The Solera process is the aging of liquids like wine, 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 oak barrels or containers used in the process. Products which are often solera aged include; Sherry, Madeira, Marsala, Mavrodafni, Muscat, Balsamic, Commandaria, Sherry Vinegar, Spanish Brandy and Spanish Rums.
This process known as solera (a Spanish word), was developed by the producers of sherry. In a Spanish sherry solera, the vintner may transfer up to a third of each barrel, each year. A solero sherry has to be at least three years old when bottled.
The traditional Sherry Solera was exposed to the sun, hence the name. The warmth of the sun encourages an active fermentation process, aging and development. This unique blending system consists of several rows of small oak barrels stacked upon one another grouped by vintages. The oldest is at the bottom and the most recent, youngest wine at the top.
At bottling, approximately one third of the contents of each of the barrels on the bottom level is removed. Sherry from the row immediately above will replace what was removed and so on until a complete transfer is made from top to bottom.
No wine-barrel is ever completely drained, so some of the earlier product / wine always remains in each oak barrel. This wine remnant diminishes to a tiny level, but there can be significant traces of this wine much older than the average, depending on the transfer fraction. In theory traces of the very first wine placed in the 'solera system' may be present even after 50 or 100 years.
On the diverse island of Sicily, where Marsala wine is made, the system is called 'in perpetuum' (from Latin - forever).
Some Sherry and especially Madeira can be labelled with the word 'Solera' and a date. This is a marketing strategy, as it simply means the year that the Solera was started, and the bottle may contain trace amounts from that year, at very best.