By Dr. Brian A. Nummer
Usually considered an unwanted infectious pest in the brewhouse, lactic acid bacteria can be harnessed as an exquisite ingredient for selected beer styles.
Since the days of Pasteur, lactic acid bacteria have been scorned, living at the periphery of civilized brewing society under the label “spoilage organisms.” Few brewers purposely invite these bacteria into their breweries to aid in the fermentation of their wort. In fact, most brewers take special care to eradicate any possibility of bacterial growth. Yet, several beer styles from Belgium and Germany are made using these bacteria to produce a tasty, characteristic sourness. Can these bacteria be all bad?
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The percentage of acids, primarily lactic and acetic, in a particular beer determines its sourness. Some beers have just a hint of tartness; others are overpoweringly sour. German Berliner Weisse and several Belgian beer styles are characterized by their sourness. Each style has a different level of sourness, and even within the same beer style this level varies. Fermentation with lactic acid bacteria is not an exact science, and one brand of beer may have different levels of sourness from batch to batch or from year to year.
Examples from Germany: German Berliner Weisse originated in Berlin and Bremen, Germany. A wheat beer made from 30–60% malted wheat, Berliner Weisse is produced using one of two different processes. In the traditional process, unboiled wort is fermented with a mixture of Brettanomyces ale yeast and heterofermentative lactic acid bacteria. After the main fermentation, the beer is blended with kräusen and bottled. Secondary fermentation and maturation then take place in the bottle for up to two years. A commercial example of this style is Berliner Schultheiss. This style is characterized by an intense, vinegary sourness, caused by lactic and acetic acids, which is complemented by an ester fruitiness from Brettanomyces.
A contemporary process (frowned upon by traditionalists) for making Weissebier was developed probably as a response to the unpredictability involved in brewing sour beers. Compared to traditional Berliner Weisse, this modern version is less sour, presenting only lactic acid and no esters. In the contemporary process, the starting gravity is low, at 7–8 °P (O.G. 1.028–1.033). The wort is split in half; one half is inoculated with homofermentative Lactobacillus sp. and the other half with ale yeast (Saccharomyces cerevisiae). Each is kept in a separate vessel and allowed to ferment at 59–77 °F (15–25 °C) for four weeks or more. After fermentation, the two parts are blended, filtered, and transferred for secondary storage at 41–50 °F (5–10 °C) for 13 months. The beer is bottled with a fresh addition of kräusen that includes yeast (no lactic acid bacteria). Once refermented in the bottle, the beer is ready to consume. A commercial example of this style is Berliner Kindl Weiss. Both of these beers are often served with sweetened raspberry or woodruff syrups to balance the acidity.
Examples from Belgium: Belgian sour beers include Iambics, witbiers, and Flanders styles. Lambic beers are spontaneously fermented wheat beers from the Brabant and Brussels regions of Belgium. Lambics provide a fascinating field of exploration for the inquisitive microbiologist.
Lambics: Lambic fermentation involves a complex sequence in which yeast strains and bacteria pass into and out of active phases. After cooling the wort, yeast and bacteria are allowed to “fall” into the open fermentors. Certain bacteria and the yeast Klockera, the most competitive, thrive for 2–3 weeks. Species of Saccharomyces then perform the main alcohol fermentation for 3–4 months. After that, lactic acid bacteria, dominated by Pediococcus damnosus, provide lactic acid fermentation for 4–5 months. This is followed by the Iambic yeasts of the genus Brettanomyces. Pediococcus bacteria and Brettanomyces yeasts ferment the complex sugars left behind by the other species. Several other bacteria and yeasts play minor roles. The process is a dynamic ebb and flow of activity from all of these various agents, each one present from the beginning of fermentation but becoming active only when conditions become favorable thanks to changes in pH and other factors resulting from another agent’s activity. The result is the wonderfully complex blend of flavors of Iambic beer. For a detailed look at Iambics, consult Jean-Xavier Guinard’s book, Lambic Beer. For brewing Iambics, he recommends the addition of three important cultures: an ale yeast, followed by Pediococcus damnosus and Brettanomyces.
