What is a solera?

A solera consists of rows of casks with wine of increasing average age. The bottom row is called the solera; above it lie the criaderas, numbered according to how close they are in age to the solera. The first criadera lies closest to the solera, the highest criadera holds the youngest wine.

Matured in American oak botas of 600 litres, i.e. 36 arrobas. For biological ageing these are filled only to about 500 litres, so that space remains above the wine. The maturing takes place in the bodegas of Jerez de la Frontera, El Puerto de Santa María and Sanlúcar de Barrameda, together the sherry triangle.

The saca is the withdrawal from the solera, the rocío the replenishment that follows. In a saca, the same portion is drawn from each cask of the solera, so the casks are left partly empty. That space is then topped up with wine from the first criadera, which in turn is topped up from the second, and so on to the top row.

As an example, the Consejo Regulador calculates with four sacas per year, each of 25 percent of the contents of the solera. Because of this systematic blending, every cask always contains a part of all preceding years. There is therefore no vintage year; only an average age can be calculated.

Less than half of the system may be drawn off per year. The pliego de condiciones of the protected designation of origin Jerez-Xérès-Sherry calculates the average ageing time as the total volume in the system divided by the volume that leaves it per year. That result must be greater than two.

This puts the legal minimum at an average maturation of two years. In practice maturation is much longer, because the characteristics of each type need time. For the designation VOS it is more than 20 years, for VORS more than 30 years. With oxidative maturation, 3 to 5 percent evaporates from the cask every year, the merma.

Biological ageing happens under a layer of yeast, oxidative ageing without. In biological ageing a velo de flor forms on the wine, a layer of yeasts that shields the wine from the air. Flor holds between 15.0 and 15.5 percent alcohol; if fortified to at least 16.0 percent, the yeast layer can no longer form and the wine continues oxidatively.

The yeast layer changes the wine profoundly. In a study of fino and amontillado, acetaldehyde rose from 72.3 milligrams per litre in young wine to 487.0 milligrams per litre in the oldest fino solera, while glycerol fell from 7,151 to 611 milligrams per litre, a drop of well over 90 percent. The same study distinguished four strains of Saccharomyces cerevisiae: beticus, montuliensis, cheresiensis and rouxii. In the young sobretabla, beticus was still completely dominant; in the oldest fino solera it was about 58 percent beticus and 38 percent montuliensis.

Oxidative ageing works the other way. Without flor the wine comes into contact with oxygen, becomes darker and the alcohol percentage rises because moisture evaporates from the cask.

The Consejo Regulador in Jerez oversees the three protected designations Jerez-Xérès-Sherry, Manzanilla-Sanlúcar de Barrameda and Vinagre de Jerez. It was founded in 1935, the first in Spain, under the wine statute of 1933.

The area covers roughly 7,000 hectares of vineyard spread over nine municipalities. Control runs through registration of vineyards, supervision of the harvest, stock control in the bodegas and laboratory analysis. For the VOS designation, at least 20 liters must remain in the aging system for every liter sold, for VORS at least 30 liters.