For the traditional Solera process used in Sherry, vinegar, and brandy production, see the Wikipedia article on Solera. In beer, particularly sour beer, a single stage Solera process has been discovered to be a reliable and easy way to continuously make sour beer. Some have argued that this is a misuse of the word "solera" and that it should only be applied to multiple tiered systems (see Misuse of the Traditional Meaning below).
The process of using a solera method in sour beer brewing has become the term used for filling a single fermenter with a sour beer, and every 6-12 months taking one third or half of the beer out for packaging (and perhaps sometimes as much as 70% is taken out). There is no defined rule for what the limit is to take out and still be considered a "solera" in beer. The maximum percentage is also variable in wine. For example, in Sherry a maximum of 35% can be removed legally, although 10-15% is more typical, but other wine styles may not have a defined percentage (ref needed) . The volume that is removed is then replaced with new beer or wort. This method provides the brewer with a "perpetual" sour beer that takes less time to age because of the blended components. Over time the beer can continue to develop and change, and the brewer has the option of trying to steer the beer by altering the recipe for the wort or beer used to refill the solera. As a rule of thumb, the larger the fermenter the better. This will allow for larger seasonal pulls from the solera, thus producing more beer. The term could also be applied to a multi-vessel solera in beer, which has been argued is a more accurate use of the term, although multi-vessel solera systems in sour beer production are less common due to the potential for overexposure to oxygen.
The barrel can be refilled with either fermented beer or wort. Choosing to refill with wort could eventually lead to a lot of trub build up, however, there might not be any negative flavor effects from doing so. Wort will ferment in the barrel and might cause a blow off unless there is adequate headspace (in which case the brewer should top up the barrel after the wort ferments). Choosing to refill with beer that is first fermented with brewers yeast or with a mixed culture might be more practical for avoiding blow off issues and for group barrel projects for homebrewers because individual brewers can brew the refill beer at any time, and then meet to refill the barrel. See Yeast Autolysis below.
Yeast autolysis is the rupturing of dead yeast cells and produces meaty or rubbery off flavors . When it comes to sour beers, Lambic producers often let their beers age in barrels on the trub for up to four years without effects of off flavors from yeast autolysis. The hypothesis is that the Brettanomyces will metabolize the off-flavor compounds produced by the bursting Saccharomyces cells, and use them as nutrients . Therefore, many people have reported that they have successfully avoided yeast autolysis in solera projects, as well as just allowing sour beer to remain in the primary fermenter for the entirety of its aging. However, as the solera ages, trub buildup can potentially become be a problem for logistic reasons (such as clogging ports). Will Meyers of Cambridge Brewing Company reported an eventual autolysis issue with his solera . Lauren Limbach of New Belgian Brewing reported tasting "goaty", "dog food", "caprilic and capric" flavors in barrels that had not been cleaned out after a few years, and upon rinsing the barrels out with de-aerated water the resulting beers were "bright and beautiful" (the barrels were filled with clean beer and not inoculated; assumably the microbes surviving in the wood were enough to produce good results)  (~37 minutes in). Some sour beer brewers strive to achieve autolysis in their beers with the belief that it could improve mouthfeel and react with other compounds to produce favorable flavors, similar to how autolysis is sometimes desired in winemaking in the form of lees aging or bâtonnage .
Whether or not the brewer elects to refill the solera with wort or fresh beer can also potentially play a role in yeast autolysis issues eventually creeping up in a solera. If flavor issues from yeast autolysis are going to occur (and those flavor contributions are not desired), adding wort will create a thicker trub, which could lead to autolysis issues faster. In either case, theoretically, these issues can be avoided. One method for avoiding off flavors from yeast autolysis is to rack some of the trub out of the solera when the seasonal pulls are taken. Another method is to rack the solera to another vessel once a year or every other year. Once removed from the trub, the Brettanomyces should be able to clean up the off flavors over time. See also Reusing a sour yeast cake.
- Will Meyers of Cambridge Brewing Co. shares his experience with autolysis and running the oldest commercial solera program for sour beer in the US.
- Mixed Fermentation Secondary Fermentation
Articles on Solera
- Starting a Solera, by Ed Coffey on HBT. This is a great article on what a solera is, and how to start one for sour beer.
- Michael Tonsmeire has several great articles on Solera. Some noteworthy articles:
- Farmhouse Beer Blog, Outside the Box Solera - Details on Sherry solera production, and how it relates to beer production.
- Adam Kielich's blog on small batch Solera and blending pulls to make a Gueueze.
- Dan Pixley's "perpetual blend" project over the last 3 years in video playlist format.
- MTF thread on planning a small vessel traditional solera.
In addition, see the Blogs Wiki page for blogs that may contain more articles on solera.
