Commercial Sour Beer Dregs Inoculation

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Beer or wort can be inoculated with commercial sour beer that is not kettle soured or pasteurized. Generally, only the last half inch of a bottle's contents, including the sediment, is used. This portion of the beer is often referred to as "the bottle dregs". It is recommended that the microbes in the beer are first reinvigorated with a small starter wort of around 1.030 gravity before it is added to the fermentation vessel. Using commercial sour beers to ferment is generally a good idea because the microbes are often stronger and more aggressive from commercial breweries as compared to mixed cultures from yeast companies (this is a generalization). It is generally advised to use as fresh of a bottle of commercial sour beer as possible, however older bottles can be used as well depending on the brewery, beer, and how the bottle was stored.

General Methods and Uses

The following information is specific to collecting microbes from commercial bottles of mixed fermentation sour beers or Brettanomyces beers. For instructions on collecting S. cerevisiae from clean beers, check out this guide.

An often asked question in the Milk The Funk Facebook group is whether or not dregs should be added to a kettle sour. While there is no reason why it wouldn't produce a good beer (assuming the dregs are in good health), it defeeats the purpose of making a kettle soured beer. The primary advantage to kettle souring is the ability to make a sour beer without risking exposing the brewer's cold side equipment to potential brewery contaminants such as Brettanomyces and lactic acid bacteria.

Making a Starter or Not

There are several things to take into consideration when deciding to make a starter or not for bottle dregs. Since Brettanomyces doesn't need many cells to have an impact unless it is being used for 100% Brettanomyces fermentation, a starter is not necessary if the bottle is young and the brewer only wants to use the Brettanomyces. If the bottle is older (6+ months from bottling), it might be a good idea to make a starter just to make sure the Brettanomyces is still viable.

If the brewer is intending to re-use the Lactobacillus or other lactic acid bacteria in the bottle, then a starter is a good idea regardless of the bottle's age. In general, a larger population of lactic acid bacteria is beneficial if lactic acid production is desired. It is possible that smaller pitch rates of Pediococcus and hardier Lactobacillus strains might still sour the beer over time, but in general making a starter will help ensure that a healthy pitching rate of bacteria is used.

Creating a Starter

After prolonged time in a bottle, microorganisms won't be at their peak vitality, and so making a starter for dregs is recommended in general. Saccharomyces strains may or may not be viable at all after an extended time in a low pH beer, but Brettanomyces and some lactic acid strains are more acid tolerant and should be viable. Making a step starter is best practice for ensuring that a high enough population of microbes are pitched. Begin by making a ~200 mL DME starter of usual strength (1.030-1.040 SG) [1]. Hops should not be used unless the brewer wants to try and suppress the Lactobacillus in the commercial beer (Lactobacillus strains for some commercial breweries are fairly hop tolerant up to 20-25 IBU. See Lactobacillus Hop Tolerance for more information). If the brewer has only one bottle or wants to keep multiple bottle dregs separate from each other, the starter wort can be poured directly into each bottle and covered with plastic wrap. Alternatively, the dregs from multiple bottles can be combined into a single vessel (such as an Erlenmeyer flask or glass jug) with the starter wort, and covered with sanitized tin foil to allow an exchange of oxygen and CO2. The mouth of the bottle may be flamed with a lighter to kill any wild yeasts that might have landed on the area. The dregs don't need to be "washed" or treated in any special way before being added to the starter wort.

Although more experiments are probably needed, agitation is believed to be an important factor for any species of microbe (yeast and bacteria). Gentle stirring on a stir plate or orbital shaker, or frequent gentle manual agitation leads to faster growth and a higher number of organisms. Agitation keeps the microbes in solution. It also maximizes the microbes' access to nutrients and disperses waste evenly. In a non-agitated starter, the microbes are limited to the diffusion rate of nutrients, leading to a slower and more stressful growth [2]. Allowing for at least some oxygen to enter the vessel will help Brettanomyces grow, and limiting oxygen will help any surviving Saccharomyces to grow. For more information regarding aeration and agitation effects on Brettanomyces growth, see Mark Trent's Brettanomyces Propagation Experiment and Effects of Mixed Cultures on Growth.

The starter should be stored at room temperature for at least 5-7 days and sometimes longer if growth does not occur within that time. It can be stored at a higher temperature if bacteria is the main microbe that the brewer is after, although avoid going above 86°F/30°C. Monitor the starter for activity during this time to ensure that there are viable microbes. Visual activity may not always be present, may be brief, or minimal. Visual indicators of successful growth include turbidity and continuously produced bubbles on the surface of the liquid. Monitoring the pH and the gravity can assist the brewer with deciding whether or not the commercial sour beer has any viable microorganisms still alive in it. If the starter krausens within 2-3 days then it might have viable Saccharomyces, otherwise strains of Saccharomyces might be dead (especially if the beer was not freshly bottled and was a low pH sour beer). Brettanomyces may not show signs of fermentation for up to 3-7 days (in some cases and depending on the viability of the microbes, signs of growth may not appear until up to 2 weeks). Once the starter starts showing signs of fermentation, it should be aged for another 3-7 days to finish fermentation. After the fermentation starts to slow, create a another 1 liter starter of DME wort, and add the original 200 mL dregs starter to it. Repeat the process above, giving the 1 liter starter another 5-7 days of growth [1]. Pitch the 1 liter starter into a ~20 liter batch, or continue to step up the starter for larger batch sizes. Avoid storing the starter for extended amounts of time because a low pH could result in lower viability over time in an acidic environment; see Storing a Yeast Cake for more information.

Supplementing another fermentation

Once the starter of the bottle dregs has sufficiently incubated, it can be pitched into another sour beer. The idea here is that the strong microbes from the commercial sour beer can enhance the overall complexity of the beer in addition to whatever other cultures have been pitched.

Using as a primary fermenter

Bottle dregs can also be used a primary fermenter. It is generally advised that mixing different bottle dregs together will provide a greater diversity and thus better results, but using a single bottle may prove to be an educational experience as well. Step the starter up to a 2 liter volume, incubate for another 7 days as above, then pitch into a ~20 liter batch. Alternatively, Jeffrey Crane describes a method of using 1 gallon of wort, which can easily be produced on a normal brew day from a larger batch, to create sour beer from bottle dregs. Essentially this process involves transferring 1 gallon of wort any time during the boil when there are less than 7-10 IBU's. Another simple option is to make 1 gallon of wort from malt extract (DME or LME). That 1 gallon of wort is then used to pitch the sour beer dregs starter into.[3] The brewer has the option of allowing the 1 gallon of sour beer to age on it's own, or it can be used as a starter itself for a larger batch. Pitch a smaller amount of the dregs starter for 1 gallon batches (~200 mL).

Wort Souring, Co-pitching, and Staggered Pitching

If there was no signs of Saccharomyces fermentation in the dregs starter then another option is to pitch the starter of dregs into the wort, and allow for a day or two to pass before pitching a fresh pitch of Saccharomyces. This has the benefit of allowing the lactic acid bacteria in the dregs starter to sour the wort before competing with Saccharomyces for sugar. See souring in the primary fermenter for more information on this technique. Co-pitching the dregs with a fresh pitch of Saccharomyces at the same time will often achieve a less acidic final product depending on how aggressive the bottle dreg cultures are. Pitching a sour culture after primary fermentation with brewers yeast has been reported to preserve more Saccharomyces yeast flavor. See the "MTF Reverse Kettle Sour" for an example. Brettanomyces isn't greatly impacted by either pitching rate or if it is co-pitched with Saccharomyces or pitched after primary fermentation with Saccharomyces, so it doesn't matter if dregs containing Brettanomyces are co-pitched with Saccharomyces or if it is pitched after a primary fermentation with Saccharomyces. See Brettanomyces primary versus secondary for more information.

Storing Dregs For Later Use

It is often the case that a potential bottle dregs beer will be consumed before the brewing of the wort in which it would be added. There are several options and suggestions on storing bottle dregs beers [4].

  • Pitch the dregs directly into a fermenter that already has beer in it.
  • Have an air-locked vessel, such as a gallon jug or an Erlenmeyer flask, with an airlock on it with a starter beer (or fresh wort) ready to collect multiple bottle dregs.
  • If the brewer wants to keep the dregs separate from other cultures or does not have another vessel available as previously described, leave the last quarter inch of the beer in the bottle itself. Recap the bottle if possible, and place it in the refrigerator. If it is not possible to recap the bottle because it is a corked bottle or a wider diameter than the brewer's bottle capper/caps, cover the bottle with plastic wrap or tin foil and a rubber band. Keeping the beer cool should prevent spoilage, although this is not guaranteed without more sanitary procedures such as purging the bottle with CO2 and capping it. Although brewers have successfully stored bottle dregs beers like this for months, it is generally advisable to make a starter for the dregs at least within a few weeks so as to avoid any potential spoilage.
  • Brettanomyces remains more viable over time if it was co-fermented with S. cerevisiae than if it was fermented by itself (100% Brettanomyces beers). Contrarily, S. cerevisiae loses viability over time faster when it is co-fermented with Brettanomyces [5]. This is something to keep in mind when using dregs.