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The Lactic Family |
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Lactic acid bacteria are Gram-positive, non–spore-forming rods (or cocci), and they are obligate fermenters. An obligate fermenter is a microorganism that must ferment a carbohydrate source. They do not grow aerobically. Lactic acid bacteria include Lactobacilli and Pediococci. Pediococci are microaerophilic; that is, they prefer an atmosphere with low oxygen levels. They grow poorly or not at all in air. The small rods often appear in groups of four (tetrads). In the past, Pediococcus was categorized as a beer sarcina (a term that included the anaerobic lactic-acid, beer-spoilage organism sarcinia). The most important species to brewers is Pediococcus damnosus, formerly called P. cerevisiae. The taxonomy has again changed, and this organism is now classified as P. acidilactici. Lactobacilli are facultative anaerobes — they can ferment in the presence or absence of air, but prefer reduced oxygen levels. Lactobacilli are also rod-shaped. One important species to brewers is Lactobacillus delbrückii. The species name delbrückii came from Max Delbrück, a founding father of microbiology, who characterized bacteria in the sour beers of Berlin (Berliner Weisse). Besides L. delbrückii, several other Lactobacilli find their way into beer, including L. brevis, L. acidophilus, and L. lactis. Some lactic acid bacteria produce diacetyl in quantities high enough to contribute to the flavor and aroma profile of the beer. Diacetyl is associated with a buttery or butterscotch flavor. In some beers this flavor in unacceptable, but for some others it is desired. Diacetyl is produced from the decarboxylation of acetolactate. Presumably, high levels of acetolactate are attributable to high levels of lactate, an ester of lactic acid present in lactic acid bacteria fermentations. Diacetyl has a taste threshold of 100–140 mg/L. Pediococci usually produce levels above the threshold, whereas Lactobacilli produce levels just below the threshold (Table I). Some lactic acid bacteria produce dextrans (long polymers of glucose). Dextrans can cover the surface of wort or beer, or form “ropes.” They are visually unappealing, but otherwise harmless, and they can also be useful in protecting against oxidation. Pediococci can produce a diffuse dextran on the surface that can appear shiny. The dextran is easily mixed back into the beer, but will reappear a short time later. In mixed-culture fermentations, the other cultures present will break down these dextrans. Some wild yeasts produce dextrans of their own. Lactic acid bacteria are divided into two categories based on their by-products of fermentation. The homofermentative group produces primarily lactic acid, whereas the heterofermentative group produces lactic acid, acetic acid, ethanol, and carbon dioxide. Either group will cause spoilage in beers in which the bacterial by-products are not desired. The homofermentative Lactobacilli and Pediococci and heterofermentative Lactobacilli each have different flavor profiles. The homofermentative Lactobacilli produce almost exclusively lactic acid. Pediococci are also homofermentative, but they also produce diacetyl, adding a buttery flavor. Last, the heterofermentative Lactobacilli produce both lactic and acetic acids (Table I). |
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Table I: Characteristics of Beer Worts Fermented by Various Cultures* |
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Culture |
Ethyl Acetate (mg/L) |
Acetic Acid (mg/L) |
Lactic Acid (mg/L) |
Diacetyl (μg/l) |
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DL-lactic acid added to beer |
18 |
95 |
370 |
12 |
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P. damnosus |
22 |
90 |
420 |
150 |
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L. brevis (heterofermentive) |
24 |
250 |
370 |
90 |
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L. brevis and P. damnosus |
22 |
595 |
490 |
140 |
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S. cerevisiae and P. damnosus |
24 |
85 |
460 |
120 |
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S. cerevisiae and L. brevis |
24 |
520 |
410 |
90 |
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S. cerevisiae, P. damnosus, and L. brevis |
23 |
360 |
490 |
110 |
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S. cerevisiae |
18 |
80 |
150 |
25 |
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* Source: Reference 4. |
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Witbier: Witbier is a tart, dry, acidic wheat beer from the Brabant region of Belgium. It is traditionally made from wheat malt and flour, barley malt, and oat flour. Perhaps the most famous witbier is Hoegaarden Wit from the town of the same name.
Traditionally, witbiers were intensely sour. Over the years, they have mellowed, probably in response to consumer preferences. Witbiers are often fermented with lactic acid bacteria and brewing yeast mixtures. Wort for Celis Wit is said to be first pitched with Lactobacillus. The process is arrested at a pH of 4.4 (presumably by filtration), and then yeast is added.
Flanders styles: Flanders-style beers are definitely the most eclectic of the sour beers. They range from brown to red in color and are characterized by a distinct sourness caused by both lactic and acetic acids. Most of the breweries making this style produce several beers from a single wort by blending, diluting, or adding fruit. Two extremes within this style are brown beers such as Liefmans (Brouwerij Liefmans, Oudenaarde, Belgium) and red beers such as Rodenbach (Brouwerij Rodenbach, Roeselare, Belgium).
Liefmans brown beer, from East Flanders, is brewed with Pilsener and caramel malts and boiled for what was once an all-night-long simmer but today is a standard 90–120 minute boil. (Unverified sources suggest the overnight boil was due to inferior equipment and not to brewing eccentricities.) The result is a brown-hued wort of original gravity 1.048. The brewery’s select culture of yeast is said to contain lactic acid bacteria. Liefmans is fermented and aged in stainless steel for up to a year. The beer is filtered, blended, pasteurized, and sweetened before force-carbonation and bottling.
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All-Grain Wort-Souring Recipe |
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Sour Wort Witbier (Makes 5 gallons) Ingredients 3 lb Pale malt 3 lb Wheat malt 2 lb Flaked wheat ½ lb Oat flakes Saaz hops (or other Noble hop), 10–15 IBUs ½ oz Coriander ½ oz Curaçao orange peel LactoCapsules (Head Start, Aeonbräu, Athens, Georgia), or Lactobacillus culture 3 oz Corn sugar Original gravity = 1.048 Final gravity = 1.009 Procedure Combine malt grists and grain flakes with strike water to achieve 122 °F (50 °C). Rest 30 minutes. Raise the temperature to 149 °F (65 °C) for 45 minutes, to 158 °F (70 °C) for 15 minutes, and to 176 °F (80 °C) for mash-out. Lauter as usual, and expect a starch haze. Add hops and boil for 60 minutes. Add bitter (Curaçao) orange peel and boil for 15 minutes. Add crushed coriander seeds and boil for the remaining 15 minutes. Cool. Inoculate with the contents of two LactoCapsules or with an 8 oz starter of homofermentative Lactobacillus. Allow the wort to sour at the temperature recommended for the lactic acid culture. When desired sourness is achieved, boil the wort for 10 minutes to kill the lactic acid bacteria. Cool. Add a starter culture of witbier yeast. Allow to ferment at 64–75 °F (18–24 °C) for one week. Age at 53–59 °F (12–15 °C ) for three weeks or more. Bottle with 3 oz corn sugar for priming (or 1 tsp/12 oz bottle). Condition as usual. |
Rodenbach’s red beer, from West Flanders, is brewed with several varieties of malt and corn grits to achieve an original gravity of 1.045. The red color derives from Vienna malts and oak barrel tannins. The beer is fermented with a mixture of yeasts, then aged in wooden casks for up to two years. Hidden in the cracks of the wood in these casks are lactic and acetic acid bacteria. The resident bacteria, in small numbers, provide acid fermentation over time. For some people, the resulting sourness is too intense; for these people, the brewery makes Alexander, a filtered, sweetened version.
Lactic acid bacteria provide a versatile tool for producing acid beers. Bearing in mind the need to isolate your lactic acid bacteria from standard fermentation yeasts — where lactic infections can be most unwelcome — several methods can be used to enhance certain beers with the sourness characteristic of lactic fermentations. (See the box, “The Lactic Family,” for a review of the flavor and fermentation characteristics of various lactic strains.)
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Malt Extract Split-Fermentation Recipe |
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Berliner Weibe (makes 5 gallons) Ingredients 3 1/3 lb Wheat malt extract Saaz hops, 6 IBUs Lactobacillus delbrückii, or other homofermentative Lactobacillus Berliner Weisse ale yeast 3oz Corn sugar Original gravity = 1.030 Final gravity = 1.006 Procedure Boil hops for 60 minutes in wort prepared from the extracts following typical brewing procedures. Typically Berliner Kindl Weisse is split 50:50. Split the wort in these proportions into two clean, sterile vessels. Inoculate the lactic wort with Lactobacillus. Ferment 2–3 weeks at 98 °F (37 °C) for Lactobacillus delbrückii or at optimal temperature for the culture being used. Ferment the remaining wort with Weissebier ale yeast at 59–77 °F (15–25 °C) for 2–3 weeks. Boil the lactic-fermented wort for 10 minutes to kill the lactic acid bacteria. This boiling step can be skipped, but live lactic acid bacteria will result in constant flavor changes as the beer ages. (The Berliner Kindl Brewery filters out the lactic acid bacteria.) Cool and combine this wort with the ale yeast-fermented wort. Transfer to the secondary and, if possible, store cool at 50 °F (10 °C) for one month or more. Bottle with 3 oz corn sugar for priming (or 1 tsp/12 oz bottle). Condition as usual. Drink as is, or add sweet fruit syrup, as desired. |
In pure cultures of sufficient biomass and at the optimal temperature for each culture, acid production is usually rapid. In worts fermented with both a lactic acid bacteria and a yeast culture, however, acid production is slower; both compete for the wort sugars, and yeast are typically much more prolific. If the lactic acid bacteria are added to the secondary — by which time all of the simple sugars have been used — they must scavenge for complex sugars, some of which they ferment through a slow process of acid production. This is the case in Iambics, where lactic acid bacteria proliferate only after most of the wort sugars have been used. It can therefore take many months to produce lactic or acetic acids at flavor threshold levels.
In addition, lactic acid bacteria are sensitive to hop acids and tannins. Hop acids are bacteriostatic, exhibiting a growth-restricting effect on many bacteria. Typically, Pediococci are less resistant to hop acids than are Lactobacilli. Experiments suggest that this feature is very strain-dependent. If a culture is very hop-acid sensitive, the methods used to acidify the wort must be altered to allow the lactic culture to ferment in a hop acid–free wort (ferment hopped and unhopped worts separately and then blend).
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Malt Extract Mixed-Fermentation Recipe |
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Me Mumm’s Sweet and Sour Stout (makes 5 gallons) It is rumored that Guinness includes a portion of lactic- and acetic-fermented wort in its stout, possibly to duplicate the historical contamination of the beer casks with Brettanomyces yeasts. I discovered another stout called Mumme in Vienna, Austria, patterned after the original stouts of the 1500s in Braunschweig, Germany. Yes, stout actually originated in Germany. This stout was marvelously sweet and sour. I was overwhelmed. Within minutes, I planned a recipe to duplicate this beer. Ingredients 3 1/3 lb Stout-flavored malt extract 3 1/3 lb Wheat malt extract 1 oz Hallertauer hop plugs for aroma Flanders lactic culture (Head Start #320L; a heterofermentative culture that will produce both lactic and acetic acid, an essential component of the recipe). A suitable stout yeast or a Lactobacillus and Pediococcus mixture (I use Head Start #190 SnP) 8oz Lactose 3oz 3 oz Corn sugar Original gravity = 1.050 Final gravity = 1.011 Procedure Prepare two 8-oz starters, one with the lactic culture and one with the yeast culture. Boil and cool the extracts as usual, adding the aroma hops in the last 5 minutes. Inoculate the wort with both cultures. Ferment at 59–68 °F (15–20 °C), or as close to that temperature range as possible. Allow fermentation to continue for 3–4 weeks to allow for the slow growth of the lactic culture. Transfer to secondary and age for one month or more. Before bottling, boil 8 oz of lactose in 2 cups of water. Transfer beer to bottling vessel leaving the lees behind and stir in lactose solution to taste. Bottle with 2 oz corn sugar for priming (or ¾ tsp/12 oz bottle). Condition as usual. Note: The culture 320L is a heterofermentative culture that will produce both lactic and acetic acid; it should not be substituted for another culture. |
To the short-cut brewer, adding 2–5 mL of food-grade, 88% lactic acid per gal of wort appears to be the simplest and most controllable method. Because of the lack of other fermentation by-products of lactic acid bacteria, however, this method produces a harsh flavor. Some brewers describe the taste of lactic acid as medicinal. With a little more time and effort, lactic acid can be easily produced with more appealing results through bacterial fermentation.
One of three methods can be used: wort souring, split fermentation, or mixed fermentation. The first two methods are recommended for rapid acid production; in mixed fermentation, however, lactic cultures compete with other cultures, which can slow acid development or produce erratic results.
Wort souring: Wort souring evolved from the sour mash, a simple technique by which a normal mash rests overnight or longer at warm temperatures to allow for growth of resident bacteria that sour the wort. The problem with the sour mash technique is its tendency toward excessive mold propagation, which alters the mashing procedure, and thus the wort sugar profile (in terms of complex versus simple sugars). Prolonged mashes at low temperatures heavily favor simple sugar production and consequently produce thin beers.
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Commercial Sources of Lactic Acid Bacteria |
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The following companies are the primary suppliers of lactic acid bacteria in the United States: |
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Aeonbräu 256 Cherokee Ridge Athens, GA 30606 Tel. 706/548-7051 Fax 706/549-0013 |
G.W. Kent, Inc. 3667 Morgan Rd. Ann Arbor, MI 48108 Tel. 313/572-1300 Fax 313/572-0097 |
Yeast Culture Kit Company 1308 W. Madison Ann Arbor, MI 48103 Tel./Fax 1-800-742-2110 |
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Brewers Resource 409 Calle San Pablo #104 Camarillo, CA 93010 Tel. 1-800-827-3983 Fax 805/445-4150 |
Williams Brewing P.O. Box 2195 San Leandro, CA 94577 Tel. 1-800-759-6025 Tel. 510/895-2739 Fax 510/895-2745 |
Wyeast Laboratories, Inc. P.O. Box 425 Hood River, OR 97401 Homebrew supply: Tel 1-800-321-0315 Large volumes: Tel. 541/354-1335 Fax: 541/354-3449 |
The wort souring technique can be used with any mash scheme. After mashing, lauter according to your normal technique. If you want a beer with body, boil the wort for 60–90 minutes to destroy the amylases as well as to extract hop bitterness. If you want a beer with a thin body (such as for Berliner Weisse), then skip the boiling step. When the wort has cooled to below 98 °F (37 °C), inoculate with a lactic acid bacteria starter. Alternatively, add freeze-dried Lactobacillus (such as LactoCapsules, Head Start Brewing Cultures, Athens, Georgia), which can be added directly to the wort without the fuss of a starter. Allow the wort to ferment for 24–72 hours. The length of fermentation depends on the level of acidity desired; more time equals more acidity. Smell and taste the wort for sourness. When the desired acidity is reached, boil the wort 10–15 minutes. Boiling the wort at this point will kill the lactic acid bacteria. Cool to pitching temperatures and proceed with normal fermentation techniques (add brewing yeast [s]).
Split fermentation: Split fermentation is a method that splits the batch of wort into separate vessels in a manner similar to the method used in Berliner Weisse production. One container is inoculated with brewing yeast. The second is inoculated with a culture containing lactic acid bacteria. Depending on the intensity of sourness you want, you can vary the portion that is inoculated with lactic acid bacteria. Generally, a 50:50 split is a good starting point; ratios can range from 20:80 to 80:20. After the fermentations are complete, boil the sour beer 10–15 minutes, cool, and blend with the normally fermented beer. Allow the yeast to finish off any unused sugars from the lactic-fermented wort.
Split fermentations offer the following advantages: Fermentations can proceed at two different temperatures (warm for the lactics and cool for the yeast); the lactic portion can be boiled to kill the culture and terminate further acid development; and the two batches can be blended to taste to achieve a desired level of sourness.
Mixed fermentation: Mixed fermentation is normal fermentation with a mixed population of lactic acid bacteria and yeast. Most traditional sour beers are produced in this manner, with a starter of this mixed culture added directly to the wort. The natural fermentation characteristics of the mix take over. An alternative is to add the yeast culture to ferment the simple sugars, and then add the lactic acid bacteria after 5–10 days (such as in industrial Iambic brewing).
The advantage of mixed fermentation is its simplicity. The obvious disadvantage is the difficulty in controlling it, and results are often erratic. The amount of sourness will depend on the action of the lactic acid bacteria in the mix (its competitiveness) and the environment in which it is fermented.