Claims of Misuse of the Traditional Meaning
The term "solera" has deep roots in the history of Sherry production dating back as far as the late 1800's. As such, the term has a fairly strict and traditional meaning in the Sherry and wine industry. See this article by Ruben Luyten for an in-depth definition of the solera system as it is known in the Sherry industry. In short, in a traditional solera system, multiple tiers of wooden casks are used with the top/youngest tier being referred to as the sobretabla, the middle tiers as criaderas, and the oldest/bottom tier being called the solera (the term solera refers to both the oldest/bottom tier of casks and the entire system). As a portion of Sherry leaves the solera casks to be packaged, they are refilled from the second oldest tier of criaderas, which are then refilled from the third oldest, until the youngest criaderas (sobretabla) is refilled with new wine. This process is called fractional blending .
There is no known use of the term "solera" in the wine industry to refer to a single tier blending system (there is a term for this in the wine industry, "perpetual blending"). As such, some brewers and likely Sherry makers and wine makers argue that unless a solera consists of multiple tiers, such as the system at Cambridge Brewing Co. for Cerise Cassée, which is the oldest known traditional solera for beer starting in 2004, that it is not a true solera. Therefore, using the term "solera" for single vessel systems misguides consumers by changing the definition of the word to include processes that do not completely resemble the solera process of Sherry making. The alternative and established term, "perpetual blending", which has been applied in wine world to refer to single vessel blending systems , has been offered as an alternative and more accurate word to use for single-vessel blending systems. The term "pseudosolera" has been proposed as well since the phrase "pseudo" has been used to refer to "pseudolambics" in the homebrewing community, although there is no precedence of using this term in the wine industry like there is with the term "perpetual blending". See these arguments made from Will Meyers of Cambridge Brewing Co. and wine blogger Otto Forsberg.
There are arguments defending the use of the term "solera" in the brewing industry and in homebrewing to refer to single-vessel blending systems. These include:
- At least one wine reference claims that the word "solera" has been used for less complex perpetual blending systems, although it isn't the true definition.
- Terms are borrowed and changed all the time, and that the nature of language is to evolve.
- The term has been used since at least 2002 by Jeff Renner in an article of Zymurgy Magazine and then later in 2006 by a homebrew club to describe a single-vessel blending system for homebrewers . A 200-year-old single vessel sour beer "solera" in Sweden was the inspiration for the latter article, however, it appears as though the owners did not refer to it as a "solera". In fact, the Swedish term that was used, which dates back to the early 17th century, was "hundred-year beer" or "hundraårig öl" (see this Ann Arbor Brewers Guild blog post and this Larsblog blog post).
- Reversing the relatively recent precedence in the brewing industry and the homebrewing hobby of calling single-vessel blending systems something other than "solera" will be impractical now.
- The brewing of the fresh beer in a steel fermenter could be viewed as the "first stage of a solera", so as long as fresh wort is not added, it is still a solera (this argument is countered by the fact that Palomino wine is first fermented in a fermentation vessel, then racked into casks, but the fermentation vessel itself is not considered a part of the solera system ).
- The term "perpetual blending" is less convenient, brief, and/or marketable than the term "solera".
A full debate of these points and counterpoints can be found on this MTF thread, with comments by Will Meyers of Cambridge Brewing Company. As with the usage of the term "lambic" and others, we encourage readers to read the arguments on both sides and develop their own informed opinions.
Videos and other resources:
Additional Articles on MTF Wiki
- Old Walloon Ale; a 200 year old solera.
- MTF Solera spreadsheet calculators (search for "solera").
- Michael Tonsmeire's Solara Aging Calculator Spreadsheet.
- Sherry presentation by Erik Wait (starts 10 mins in).
- Sherry Notes website. "The Solera System". 08/28/2013. Retrieved 03/06/2018.
- Freccia, Nico. Beat Yeast Bite. Brew Your Own Magazine. April 1997. Retrieved 2/20/2015.
- Tonsmeire, Michael. Second Pull Wine Barrel Solera. The Mad Fermentationist Blog. Retrieved 2/20/2015.
- Tonsmeire, Michael. Second Pull Wine Barrel Solera. The Mad Fermentationist Blog. Comments section, July 30, 2014 at 9:39 PM. Retrieved 2/20/2015.
- Lauren Limbach. Craft Beer and Brewing Magazine Podcast. Episode 21. 02/16/2018.
- Lars Meiner, Richard Preiss, and Alex Seitz. milk The Funk Facebook group thread on autolysis. 01/03/2019.
- Ruben Luyten. "The Solera system". The Sherry Notes websites. 08/28/2013. Retrieved 10/30/2018.
- Peter Liem. Champagne Guide. "Glossary of Terms". 2009. Retrieved 03/06/2018.
- Jeff Renner. "Solera Ale: Beer That Gets Older As You Drink it". Zymurgy Magazine. Jan/Feb 2002.
- Ann Arbor Brewers Guild. Walloon Solera Ale in Sweden. 2006. Retrieved 03/06/2018.
- Sogrape Vinhos website. "Making Sherry Wine". Retrieved 03/06/2018.