Potential Problems and Issues

Starter made from bottle dregs from Monte Subite Geuze. The green colored stuff is mold. Photo provided by Matt Fanning‎.

The Dreaded Killer Champagne/Wine Yeast

In general, wine yeast is not a concern when it comes to using bottle dregs (even when making a starter). Many commercial sour beers are bottle conditioned with fresh wine or Champagne yeast. If this yeast is still viable, it is possible that it could contribute to the fermentation profile if pitched during primary fermentation when simple sugars are still available, however if the Champagne/wine yeast is pitched after primary fermentation then it usually won't have a big impact. While killer wine strains can kill ale or lager yeast (killer Saccharomyces strains do not kill Brettanomyces or lactic acid bacteria), and potentially lead to autolysis off-flavors, reports of autolysis in aged sour beer are next to none, and in general autolysis is not a concern in beers containing living Brettanomyces since Brettanomyces can consume the compounds that are released during autolysis [6]. If pitched into a beer that has already fermented the maltose and glucose, wine and champagne yeast won't have a significant food source, and therefore won't have an impact on the beer as far as producing flavor byproducts (most wine and champagne yeast strains are not efficient at fermenting maltotriose).

Champagne/wine yeast could produce flavor compounds such as esters and thiols if it is introduced into wort during primary fermentation. When dregs containing viable Champagne/wine yeast are added to a starter, then that yeast will often grow into a healthier population, which could then have a large flavor impact from the Champagne/wine yeast if the starter is pitched into primary. If flavor contributions from bottling yeast are a concern (esters, thiols, etc.), then cooling the beer and leaving the sediment behind before building a starter may help to alleviate the problem, however, at least some the wine/Champagne yeast will probably still be in suspension in the beer. Dregs or a starter from dregs containing Champagne/wine yeast could be added after primary fermentation, which would eliminate any potential for the Champagne/wine yeast having a significant impact on the flavor as far as producing esters from fermentation. Additionally, the long term survival of many wine strains of Saccharomyces yeast is usually limited in a low pH sour beer, so if the bottled beer has aged for 6+ months then the chances of viable wine yeast being in the bottle are low [7].

Many brewers have had success using bottle dregs that were conditioned with killer strains of Champagne or wine yeast, and many consider it to be not a big concern due to the reasons previously mentioned. If the brewer wants to guarantee that only Brettanomyces and/or bacteria are cultured, then they must use isolation techniques to do so [8], or verify with the producer of the beer being used to see if Champagne or wine yeast was used for conditioning.

See the Packaging and Re-yeasting page for more details on killer wine yeast strains.

Potential for Mold Growth

Mold growth, although rare, is a possibility. If mold begins to grow, throw out all of the beer and start over. See Mold for examples of identifying mold versus pellicles.

How Old is Too Old?

A frequently asked question is, "How old of a bottle can I use to grow dregs?" This is ultimately an impossible question to answer, but the general advice is to try and use a bottle that is as young as possible, preferably less than one year old. Although Brettanomyces and lactic acid bacteria can survive for long periods of time in bottles, the conditions are still stressful on these organisms and not enough information on viability over time has been collected to answer this question for every given bottle of sour beer. Old bottles might not lead to growth in the starter. Signs of fermentation should be observed, including visual signs like cloudy wort, bubbles or a pellicle on the surface, and measured signs such as a drop in pH and gravity in the starter (measured signs of fermentation are better indicators of active fermentation than visual indicators). That said, Brettanomyces has been cultured from bottles of beer that are 10+ years old. For example, microbiologist Richard Preiss has been able to culture Brettanomyces from very old bottles of commercial German Berliner Weisse. Culturing microbes from very old bottles like these often requires agar plates and streaking techniques. If there are not enough viable cells in an older bottle of commercial sour beer, using a simple starter to grow them may fail because the starter may fail to ferment fast enough before other microbes such as mold take hold. If this is the case, for safety reasons do not consume the wort and consider growing the microbes from a different, fresher bottle. Using autoclaved or pressure cooked wort and jars might help with preventing other microbes from growing.

Is There a List of Viable Bottles?

The only list of viable commercial bottles that we know of is Michael Tonsmeire's list of unpasteurized commercial sour beers. Tonsmeire has stated that he will no longer maintain this list because of the effort needed to do so. For this reason, we also do not track unpasteurized commercial sour beers. If you are curious if a particular brewery pasteurizes or kettle sours their sour beer, it is recommended that you contact the brewery and ask them if the bottles dregs can be used to make sour beer. Most breweries will be friendly enough to answer your question.

See Also

Additional Articles on MTF Wiki

External Resources