Lactic acid bacteria can be obtained from several sources; however, only strains that are considered “beer spoilage organisms” will actually grow in wort. It is therefore not recommended to use yogurt, sourdough bread, or similar cultures. These cultures simply are not adapted to life in wort. You may be successful culturing lactic acid bacteria from bottle-fermented sour beers. Lactic acid bacteria, however, frequently die in aging bottled beers, and this procedure is rarely successful. Representatives of lactic acid bacteria should be taken only from the freshest bottles. An alternative is to purchase or make mixed cultures by combining a lactic acid culture and a selected yeast culture. Many homebrew suppliers carry Pediococcus. Few, if any, however, carry Lactobacillus strains. Lactic cultures for brewing can be obtained from various yeast suppliers, including the author at Head Start Brewing Cultures (see box, “Commercial Sources of Lactic Acid Bacteria”).
Culturing lactic acid bacteria, compared to yeast, requires special media, temperatures, and incubation.
Media: Lactic acid bacteria are preferably grown on or in MRS (modified De Man, Rogosa and Sharpe) agar or broth. This medium contains several complex nutrients that provide vitamins and amino acids. The medium is prepared from dehydrated powder available from culture suppliers or in large quantities (500 g) from microbiological media sources.
Temperature: L. delbrückii requires an incubation temperature of 98 °F (37 °C). It can be tricky to obtain 98 °F (37 °C) without proper equipment (other cultures are not so temperamental and can be incubated at room temperature). Jury-rigging for temperature control is a matter best left to the brewer’s imagination.
Incubation: In liquid (broth) media, the culture simply grows as a white sediment at the bottom of the container. Homofermentative lactic acid bacteria are not fermentative and consequently evolve no gas. (Heterofermentative lactic acid bacteria will produce some carbon dioxide.) A distinct odor shift from sweet to sour is another sign of activity. Since these organisms favor low levels of oxygen, shaking the medium to incorporate air is detrimental.
When incubating an agar slant or plate, the culture requires a reduced oxygen content to grow. A near-perfect environment is obtained by placing the slants or plates into a gasketed, sealable crock or jar. A tight seal is required. Before sealing the cultures in the container, place inside a shot glass half filled with water. Drop in two Alka Seltzer tablets, then quickly seal the container. The carbon dioxide that evolves provides the preferred atmosphere. Incubate cultures for 3–10 days.
To make a starter culture of lactic acid bacteria, begin by combining 2 tablespoons of dried malt extract, 2 g of MRS medium powder, and 8 oz of water (tomato juice [10%] may serve as an alternative medium if MRS is unavailable). Boil 10 minutes to sterilize. Pass the starter through a coffee filter to remove trub, if you like, and then briefly reboil it. Pour the starter into a clean and sterile container. Cover with a sterile lid and allow to cool. Once cool, inoculate the starter with a lactic acid bacteria culture and seal the container.
As mentioned earlier, heterofermentative lactic acid bacteria produce some carbon dioxide and should not be sealed tight. Allow the culture to grow until a visible white sediment forms (as opposed to trub, which is more yellow), or until the culture smells or tastes sour. Once sourness is detected, use the starter within a few days. If the culture will not be used immediately, store refrigerated and make new starter just before brewing. Lactic acid bacteria are finicky and fastidious; storage of just a few weeks can kill the culture.
Because lactic acid cultures accumulate acid in their medium, they are difficult to keep and die quickly in storage. As with Brettanomyces, the incorporation of 1% calcium carbonate (CaCO3) to buffer the acid in the medium is advantageous, and the use of only 100% MRS medium for storage is recommended. Transfer cultures monthly.
To prepare frozen cultures for backups, add sterile glycerol to a final volume of 10% to a densely populated, actively growing lactic acid bacteria culture. Quickly freeze the mix in the main compartment of a freezer (avoid the door area and self-defrosting freezers, because of higher temperature fluctuations in these areas). Properly frozen cultures remain viable from several months to several years. To revive the culture, remove it from the freezer, thaw it, and inoculate a starter culture with the entire contents. Avoid refreezing.
Although challenging, the use of lactic acid bacteria opens the doors to replication of some very interesting traditional beer styles that otherwise go unexplored by craft brewers. Most notable among beer styles suitable for souring are those from Germany and Belgium. With a firm foundation in the behavior and handling requirements of lactic acid bacteria, craft brewers can successfully produce exceptional beers of unique character.
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