Brewing beer with wild yeast and bacteria adds a new level of complexity to an already complex process. Making beer with these specialty cultures is less precise and much less predictable than brewing with a single yeast strain. The rewards however can be tremendous if a brewer has patience.
The most important factor to keep in mind is that these cultures take time to fully develop and do their jobs. A good lambic or sour style beer usually takes 1 to 2 years to fully develop with the Mixed Culture Souring Method. The temperature at which the beer is fermented and then stored will play a large role in determining how quickly the characteristic aromas, flavors and acidity develop.
WILD YEAST AND BACTERIA IN BREWING
In the traditional setting nature acts as the source of the desired cultures. Coolships and fermenters that are open to the air allow a host of microbes to enter the wort. It is the specific combination of these microbes and their sequence of activity in the fermentation that creates the unique and complex profile of lambic and sour beers.
The following is a list of the cultures involved in true lambic fermentations and the sequence of activity, from Jean Xavier Guinard’s book Lambic:
Enteric bacteria: 3 to 7 days
Kloeckera apiculata: 3 to 7 days
Saccharomyces species: 2 weeks
Lactic acid bacteria: 3 to 4 months
Brettanomyces yeast: 8 months
Oxidative yeasts: 8 months
Each one of these groups of organisms adds some character to the finished beer; however three groups in particular do the lion’s share of the fermentation and contribute the bulk of the flavor characteristics. Typically, it is impractical to try and manage all of these cultures. The enteric bacteria and the Kloeckera apiculata are not readily available and contribute the least amount of character to the final beer. The Oxidative yeasts are also not readily available and also contribute very little to the final profile of the beer. That leaves the Saccharomyces yeast, the lactic acid bacteria, and the Brettanomyces yeast.
The bulk of the available sugars in the wort are fermented by a Saccharomyces strain. In modern lambic brewing, the rate at which fermentation begins is much quicker than in a traditional lambic. The brewer has a large number of choices on how to begin the fermentation. The easiest choice is to use a prepared lambic culture that contains the Saccharomyces culture along with the lactic acid and Brettanomyces cultures. The other choice is to use any other commercially available Saccharomyces culture. If the prepared lambic culture (3278 Belgian Lambic Blend) is used, it is important to keep the primary fermentation temperature relatively low (68-72 °F/20-22 °C) in order to keep the lactic acid cultures in check. If the temperatures get too high, the bloom of the lactic acid cultures can inhibit the Saccharomyces cultures.
If the brewer chooses not to use a prepared lambic culture, then any Saccharomyces culture can be used for primary fermentation. The primary fermentation should be allowed to progress for about two weeks before adding the lactic acid cultures. This allows plenty of time for the primary culture to establish itself and complete the bulk of the fermentation.
When the main fermentation is complete and the Saccharomyces population in suspension begins to decrease, the lactic acid bacteria start to increase in population. If a prepared lambic culture was not used, this is the time to add the lactic acid bacteria. It is important to note that Lactic Acid Bacteria is very sensitive to even moderate levels of IBU. Keep IBU levels below 10. The lactic acid cultures responsible for souring a lambic beer are 5733 Pediococcus damnosus and 5335 Lactobacillus buchneri. The temperature of the fermentation should be allowed to rise to allow the lactic acid cultures to establish themselves. The sourness will continue to increase for up to 2 years.
The final addition to a lambic beer are the Brettanomyces yeast. The available cultures are 5112 Brettanomyces bruxellensis and 5526 Brettanomyces lambicus. If a prepared lambic culture has not been used, the Brettanomyces cultures can be added any time after primary fermentation is complete. The Brettanomyces cultures are slow growers that are able to ferment complex sugars that Saccharomyces is not able to utilize. These cultures do not add a significant amount of alcohol to the beer, but they are the primary contributors to the aroma of the finished beer. The characteristic horsey aroma and flavor are by-products of Brettanomyces metabolism. These cultures also produce large amounts of ethyl lactate and ethyl acetate along with some acetic acid. These cultures can remain active for 16 months.
The lactic acid cultures and the Brettanomyces cultures are both slow growing cultures that have complex nutritional requirements which makes growing and maintaining the cultures problematic. It is extremely important that brewers using these cultures understand that the beer will take 1 to 2 years to develop the desired characteristics.
A turbid mash is desired. To achieve it, high sparging temperatures of 200 °F (93 °C) are used to wash the sugars and tannins, in which the resulting wort product will preserve enough dextrins and starches that will nourish other microorganisms in subsequent phases.
Brettanomyces are not demanding in nutritional requirements which allows them to survive under minimal nutrient environments. The nitrogen source can be very low – as low as 6 mg N/L of yeast assimilable nitrogen (YAN). Without oxygen, nitrogen is required for subsistence as well for cell growth.
The amino acids Glutamine, Glutamate, and Aspartate are the first preferred source of nitrogen; other nitrogen sources will be strain dependent.
Depending on the brewing system at the brewery, if wort is cooled overnight in a coolship it may absorb 4 ppm. Additional oxygen is added when filling the barrels. During a Lactobacillus souring the oxygen level is lowered and for Brettanomyces during aging is also maintained at low levels.
Brettanomyces grow very slowly and required low oxygen levels. Many studies have shown that higher oxygen aeration will yield in greater acetic acid production and low ethanol production.
- Brettanomyces management in the brewery
To avoid cross contamination a rigorous and detailed CIP procedure should be establish at the brewery. It is recommended to have separate equipment to prevent contaminations and the use of heat or steam.
There are many different fruits that can be used to blend the acidity. The most common fruits added are cherries, raspberries, apricots, peaches, and grapes. Important factors to consider when selecting the fruits are:
- Sugar content:
Depending on when the fruit is added to the beer, the sugars present in the fruit – generally fructose and glucose – will feed the yeast or bacteria remaining in the fermentation during the addition.
There is a great variation of the kind of acids in fruits, which if added to the beer under fermentation will lower the pH.
- Cherries Malic Acid
- Peaches Malic Acid
- Apricots Malic Acid
- Raspberries Citric Acid
- Grapes Tartaric Acid
Tannins are extracted from the grain during sparging, hops during boiling, and fruit pulp which gives a pleasant astringency flavor to the palate. Fruit stems and pits will impart a harsh flavor to the beer.
- Sugar content: