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: ''This article is about sour brewing methods using commercial cultures. For other sour brewing methods, see [[Brewing Methods]].''

For the purposes of this article, we are defining a ''mixed fermentation'' as any fermentation that consists of a combination of [[Saccharomyces]], [[Brettanomyces]], [[Lactobacillus]], and [[Pediococcus]] cultures. Broadly speaking, there are two styles of mixed fermentations: "sour" fermentations and "funky" fermentations. "Sour" fermentations are characterized by their higher acidity and tart flavor, and require the use of a Lactic Acid Bacteria (LAB; generally Lactobacillus and/or Pediococcus). "Funky" fermentations generally do not contain LAB, but instead use a combination of Saccharomyces and Brettanomyces. Funky beers may be slightly tart, but are generally not considered sour. For both categories, the primary fermentation will be completed by yeasts such as Saccharomyces and/or Brettanomyces. For ''Funky Mixed Fermentations'', see the [[Funky Mixed Fermentations]] page. For 100% Brettanomyces fermentations, see the [[100% Brettanomyces Fermentation]] page.

It is important to mention that sour brewing in general has very few well-established "rules". Many methods can be used in conjunction with other [[Brewing Methods]], and new methodologies are constantly being developed. Many of the methods used are determined by the types of microbes the brewer is working with. An article of this length cannot encompass all mixed fermentation methods. Instead it will provide a "big picture" view of the general methodologies. Towards this end, we divide mixed fermentation methods into two approaches: the traditional long fermentation method and an increasingly popular, short fermentation method. They are divided here as a device to illustrate the philosophy of each and facilitate the discussion of the techniques used for each methodology. The distinction of these two methods is however somewhat artificial, indeed many brewers use elements of both approaches to achieve their desired results. More detailed information can be found in Michael Tonsmeire's pivotal book on sour brewing, "American Sour Beers".

Sour fermentations require at least one Lactic Acid Bacteria (LAB), such as [[Lactobacillus]] or [[Pediococcus]], and at least one yeast such as [[Saccharomyces]] or [[Brettanomyces]]. Many yeast companies offer [[Mixed Cultures]] that provide all of the microorganisms necessary to make a sour beer. The results of these commercial mixed cultures can be as varied as the cultures themselves. For example, some of these commercial mixed cultures produce lightly tart beer that may exhibit minimal funky flavors; others may produce intense sourness and assertive funk. This is dependent on the types of microbes in the mixed culture, their ratios, how old the cultures are, and what methods the brewer uses to encourage or discourage certain flavors. The brewer must understand that all of these microbes are complex organisms (some more complex than others). Not only do different species behave differently and produce different results under different conditions, but different strains of the same species also can also behave differently and produce different results under different conditions. Just as strains of Saccharomyces cerevisiae produce different results in clean beers (e.g., California Ale yeast versus Belgian Ale yeast), strains of Lactobacillus spp. and especially Brettanomyces spp. can also vary widely.

Beer styles that can be brewed using this method include [[Berliner Weissbier]] and the subcategories of [[American Wild Ale]], which include [[Mixed-Fermentation Sour Beer]] and [[Soured Fruit Beer]] <ref>[http://www.bjcp.org/docs/2014%20BJCP%20Style%20Guidelines%20%28DRAFT%29.pdf BJCP 2014 Style Guidelines Draft.]</ref>. European sour styles such as [[Lambic]], [[Gueuze]], and [[Fruit Lambic]], technically can only be produced by [[Spontaneous Fermentation]]. [[Flanders Red Ale]] and [[Oud Bruin]] styles can be brewed using pure cultures, but can also be brewed using spontaneous fermentation or a mix of using pure cultures and spontaneous fermentation.

The most basic method for making a mixed fermentation sour beer is to brew some simple wort (fresh extract or all grain) that is low in IBU's. Iso-alpha acids can inhibit many species and strains of LAB. Keeping the wort less than 6 IBU's is recommended in general, unless the brewer has information about their LAB culture that indicates that they can tolerate more. Mash hopping is one technique that can be used to limit the IBU's by about 70% <ref>[http://www.homebrewersassociation.org/attachments/presentations/pdf/2014/Putting%20Some%20Numbers%20on%20First%20Wort%20and%20Mash%20Hop%20additions.pdf Putting Some Numbers on First Wort and Mash Hop additions. David Curtis. NHC 2014.]</ref>. If hops are not required (commercial brewers may be required to use hops, while homebrewers aren't), they can be completely excluded from the recipe.

This method has been used by some brewers before a lot was known about sour brewing microbes and before some of the more current techniques were popularized (such as [[Sour Worting]], or the other methods mentioned in this article). This method is best described by the following: Brewer X has a beer, and perhaps that beer didn't come out the way they wanted, or perhaps there is some other motivation, but Brewer X decides that they want this beer to be sour. Great idea, right? Perhaps. [[Lactobacillus]] tends to prefer simple sugars, which are required for lactic acid production. [[Pediococcus]] can break down some of the larger chain sugars that are left over in the beer from the [[Saccharomyces]] fermentation, but there may not be enough of those sugars to create even a moderate level of acidity. If the beer contains more than 6 IBU's, the bacteria may be largely inhibited. The [[Brettanomyces]] in general should create a nice range of flavors (see [[Funky Mixed Fermentations]]) in secondary, so this could benefit the beer. However, due to the lack of sugars, finished beers which a brewer may attempt to sour often do not get very sour at all. Some brewers have reported good results with this technique (particularly professional brewers who use barrels; see "American Sour Beers" by Michael Tonsmeire for more information), but in general if the brewer is going for a truly sour beer, it is better to use another method.

If this method is used, it generally requires 6-12+ months, which is required for the Brett and Pedio to break down the remaining complex sugars.

The grain bill and production for the wort doesn't have to be complex. For sour blonde ales a simple grain bill of about 70% Pilsner malt and 30% malted wheat can be used (these can be replaced by Pilsner and wheat unhopped extracts for the extract brewer. See [http://byo.com/videos/item/975-lambic-brewing Lambic Brewing by Steve Piatz] for a good extract recipe). Some crystal and a small amount of roasted malts an be used for sour brown ales. Some higher chain sugars or even starches can be included for beers that will be aged for a long time and include Brettanomyces, or Brettanomyces and Pediococcus (Pediococcus generally should not be used without Brettanomyces. See the [[Pediococcus]] page for more details). Performing a [[Turbid Mash]] is the traditional way to include starches in the wort, however other methods such as steeping some oats or flaked wheat during the boil can also impart starches that won't be converted to sugars by the mash <ref>[http://www.homebrewtalk.com/f127/adjuncts-starches-sour-beer-448529/ Homebrewtalk Discussion started by Amos Brown aka 'Metic']</ref>. This step is completely optional, however it may be very beneficial to make sure some higher chain sugars or starches are available in the wort if the brewer wants to rely on Pediococcus for producing most of the acidity. Extract brewers can use 0.25 lbs. (0.11 kg) of Maltodextrin <ref>[http://byo.com/videos/item/975-lambic-brewing Lambic Brewing. Piatz, Steve. Brew Your Own Magazine. October, 2004.]</ref>, or hot steep a pound of flaked wheat, flaked oats, or carapils malt.

Once the wort is produced and chilled, the mixed culture can be pitched as normal. If using a mixed culture from a commercial yeast lab, it is usually recommended to not make a yeast starter with it because the starter will throw off the intended ratios of the microbes. Instead of buying a single mixed culture, a brewer can create their own mixed culture by combining their own ratios from single cultures of yeast and bacteria. A single mixed culture can be supplemented by adding pure cultures from different yeast labs as well. Another suggestion that often helps produce a higher quality sour beer is to supplement the mixed culture with a [[Commercial Sour Beer Inoculation]]. In general, the more diversity of microbes, the more complex a sour beer can potentially be. Using a [[Sour Worting]] method in conjunction with this method can help increase acidity that may not otherwise be produced by some commercial blends (Wyeast Roeselare is known for this characteristic, for example).

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Aging is generally required for mixed fermentations that include Brettanomyces. Expect an aging time of at least 9 months, but preferably 12-18 months or longer. Sour beer should be aged in an environment that minimizes high temperatures and exposure to oxygen. Drastic temperature fluctuations and changes in atmospheric pressure will cause a vacuum inside of the fermentation vessel causing water airlocks to "suck back" air into the fermenter. This could potentially contribute to acetic acid and ethyl acetate (nail polish aroma in high concentrations) production by Brettanomyces, however the development of a pellicle can help protect against this. Filling the carboy to the neck will also help minimize the surface area of the beer that can be exposed to air. Avoid over sampling the beer (once every 3 months at the very most). It should also be noted that micro-oxygenation is helpful for creating certain flavors in sour beer, and many homebrewers have reported not having any issues with over exposure to oxygen using water-based airlocks.

The short fermentation method refers to an approach for making sour beers that involves successive inoculation of microbes by the brewer to a wort designed for faster attenuation. This approach accomplishes the souring and full attenuation of the wort in a shorter time frame than the traditional method.

