The Solera system is a dynamic aging method that was developed in Southern Spain, in the mid 18th century. Solera describes a fractional blending system originally used for sherry production, where a number of casks containing the wine are stored for consecutive ages. A group of containers, criaderas (nurseries) are filled for each aging interval, where the oldest scale or stage is the solera (floor). After last barrel is filled, the oldest solera is tapped in succession down to the youngest barrel which is refilled with new product. Typically, there are 3 to 4 stages to the filling system.
For sherry makers, the solera system offers continuity and consistency, where the young wine provides the essential micro-nutrients to support and feed the yeast. The solera system in sherry production is regulated, where a max of 35% of the barrels can be emptied from each barrel, but generally only 10-15% is withdrawn at any given time.
Solera methods have also been applied to aging port, whiskey, rum, brandy, sherry vinegars – and beer. For many years, Belgian brewers used solera to make Flemish red and oud bruin style beers. With this system, beer can be made quickly and continuously, where the untapped beer from the cask acts as a jump starter (think sourdough yeast starter) because it keeps the bacteria and yeast alive. Contemporary brewers are using the solera system to give beer an aged character with more depth than could be achieved through conventional barrel aging.
There are no regulations regarding solera beer production and therefore no defined rules of how much beer can be emptied. The main struggle in making solera beer is maintaining a consistent blend. Beer does not have the alcohol level of distilled spirits, nor the acidity of wine to protect beer from infection. Sour beer or wild yeast ales tend to be the best candidates for solera beer, as increased levels of acidity and use of yeast strains like brettanomyces help to prevent against infection.
For aging spirits and wine, the solera system allows for producers to make a consistent product that can be drawn off at regular intervals. Although beer is less stable, brewers have been able to experiment with this system to make some beautifully complex and very unique beer that they wouldn’t have been capable of achieving through regular barrel aging.