Solera fermentations have been growing in popularity ever since Will Meyers implemented his system at Cambridge Brewing Company to age Cerise Cassee, an Amber colored sour wild ale with cherries. Solera is a technique originating in the production of Sherry but is also used in the aging of vinegar, brandy, wine and many other liquids for where there is a desire to blend old and new vintages. The solera system can be applied to any type of beer which would do well with aging and blending, but has become most popular in the sour/wild brewing community.Traditionally a "Solera", which literally translates to "on the ground" in Spanish, is comprised of a series of oak barrels all filled at different intervals with product on a yearly cycle. At the end of each cycle a portion is pulled from the oldest barrel (called the Solera) and packaged. Then, each of the younger barrels (called the criadera, or "nursery"), are transferred down the line to refill the Solera. No barrel is ever fully drained, as the youngest barrels are topped off with fresh liquid to age and wait for next years cycle. The result is an evolving product of an average age year after year. On a more manageable homebrew level the same system can be maintained in a standard 5 gallon size using carboys, kegs, etc contained in one vessel.There are a few variables to decide on when planning your own Solera, starting with choosing the base beer style. I use a pale Lambic-esque wort in my own cellar, but you could brew a Flanders Red/Brown, a Stout, really any type of beer you want. The goal here is to create a wort that is low in IBU's, to not inhibit the Lactobacillus, while leaving enough long chain unfermentable sugars for the yeast and bacteria to acidify over time. Keep in mind that when brewing a sour/wild ale, you want to make sure the flavors of the base beer compliment the flavors coming from the blend of yeast and bacteria. Try to keep it simple, stay away from using spices, or over hopping, you can play with those things when you package the beer.With your base beer planned out, you will need to decide what type of vessel to age the Solera in. Some vessels to consider are glass carboys, Better Bottles, kegs (corny or Sanke), oak barrels, whatever size you're most comfortable maintaining. Oxygen permeability of the vessel is always a concern when aging beer this long, but especially with Brettanomyces and bacteria in the mix. When exposed to high levels of oxygen Brett can create acetic acid (vinegar flavor) so you'll want to keep oxygen exposure minimal. Furthermore, acetobacter, which has the ability to convert ethanol to acetic acid when exposed to oxygen, is a spoiling organism that can wreak havoc on your Solera. Although it can be appropriate in some styles at lower levels, you want to avoid as much acetic acid as possible in a Lambic type wort.Stay away from using a plastic bucket or even a 5 gallon oak barrel (whose staves are much thinner than 50+ gallon oak wine/spirit barrels) both are more oxygen permeable than glass carboys, Better Bottles, and larger oak barrels (o2 Permeability chart from Wild Brews). I use a 15.5 gallon sanke keg to age my Solera as it is only oxygen permeable through the orange carboy cap on top and can store a fairly large volume.With regard to microbes, they are as important as the vessel you are aging in, and probably more important than your base recipe. The players at work here are Saccharomyces (brewers yeast), Brettanomyces (a type of yeast known for its "wild" character), Lactobacillus and Pediococcus (both of which produce lactic acid). Saccharomyces will do the bulk of the fermenting in the early stages while Brett/Lacto/Pedio will acidify and add a wild "funky" character to the beer. Note, Brett alone will not make a beer sour, that's what the Lacto and Pedio do. For further reading on these organisms, see American Sour Beers by Michael Tonsmeire.In my opinion, the best option is to buy a blend of these yeasts from a major yeast lab. I suggest WLP655 Belgian Sour Mix, Wyeast 3763 Roselare, ECY01 Bugfarm or The Yeast Bay Melange, to name a few. I am a firm believer in biodiversity for complexity, so as you age the beer add the dregs of some of your favorite sour beers right into your Solera, If you're interested in what dregs to use, Tonsmeire maintains an extensive list of viable sour dregs in commercial beers . I also like to add some medium toast oak cubes to my Solera, however make sure they've been boiled long enough so there is little to no oak flavor left. The wood can be a nice home for Brett and bacteria to live. In addition, Brett can also metabolize cellobiose in wood and create some unique flavors.The hardest part after having brewed the first batch for the solera is the waiting. Resist the urge to open the vessel unless you're adding dregs or removing a sample. Remember oxygen could turn your project into vinegar. Carefully remove samples periodically take good notes on the tasting, take a gravity reading, and take a PH reading if you have a meter. If the gravity is stable after 1 year (it should be), you can pull a portion out to be packaged. I generally bottle or keg the first cycle straight so that you can get to know the base beer that is in your Solera. Usually I will remove one third to one half of the old beer leaving the rest in the Solera to be topped off with fresh beer or wort. Tonsmeire created a handy spreadsheet to calculate the average age of your Solera based on the amount removed and then topped off.After tasting your first pull, give some thought as to whether you would want to modify the top off batch to adjust the beer for next years cycle. Ask yourself: Is it too sour? not sour enough? too funky? Prior to a recent cycle I felt my Solera was overly sour and needed some Brett complexity so I modified the top off batch to adjust. I mashed the beer at 160F and pitched WLP530 along with The Yeast Bay Brussels Blend, known for some classic Brett funky aromas, and increased the bittering addition slightly. Once the krausen started to fall I racked the fermenting beer into the Solera. Some people will drastically change their top off wort every cycle, and some people will keep it virtually the same each year. On a recent episode of Basic Brewing radio a National Homebrewers Conference attendee mentioned they increased the color on every top off batch from pale all the way to black, what a unique idea!After the first cycle is complete you will have a good grasp on your Solera and will easily be able to determine when your next cycle should be. Whether you pull some to age on fruit, blend some into some fresh beer, bottle it straight, you can end up with many interesting vintages as your Solera progresses. One thing to keep in mind is that your Solera may have an expiration date - if it becomes overly vinegary (acetic acid) or has aromas of nail polish remover (ethyl acetate), you may want to consider ending your Solera and starting over from the beginning. Take good notes, plan ahead, be patient, and let the beer point you in the right direction. Remember don't remove that bung too often and keep those airlocks full. Funk it up!