Beer School : Catching the beer bug

In this month’s Beer School, Louise Crane peers down the microscope for a closer look at the friendly microbes behind so many brewing triumphs… and tragedies

Bacteria are the most fascinating, dual-personality organisms. One one hand there’s ‘bad’ bacteria that can enter your bloodstream, causing infection, septic shock, and death. Or, less morbidly, a couple of pus-filled pimples on your face. It’s these effects that makes us squeamish when we hear the names E. coli and C. diff. But then there’s ‘good’ bacteria. touted as “healthy” by yoghurt manufacturers; the more ‘good’ bacteria, the better it is for your gut. Brewers contend with many forms of both good and bad bacteria, warding off bad with antibacterial hops and good hygiene, and encouraging the growth of other bacteria for ‘good’, tangy sour notes to their beer.

Reflection Better

Three main types of bacteria are used when making beer: Lactobacillus, Pediococcus, and Acetobacter. Lactobacillus and Pediococcus are both classed as “Lactic acid-producing bacteria”, or LAB for short. “LAB are everywhere around us in nature, on our food, on our skin, on plants, on fruit, and more,” explains says Milk The Funk’s Dan Pixley, a group of brewing enthusiasts who have a wiki and podcasts to explore and explain the use of bacteria and wild yeasts to make beer.

There are many species of Lactobacillus including L. acidophilus (it creates the tartness of natural yoghurt), L. casei (often used for ripening cheddar cheese), L. helveticus (not named after the font, but an ancient region) and L. kimchi (don’t worry, this one is obvious, it’s named after the Korean fermented-vegetable food kimchi in which it’s found). Most cannot abide the antibacterial conditions hops create, but some have adapted and thrive in beer and breweries.

All Lactobacillus bacteria use sugars to make lactic acid, and some of these tiny, rod-shaped organisms can make acetic acid with access to fructose and oxygen. The acids they produce lower the pH of wort or beer within one or two days, and according to the adventurous, Somerset-based Wild Beer Co, these microcritters, “churn out a smorgasbord of flavours and aromas. The result is a brew that has all the complexity of a fine wine and a delicious sour zing.”

Brett Ellis, co-founder of Wild Beer Co. continues: “The bacteria create flavorsome acids that heighten our pallet and end up providing an astonishing canvas on which to layer other flavors such as fruits, spices and or herbs. Our house culture has been built up over the last five years, from the very first time I used chopped-up apples floating around in the wort. We’ve kept this apple beer as a starter, in much the same way as sourdough has a ‘starter culture’. The opportunity for creativity is limitless. ”

Wild Beer also works with the LAB genus Pediococcus. It too is rod-shaped and 90% of the sugar it gobbles up is made into lactic acid. You can taste its work in sauerkraut, which it produces by fermenting cabbage. Pediococcus species and strains are resistant to hop compounds up to a point. P. damnosus can ferment glucose, sucrose, galactose and even maltose in some strains, though this is not a big help to brewers, because the metabolite malic acid can harm the beer. Together with Lactobacillus, Pediococcus bacteria are reported to be responsible for about 70% of all beer spoilage incidents caused by stray microbes.

Oak barrels, Wild, sour

Pediococcus leaves behind a buttery flavour, particularly in young beers, which is down to the diacetyl it produces, a diketone that tastes like - you guessed it - artificial butter. Pediococcus bacteria are not able convert diacetyl to other, more discrete byproducts on their own, and in beers other than certain Scottish and English styles, the sweet, warm popcorn smell is considered a flaw. The other major danger of using Pediococcus is its ability to cause “ropiness” or “la maladie de la graisse”, aka fat, sick beer with filmy, gummy beta-glucan exopolysaccharides produced with sugar residues. They change the mouthfeel of beer, making it more viscous, and occasionally form gluey strands. Brettanomyces can save the day (and a spoiled beer) though, swooping in to break down exopolysaccharides and diacetyl following a period of rest. That’s why you’ll often find Brett and Pediococcus used together, “in symbiosis”, so that Pediococcus can get to work without ruining the batch, and to give Brett a fuel source.

