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'''Mixed fermentation''' (also referred to as "mixed culture fermentation") is any fermentation that consists of a combination of ''[[Saccharomyces]]'' (brewer's yeast), ''[[Brettanomyces]]'' (wild yeast), ''[[Lactobacillus]]'' (lactic acid bacteria), and ''[[Pediococcus]]'' (lactic acid bacteria), or other [[Nonconventional_Yeasts_and_Bacteria|microbes that are unconventional in brewing]]. Broadly speaking, there are two styles of mixed fermentations: mixed fermentations with lactic acid bacteria (''Lactobacillus'' and/or ''Pediococcus'') and mixed fermentations without lactic acid bacteria. Mixed fermentation sour beers are characterized by their higher acidity and tart flavor caused by the production of [[Lactic Acid|lactic acid]], and require the use of a lactic acid bacteria (abbreviated as '''LAB'''; generally ''Lactobacillus'' and/or ''Pediococcus''). These beers generally fall within a pH range of 3.0-3.7 (although [[Titratable Acidity]] is more accurate for measuring perceived sourness). Mixed fermentation without lactic acid bacteria are usually fermented with a combination of ''Saccharomyces'' and ''Brettanomyces''. Mixed fermentation beers without lactic acid bacteria may be slightly tart from the [[Acetic Acid|acetic acid]] production of ''Brettanomyces'', but are generally not considered to be sour if well brewed because they lack lactic acid and too much acetic acid is considered a flaw. For both categories, the primary fermentation will be completed by yeasts such as ''Saccharomyces'' and/or ''Brettanomyces''.
This page will focus on information for mixed fermentation sour beers using pure laboratory culturesand where the lactic acid bacteria is allowed to co-exist with yeast (e.g. not [[Wort_Souring#Souring_in_the_Boiler_.28Kettle_Sour.29|Kettle Sours]]). For mixed fermentation beers without lactic acid bacteria, see the [[Brettanomyces and Saccharomyces Co-fermentation]] page. For 100% ''Brettanomyces'' fermentations (technically not a "mixed" fermentation), see the [[100% Brettanomyces Fermentation]] page. 100% ''Lactobacillus'' or ''Pediococcus'' beers do not exist because they do not fully attenuate wort (see [[Lactobacillus#100.25_Lactobacillus_Fermentation|100% ''Lactobacillus'' fermentation]] for details). Other alternative yeast and bacteria can also be used, however this is currently not common even for brewers who make wild/sour beers. For example, [[Spontaneous Fermentation|spontaneous fermentation]] and [[Wild_Yeast_Isolation#Growing_and_Testing_Without_Plating|wild yeast captures]] usually contain a plethora of [[Nonconventional_Yeasts_and_Bacteria|yeast and bacteria that are not conventional to modern brewing]].
It is important to mention that mixed fermentation brewing in general has very few well-established rules and definitions. While we may categorize techniques for the sake of keeping some sort of manageable structure to this wiki, many methods can be used in conjunction with other [[Brewing Methods]], and brewers sometimes use same/different terminology for the same/different things (for example, the use of the term "wild beer" by professional brewers can mean "any mixed fermentation beer", or can also mean "mixed fermentation beer brewed with wild caught microbes"). New methodologies are constantly being developed that combine elements of more established techniques, as well as slight changes to established techniques. Definitions equally evolve over time. 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. Examples of how techniques can overlap to create new techniques can also be found in Michael Tonsmeire's pivotal book on mixed fermentation brewing, "American Sour Beers". An archive of various sour beer terminology discussions and debates can be found [[Sour_Beer_Terminology|here]].