Mixed Fermentation

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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 Mixed Fermentation - The Basics

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.

BJCP 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 [1]. Belgian 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.

Traditional Method - Long Fermentation

Introduction

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% [2]. 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. The wort is often mashed at a high temperature to encourage the inclusion of complex carbohydrates in the final wort. The wort is then Primary fermented with a Saccharomyces strain to achieve the majority of attenuation, leaving behind the complex carbohydrates. The primary fermentation is then inoculated with a mixed culture of Brettanomyces, Lactobacillus and Pediococcus, either by moving the wort into barrels with active cultures or by inoculating the primary fermentation vessel (i.e. glass carboy when the method is used by home brewers). This inoculation then starts a secondary fermentation of the remaining complex carbohydrates which follows a slow progression between the microbes that are primarily active. This secondary fermentation may not show readily apparent sighs of active fermentation as in the primary fermentation but is often accompanied by the slow evolution of CO2 in the first 8 weeks and the eventual formation of a pellicle which may form quickly or very slowly. This method is still the most commonly used by commercial producers of modern and traditional belgian sour beer, with variations on the process occurring widely. While still widely used by homes brewers, fast fermentation methods such as Sour Worting other methods mentioned in this article are ever increasing in their use.

Wort Production

The grain bill and production for the wort doesn't have to be complex, in fact many sour breweries produce their full line of sours from 2-3 base sour recipes which are then modified after aging by blending, the addition of fruit, dry hops or simply packaging them without alteration. 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 Lambic Brewing by Steve Piatz or AmandaK's lambic-style extract recipe 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 [3], or running off over a bag of flaked oats or wheat on the way to the kettle can also impart starches that won't be converted to sugars by the mash (see alternatives to turbid mashing). 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 [4], or hot steep a pound of flaked wheat, flaked oats, or carapils malt.

Bug Inoculation

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).

Stages of Fermentation

 
Conceptual graph of traditional souring microbe and wort 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 [5] and Wild Brews [6] .

Primary Fermentation

Primary Fermentation by Saccharomyces is generally conducted in the same way for a sour beer as for a non-sour beer. Depending on the intended final result the brewer might select a neutral ale strain (WLP 001-Wyeast 1056, WLP036-Wyeast 1007) to provide a neutral background for the souring microbes to act on. Alternatively, the brewer may use a belgian-farmhouse strain (see Saccharomyces page for a comprehensive list) to increase the phenol characters of the beer which can then be acted on by Brettanomyces. This primary fermentation can take place in any vessel suitable for a normal Saccharomyces fermentation, as always fermentation temperature control is of critical importance and temperature profiles for this fermentation step should match those suggested for the strain of Saccharomyces selected for this step. Once active fermentation has subsided the mostly attenuated wort can then be moved on to the secondary fermenting vessel. There is some variation in common practice as to whether or not the primary fermentation yeast should be carefully settled out, moving over bright clear beer only, or if un-settled cloudy high yeast population wort is moved to the secondary vessel. New Belgium moves their lager primary fermented beer after centrifuging, indicating that this centrifuged beer exhibits cleaner characters from secondary fermentation faster than un-centrifuged beer, allowing the resulting sour beer to be ready for packaging more quickly [7].

Secondary Fermentation

After primary fermentation the mostly attenuated beer is moved to a secondary fermentation vessel. Often in traditional commercial production secondary fermentation is conducted in wine barrels, however home brewers can accomplish this phase in glass or plastic carboys with low oxygen permeability. A mixed culture of Brettanomyces, Lactobacillus and Pediococcus is then introduced to the beer. If barrels are being used these "bugs" may simply come from the walls of the barrel, originating from a previous batch. Alternatively, the brewer might inoculate the wort with a mixed culture directly, either with a house culture or by introducing the dregs of sour beer. Upon their introduction these new microorganisms begin converting the longer chain sugars left over from the primary fermentation. These sugars are primarily converted into alcohol and lactic acid, increasing the degree of attenuation and lowering the pH of the beer. In the presence of oxygen acetic acid is also produced which in low amounts can be complementary, adding to the complexity of the beer.

Some brewers (mostly homebrewers) do not find it necessary to move the mostly attenuated beer into a secondary vessel. Instead, the mixed culture is pitched directly into the primary fermenter. While yeast autolysis is a concern in regular brewing, it is less so in mixed fermentations that contain Brettanomyces. Lambic brewers, for example, perform a primary fermentation in barrels, and leave the beer in the barrels during the beer's entire aging process, which is usually 1-3 years [8]. Yeast autolysis releases trehelose, acids, and other compounds, which are metabolized by Brettanomyces [9]. Maintaining a Solera may be an exception to this (see the Solera page for details). The advantage of not moving the beer into a secondary vessel is that less overall oxygen is introduced into the beer (oxygen exposure will contribute to more acetic acid production), and may be the best option if the brewer does not have a closed/CO2 system to prevent exposure to oxygen during transferring.

Aging

Aging is generally required for mixed fermentations that include Brettanomyces. The necessary/ideal amount of aging time will depend on the microbes pitched, the pitching rate, and the desired final beer. Keep in mind that the beer will also continue to develop once packaged. For more straightforward beers with highly attenuative primary strains (like tart saisons), a reasonable final product with tartness and brett character can be reached in a few months. For more complex and/or acidic beers (such as Flemish reds or beers inspired by lambics) you may expect an aging time of at least 9 months, but quite possibly as long as 12-18 months or longer. In general longer aging will allow more complex expression of the spectrum microbes present.

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.

