldaña of the Consejo Regulador wrote in 2019 that fermented Palomino musts reach between 11 and 12.5 degrees of alcohol, too high for normal fermentative yeasts, after which flor yeasts combine ethanol with oxygen to create acetaldehyde
• For Fino, the pliego also allows fortification during ageing, to raise the alcoholic strength when the action of the flor yeasts lowers it

Biological ageing follows a complete alcoholic fermentation of the must; the Consejo Regulador describes flor yeasts as carrying on their activity even when all the fermentable sugars in the must have been exhausted, feeding off the alcohol created during fermentation. Flor develops spontaneously from Saccharomyces cerevisiae strains naturally present in the wine and bodega environment, rising to the surface and forming a dense white biofilm that protects the wine from direct oxygen contact. Saldaña wrote in 2019 that small floating islets appear a few weeks after fermentation ends and that by around October the whole surface of the wine is covered. In 2020 he added that each of the four strains identified in the 1970s, beticus, cheresiensis, montuliensis and rouxii, differs in its capacity to produce acetaldehyde or consume acetic acid, which underlies differences between Jerez, El Puerto de Santa María and Sanlúcar. Flor viability depends on a continuous supply of nutrients, which is provided by the solera and criaderas system: a dynamic, fractional blending process in which wine is periodically drawn from the oldest tier for bottling and replaced with younger wine cascading down from successive criaderas above. The Consejo explains that small quantities of young wine reaching the oldest scales supply the micro-nutrients the flor needs, and that each transfer dissolves some oxygen, stimulating regeneration of the veil.

• The pliego calculates the average age of a criaderas and solera system as t = Vt/Ve, the total volume in the system divided by the volume extracted for sale in a year; the Consejo Regulador requires this ratio to be greater than two
• Flor acquires a particularly vigorous aspect in spring and autumn, times of the year the Consejo Regulador says coincide with ideal conditions of temperature and humidity; Saldaña adds that in the oldest scales the flor turns darker and thinner and can sit slightly submerged at certain times of the year
• Dead flor yeasts fall to the bottom of the butt as a sediment called cabezuelas, which, Saldaña writes, release vitamins, amino acids, proteins and enzymes as they decompose

Under flor, the Consejo Regulador explains, the yeast consumes alcohol, glycerine and volatile acidity and brings about a substantial increase in acetaldehydes, which are responsible for the sharp sensation on the nose of biologically aged wines. Saldaña notes that colour is the one aspect that remains largely unaltered, thanks to the veil's protection from oxidation. The Consejo's own figures show the difference: volatile acidity below 0.25 grams per litre for Fino and Manzanilla, against below 0.8 grams per litre for Amontillado, Oloroso and Palo Cortado. Oloroso, fortified to 17% by volume from the start, ages in constant contact with oxygen, and the gradual loss of water through the cask walls concentrates it, which is why, the Consejo notes, Olorosos sometimes reach even above 20% alcohol. Amontillado begins under flor, as Fino and Manzanilla do; at a given moment the flor begins to disappear, giving way to oxidative ageing that gradually darkens the wine. Palo Cortado comes from very fine must initially fortified to 15% by volume: after the sobretablas stage, tasters mark casks in which the flor has been maintained and which show very specific characteristics, and the wine is fortified again to above 17% by volume, redirecting it to oxidative ageing. The Consejo describes the result as combining the delicate bouquet of an Amontillado with the body and palate of an Oloroso.

• Butts lose 3 to 5 percent of their wine each year through evaporation, known as merma; because the loss is mostly water, the Consejo Regulador says, the remaining components are concentrated
• The sobretablas stage lasts between six months and a year, after which tasters make a second classification butt by butt, chalking the palma symbol on butts where the flor is still present and flourishing
• Where the flor has weakened or disappeared after sobretablas, the Consejo Regulador describes fortifying the wine to 17 per cent by volume so that oxidative ageing gives rise to oloroso

The pliego de condiciones of the DOP Jerez-Xérès-Sherry, consolidated under an Andalusian order of 13 May 2026, sets a minimum average ageing of two years for its vinos generosos and defines them by ageing: Fino is exclusively biological, Amontillado combines a first biological phase with an oxidative one, Palo Cortado is predominantly oxidative after the disappearance of the initial veil of flor, and Oloroso is exclusively oxidative after that veil disappears. Ageing takes place in oak butts of no more than 1,000 litres, using either the criaderas and solera system or the static añada system. The pliego lets Fino be aged in any town of the delimited zone except Sanlúcar de Barrameda, while the Consejo Regulador states that Manzanilla can only be aged in bodegas in Sanlúcar, which is why it is a Denomination of Origin in its own right, Manzanilla-Sanlúcar de Barrameda, sharing the production zone and the Consejo Regulador with Jerez. The V.O.S. and V.O.R.S. categories, created by the Consejo Regulador in 2000, certify individual sacas with an average age of over 20 and over 30 years; they are open only to Amontillado, Oloroso, Palo Cortado and Pedro Ximénez, and the Consejo explains that biological ageing cannot be prolonged excessively, because after an average of seven or eight years the flor yeast grows weak.

• Pliego analytical limits (consolidated 2026): Fino 14% to 17% vol; Amontillado 16% to 22%; Oloroso and Palo Cortado 17% to 22%; all with no more than 4 g/L of sugar
• V.O.S. and V.O.R.S. wines must pass an independent Tasting Committee whose members have no connection with sherry firms; for every litre of over-20-year-old sherry sold, at least 20 litres must remain in the ageing system, and at least 30 litres for over-30
• The pliego restricts the label terms Viejo or Old to biologically aged wines of 7 years or more and other wines of 12 years or more, and Fino Viejo requires an average ageing of at least seven years

González Byass's own site says Manuel María González Ángel entered the wine business in 1835, aged 23, guided by his uncle José María Ángel y Vargas, the Tío Pepe of the brand, and that in 1844 he wrote to his agent in Great Britain, the English wine merchant Robert Blake Byass, requesting butts of the 'exceptionally pale wine that you recommend so highly'. The house describes Tío Pepe as 100% Palomino Fino, biologically aged for at least four years in American oak following the solera system, at 15% vol. Bodegas Hidalgo La Gitana in Sanlúcar de Barrameda was founded in 1792 by Pantaleón José Hidalgo and is run by the eighth generation of the family, according to the bodega; its Manzanilla La Gitana spends three years in the Bodega San Fermín in Sanlúcar's lower quarter, then two to three more in the San Luís bodega, five to six years in all through criaderas and soleras under flor. Lustau traces its origins to 1896, when José Ruiz-Berdejo began making wines sold on to larger houses as an almacenista; its first Almacenista bottlings appeared in 1981, and the company merged with the Luis Caballero group in 1990, according to Lustau.

• González Byass says the support of José María Ángel y Vargas was crucial to the creation of the house's first Fino solera, which the founder named the Solera del Tío Pepe
• Hidalgo says its La Gitana soleras date from 1792 and that the grapes come from its Finca El Cuadrado in the pago of Balbaína Alta
• Lustau's Almacenista line currently includes eight wines aged by five producers, according to Lustau, bottled under the almacenista's name, as in Lustau Almacenista Manuel Cuevas Jurado Manzanilla Pasada de Sanlúcar

The pliego de condiciones describes albariza as a soft white marl with a great capacity to retain moisture, its calcium carbonate normally at least 25% and up to 40%, alongside clay and silica from diatom and radiolarian shells; the Consejo Regulador's viticulture page gives the same upper figure of up to forty percent. Two prevailing winds shape the region: the Consejo describes the westerly Poniente as cool and humid, with humidity reaching ninety-five percent, and the south-easterly Levante as hot and dry, at around thirty percent. The pliego singles out the Poniente's moderating influence as especially relevant to biological ageing, which needs moderate temperatures and high humidity inside the bodega. Bodegas are oriented to let moist Atlantic breezes in while blocking the dry levante, with walls usually at least 60 centimetres thick and floors of albero earth that can be sprinkled with water to regulate temperature and humidity. In Sanlúcar, the Consejo explains, the Guadalquivir, the Atlantic and the marisma produce milder temperatures and higher humidity than in the rest of the region, fostering a flor peculiar to the town. Saldaña wrote in 2020 that the proportions of flor strains depend on each town's climate and each bodega's microclimate, and that biologically aged wines differ even between butts a few metres apart in the same bodega, which some writers have called the 'terroir of the bodega'.

• Sarah Jane Evans MW wrote in Decanter that Sanlúcar shows the maritime influence most strongly, that Jerez finos are bolder and more structured, and that El Puerto lies between the two
• The Consejo Regulador reports average rainfall of six hundred litres per square metre per year, usually falling in autumn and winter, and an average annual temperature of 17.3ºC
• Andrew Jefford described the flor film in Decanter as ranging from a thick blanket in Sanlúcar to thin pond scum in Tokaji and the Jura

Primary sources (Consejo Regulador Jerez-Xérès-Sherry and its pliego de condiciones, MAPA, and producers' own sites): https://www.sherry.wine/sherry-wine/production/fortification, https://www.sherry.wine/sherry-wine/production/ageing, https://www.sherry.wine/documents/597/21052026_Pliego_Jerez_1wBQg8y.pdf, https://www.sherry.wine/sherry-wine/special-categories, https://www.sherry.wine/sherry-wine/dry-sherry-wines/manzanilla, https://www.sherry.wine/sherry-wine/dry-sherry-wines/fino, https://www.sherry.wine/sherry-wine/dry-sherry-wines/amontillado, https://www.sherry.wine/sherry-wine/dry-sherry-wines/palo-cortado, https://www.sherry.wine/sherry-wine/dry-sherry-wines/oloroso, https://www.sherry.wine/sherry-wine/production/viticulture, https://www.mapa.gob.es/es/alimentacion/temas/calidad-diferenciada/dop-igp/detalle/vcprd/dop_jerez, https://www.sherry.wine/news/biological-ageing-sherry-veil-flor-part-1, https://www.sherry.wine/news/biological-ageing-sherry-veil-flor-part-2, https://www.sherry.wine/news/what-is-fortification-in-sherry-wine, https://www.sherry.wine/sherry-wine/production/vinification, https://www.tiopepe.com/int-en/our-story, https://www.gonzalezbyass.com/en/news/wineries/tio-pepe-jerez-world-story-icon, https://bodegashidalgolagitana.com/historia/, https://bodegashidalgolagitana.com/tienda/manzanilla-la-gitana/, https://lustau.es/en/about-lustau/, https://lustau.es/en/blog/sherry-almacenistas/, https://www.tiopepe.com/int-en/products/tio-pepe. Secondary sources: https://www.decanter.com/wine-news/opinion/jefford-on-monday/wines-of-jura-comparison-385099/, https://www.decanter.com/learn/know-your-sherry-279407/. Last verified 2026-09-28