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Solera is a process for aging liquids such as wine, beer, vinegar, and brandy, by fractional blending in such a way that the finished product is a mixture of ages, with the average age gradually increasing as the process continues over many years. A solera is literally the set of barrels or other containers used in the process. Products which are often solera aged include Sherry, Madeira, Port wine, Marsala, Mavrodafni, Muscat, and Muscadelle wines; Balsamic, Commandaria, and Sherry vinegars; Spanish brandy; beer; and rums.
In the solera process, a succession of containers are filled with the product over a series of equal aging intervals (usually a year). One container is filled for each interval. At the end of the interval after the last container is filled, the oldest container in the solera is tapped for part of its content, which is bottled. Then that container is refilled from the next oldest container, and that one in succession from the second-oldest, down to the youngest container, which is refilled with new product. This procedure is repeated at the end of each aging interval. The transferred product mixes with the older product in the next barrel.
No container is ever drained, so some of the earlier product always remains in each container. This remnant diminishes to a tiny level, but there can be significant traces of product much older than the average, depending on the transfer fraction. In theory traces of the very first product placed in the solera may be present even after 50 or 100 cycles.
The age of product from the first bottling is the number of containers times the aging interval. As the solera matures, the average age of product asymptotically approaches one plus the number of containers (excluding the top container) (K) divided by the fraction of a container transferred or bottled (α), or (1 + K/α). 
For instance, suppose the solera consists of three barrels of wine, and half of each barrel is transferred once a year. At the end of the third year (and each subsequent year), half the third barrel is bottled. This first bottling is aged three years. The third barrel is then refilled with by transferring half of the wine from the second barrel. The wine transferred from the second barrel has an average age of 2.5 years (at the end of year 2, after barrel transfers, it was half 2-year old wine, half 1-year old wine, for an average age of 1.5 years; at the end of year 3, before barrel transfers, it will have aged another year for an average age of 2.5 years). The second bottling will then be half 3.5 years old and half four years old (the wine left in the last barrel at the previous cycle), for an average age of 3.75 years. The third bottling will be an average age of 4.25 years (one half wine that was left over from the second bottling - average age 4.75 years, and one half wine transferred from the second barrel after the second bottling - average age 3.75 years). After 20 years, the output of the solera would be a mix of wine from 3 to 20 years old, averaging very slightly under five years. The average age asymptotically converges on five years as the solera continues.
The output of the solera is the fraction of the last container taken off for bottling each cycle. The amount of product tied up in the solera is usually many times larger than the production. This means that a solera is a very large capital investment for a winemaker. If done with actual barrels, the producer may have several soleras running in parallel. For a small producer, a solera may be the largest capital investment, and a valuable asset to be passed down to descendants.
Wine produced from a solera cannot formally have a vintage date because it is a blend of vintages from many years. However, some bottlings are labeled with an age for marketing reasons. It is unclear whether such age indications denotes the average age, or the age of the oldest batch.
Solera in different countries
This process is known as solera in Spanish, and was developed by the producers of sherry. In a Spanish sherry solera, the vintner may transfer about a third of each barrel a year. A solero sherry has to be at least three years old when bottled.
In Sicily, where Marsala wine is made, the system is called in perpetuum.
Solera vinification is used in the making of Mavrodafni ("Black Laurel"), a fortified red dessert wine made in the Northern Peloponnese in Greece. Exceptional Mavrodafni vintages are released every 20 or 30 years: they are of minimal availability and highly expensive.
Glenfiddich, a Speyside distillery in Scotland has a 15 year old whisky that uses a process that is similar to the solera process. The whisky is labelled as their "15 year old single malt Scotch Whisky".
Possible solera abuse
Italian labeling laws permit blended vinegar to be labeled with the age of the oldest vinegar in the blend. Some balsamic vinegar producers have established solera aging facilities, and claimed the age of the entire solera as the age of the vinegar produced. In the case of the more strictly-controlled and more expensive vinegars, such as aceto balsamico tradizionale, this labeling practice is not permitted.
- Website of the Denominación de Origen of Jerez
- Website of the Denominación de Origen Montilla-Moriles: Criaderas and Soleras
- Aging: the Solera Method demystified... by The Rum Project