In the traditional or long ferment method, the selective availability of carbohydrates to particular microbes allows the activity of those microbes to occur in a natural succession. As the microbes with better competitive ability run out of metabolic resources microbial groups with lower competitive advantage, but wider access to metabolic resources, begin their primary activity. In the short fermentation method, the brewer controls the phases of microbial activity. This allows the brewer to introduce the microbes with the lowest competitive ability to the wort first, allowing them to act on the simplest sugars and establishing their population in the absence of better competing microbes. The order of primary microbial activity in the short fermentation method is, therefore, often the opposite of the order typically observed in the long fermentation method. Further, this approach allows the brewer to maintain temperature profiles that are optimal for each microbial phase. Since the phases are controlled by the brewer, there isn't a need for the longer chain sugars that are generally included in the wort designed for long fermentation because the microbes with lower competitive ability have already been established in the beer by the time the better competing microbes are introduced. For this reason, the beer can fully attenuate within 3-4 weeks of it's final inoculation in many cases. It is important to note however that care should be taken in the decision to bottle these beers in such a short time frame (See bottling section below). Once the beer has reached full attenuation the beer can be packaged, in some cases this can be only 6-8 weeks from brew day. A number of biochemical reactions that effect flavor and aroma may still take place over a period of weeks, to months or even years, however, most of these reactions do not involve the evolution of carbon dioxide so these reactions may take place in the bottle.

The grain bill for a short fermentation sour can be based on nearly any style. In contrast to the low fermentability wort used in the long ferment method, the wort used in the short fermentation method is generally designed to be highly fermentable. This is because the order and timing of microbial inoculation, rather than natural succession of the microbial community, is used to control acidity and fermentation characters. A few modifications to grain bills can be made to increase the fermentability of the wort and accomplish the full attenuation of the wort in a relatively short period of time. These modifications include lowering or removing crystal malts from the recipe and mashing for 90-120 minutes at 149F. Extract brewers can steep 2-3 lbs. of crushed, malted 6-row at 149F in their kettle with their extract in order to increase the fermentability of their wort.

[[File:Fast Sour Graph.jpg|thumb|upright=2.5|Conceptual graph of Fast Souring microbe and media dynamics|Conceptual graph of fast souring microbe and media dynamics. Y-axis for each microbe group depicts relative activity which combines in a conceptual sense: growth, acidification of wort, attenuation and production of flavor compounds. Plot drawn by Drew Wham based on concepts discussed in American Sour Beer <ref> Tonsmeire, M. (2014). American Sour Beers. Brewers Publications </ref> and Wild Brews <ref> Sparrow, J. (2005). Wild Brews: Beer Beyond the Influence of Brewer's Yeast. Brewers Publications</ref> . ]]

Matt Miller outlines a ''three stage fermentation'' process on his blog article [http://sourbeerblog.com/understanding-brewing-blending-lambic-style-kriek/ Understanding, Brewing, and Blending a Lambic Style Kriek] <ref>[http://sourbeerblog.com/understanding-brewing-blending-lambic-style-kriek/ Understanding, Brewing, and Blending a Lambic Style Kriek. Miller, Matt. Retrieved 1/24/2015.]</ref>. See the article for a much more detailed process. Matt was also interviewed about his process by James Spencer on the [http://traffic.libsyn.com/basicbrewing/bbr01-22-153stepkriek.mp3 BasicBrewing Radio podcast]. In summary, his process is as follows:

# For the last two months of aging, fruit, spices, and/or oak can be added directly into the fermenter. Also consider [[Blending]] with other sour beers.

Reusing a sour yeast cake can often provide great results. Brewers have reported success repitching on very old yeast cakes (2 years) without getting off flavors from yeast autolysis. After several months, [[Saccharomyces]] tends to die off due to the low pH in a sour beer. The bacterias and [[Brettanomyces]] tend to survive the lower pH, and their cell counts can be high in even an old yeast cake. By pitching new wort on an old sour yeast cake, these microbes (particularly the [[Lactobacillus]]) have access to the simple sugars in the wort <ref>[http://www.themadfermentationist.com/2009/11/brewing-sour-beer-at-home.html Tonsmeire, Michael. The Mad Fermentationist. Brewing Sour Beer at Home. Last paragraph in the "Inoculation" section. Retrieved 2/19/2015.]</ref>. In general, washing the yeast cake is not necessary.

As with all methods, the species and strains of the microbes being used should always be taken into consideration. Experimentation and repeated processes should be carefully employed by the brewer in order to find the best results for their cultures. For example, using different strains of Saccharomyces cerevisiae as the primary fermenter can produce widely different results.

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[[Category:Techniques]]