Modern Method - Fast Fermentation

Introduction

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.

Wort Production

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.

Multi-Stage Fermentation

 
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 [10] and Wild Brews [11] .

Matt Miller outlines a three stage fermentation process on his blog article Understanding, Brewing, and Blending a Lambic Style Kriek [12]. See the article for a much more detailed process. Matt was also interviewed about his process by James Spencer on the BasicBrewing Radio podcast. In summary, his process is as follows:

  1. Produce a low hopped wort (see the Standard Method above).
  2. After boiling the wort, cool it to 110-120°F (43.3-48.9°C), and run it into the fermenter.
  3. If possible, purge the fermenter of oxygen with CO2. Pitch a pure culture of Lactobacillus, and if possible hold the temperature between 110-120°F (43.3-48.9°C) for 2-4 days (see the Souring in the Primary Fermenter page for more details).
  4. After 2-4 days, cool the wort to 65-70°F (18.3-21.1°C), oxygenate the wort, and pitch a starter of Saccharomyces.
  5. After primary fermentation has finished, transfer the beer to a secondary vessel for aging.
  6. Add one or more cultures of Brettanomyces. Optionally, also pitch a culture of Pediococcus and/or bottle dregs from commercial sours (see Commercial Sour Beer Inoculation for more details on using commercial bottle dregs). For more funky Brett flavors, do not make a starter for the Brett.
  7. Age for 6-18 months, or longer if desired.
  8. 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.

Fermentation in Less Than 3 Months

Some brewers have been experimenting with mixed fermentations that can finish within 3 months. This approach to mixed fermentations takes some knowledge of the cultures being used and is considered an advanced topic. In general, use cultures that don't produce a lot of off-flavors early on in fermentation. Omega Yeast Lab's Lactobacillus blend (OYL-605) and The Yeast Bay's Lorchristi Brettanomyces blends are good choices, for example. Warning: if mixed fermentation beers are bottled too early, they can result in bottle bombs or gushing bottles. Gareth Young offers his advise to brewers wanting to try mixed fermentations that finish within 3 months [13]:

I typically turn around funky beers, especially roughly saisonish things, pretty fast. One of the things I like about them is the way they change over time, so I like being able to drink them when they're very young, then drinking them sporadically over the next months as they develop.

Looking through stuff I've posted here, I've found one that I posted about 5 weeks from brew day, and it was good a while before that. It was also dry-hopped and had honey added, which meant it took longer. https://www.facebook.com/groups/MilkTheFunk/permalink/1108334109194802/

Here's some stuff about my process:

1) As you suggest, lower ABV stuff will be quicker. It doesn't need to be super low, but below 6% is probably wise.

2) Make your wort very fermentable. The more fermentable, the quicker it will hit a stable gravity. I do this by mashing at 63 for two hours, then not sparging too hot, so it's becoming more fermentable all the way to the kettle. Also, don't use any grains with unfermentable (or slow-fermentable) sugars. Just base malt, and maybe some oats/wheat or something.

3) Pitch plenty of yeast. I'd use a culture with lots of healthy Saccharomyces and lots of Brettanomyces, and maybe bacteria, right at the start of primary. This lets Brettanomyces character develop more quickly and helps you hit a stable gravity quicker. I always do this, and there are Brettanomyces aromas coming out of the airlock almost immediately. If you reuse this culture, you'll start selecting for things that get the job done quickly.

4) Pitch your culture low, but then let it rip, and warm it up a bit towards the end of fermentation, if necessary. That should get most of the sugars fermented pretty fast but without the off flavours you can get from pitching hot. Brettanomyces can often clear those up, but it takes time. The less mistakes you leave for your bugs to clear up, the sooner it will be ready.

5) Bottle as soon as you hit a stable gravity. If you do it right, your final gravity should be 0.998-1.002 pretty quick. If you bottle soon, I find you don't get any THP (presumably because there's enough healthy Saccharomyces around to ferment the priming sugar without producing it). It was only when I decided I wanted my beer to be a bit clearer, and so started leaving it in primary until it cleared, that I started regularly seeing small amounts of THP developing early in bottle conditioning.

For 4-5%-ish beers, the soonest I've started drinking was 1.5 weeks, but I've started in 2 weeks quite a few times (edit: 4 weeks sometimes). Typically, they hit the final gravity I want within a week, then I bottle them and they're properly carbonated in another week (sometimes less). They obviously develop and get "brettier" and more refined over time, but they're still good, and still funky that quickly.

Finishing Mixed Fermentations

Reusing a Sour Yeast Cake

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 [14]. In general, washing the yeast cake is not necessary.

Some brewers will harvest a certain amount of trub from their fermenters (500ml for example), and use only this amount to inoculate a new batch of beer. This will allow the brewer to control the amount of dead trub material that goes into the fresh beer. Michael Tonsmeire often advises that the brewer also pitch a fresh culture of Saccharomyces [15].

Some brewers have good success reusing a yeast cake or a portion of a yeast cake by leaving the wort in contact with the old yeast cake for 3-4 days. After 3-4 days, a fresh culture of Saccharomyces is pitched to finish the fermentation. The 3-4 day head start gives the souring bacterias a head start and results in a low pH beer. The harvested yeast cake can also be pitched at the same time as a fresh culture of Saccharomyces, or afterwards. The decided timing on when to pitch the harvest sour yeast cake will effect the outcome of the sour beer. Early pitching of the sour yeast cake generally produces a more acidic beer, and later pitching generally produces a less acidic beer.

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.

Bottling and Kegging

See the Packaging page.

See Also

References

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