The final form of bacteria to explore is Acetobacter. It’s less common than the previous two, and it converts ethanol into acetic acid, not lactic acid, which makes it an AAB not an LAB. Continuing the caps, It’s found in the SCOBY (symbiotic culture of bacteria and yeast) of live kombucha, a funky fermented tea drink, and since it’s airborne, even on the wind. One gust and it can infiltrate even the most meticulously sterilised equipment. You’ll find Acetobacter aceti, Acetobacter pasteurianus in barrel-aged beers, as well as the closely related AAB, Gluconobacter oxydans.

 It’s not often that a vinegar tang is desired in a beer, but when it is, Acetobacter is the microogranism that causes your mouth to pucker up. It can ruin a batch without careful management, but it needs oxygen to get to work, and in a brew’s primary fermentation, oxygen is in short supply while CO2 rules the waves. This bacteria is not usually a problem at that end of production, but when it comes to conditioning, kegging and bottling, it will quite happily turn all the lovely alcohol into vinegar. “It is difficult to keep its flavour to a small nuance once it starts to develop, because it can happen quickly. The main way of controlling it is to control how much oxygen is touching the beer,” explains Brett..


Flanders Red, a wine-like beer from West Flanders, Belgium is the most famous Acetobacter style. This ale is fruity, with plum, prune and raisin licking your lips and sometimes, if you’re lucky, a kiss of tart raspberry. It is aged for a year or more in wooden barrels or casks, and it’s at this stage that Acetobacter slips out of the old wood it calls home to produce its characteristic zing. By the end of what is a tertiary fermentation, acetic acid concentrations can be as much as 1600ppm, while the beer becomes a beautiful rose gold. Or, it’s all turned to vinegar, such is the risk of stashing away your filled, bacteria-laden barrels for years on end.

Lactobacillus are also involved in Flanders Red, but at the primary fermentation stage where they begin to grow from a small presence in the yeast slurry. Strains of L. delbruekii are the most common. They’re still present in the secondary fermentation, which lasts four to five weeks. L. plantarum, L. coryneformis and L. brevis are also along for the ride at this point, having been lurking since the end of the first fermentation. In light beers with a low sugar content, they rapidly slurp up the newly formed alcohol to produce lactic acid, and grow to become the dominant microbe over yeast. Lactobacillus is also the primary souring microbe in Berliner weisse and Gose, where its rapid metabolism of ethanol to lactic acid means a short aging time.

Pediococcus takes much longer to cause any dramatic lowering of pH, and this gives the primary yeast strain longer to complete its fermentation before it is killed off by the strongly acidic conditions. The bacteria is responsible for the funky aromas and flavours of lambic beers of Belgium. Lambics are the original ‘wild’ beers, not inoculated with any specific culture, but instead, the wort is left in shallow, open vessels known as coolships, to mingle with the air of the brewery and whatever microbes it might host.

Branded Barrel

These three bacteria general are by no means the only ones to take part in souring lambics. Once the cooling wort has spent a night open to the atmospheric yeast and bacteria’s mercy, it is pumped into oak barrels that are up to 100 years old, to ferment for as many as three years. For the first month, enterobacteria, including Escherichia, Citrobacter, and Pectobacterium produce 2,3-butanediol, ethyl acetate, higher alcohols, and acetic, lactic, and succinic acids. It’s only after this happens that LAB, primarily Pediococcus, come to the fore, along with Saccharomyces yeast. Brettanomyces finishes off, mopping up any gunk produced by Pediococcus during the main fermentation. Then it is ready to be drunk, or blended with one-year-old lambic and allowed to re-ferment in the bottle to produce gueuze. This beautifully-scented beer exhibits a markedly different aroma to lambic due to regrowth of Brettanomyces in the bottle - up sparkles flavors of champagne or fresh lemonade.

You might think that by allowing wild yeast and bacteria entry, breweries face a problem with consistency. But batch after batch, lambics show similar successions of microbial communities, evidence that breweries must have their own unique micro-organism identity. Brett volunteers: “Consistency is important in wild brewing and blending, but if you try to force your wild ale through your ‘pale ale or lager’ expectations of stability and predictability you will hit a wall. Wild beer is a square peg and standard beer stability is a round hole! Every blend of our wild beers will be different, but, for example, with our Tepache (brewed by Stu Winstone), we are able to clean our Brettanomycess and L. plantarum cultures off of our equipment easily and get repeatable results on the next batch. Having the right tools to monitor the beer is vital, and we are continually investing in our lab and sensory training.”


When brewing with bacteria (and wild yeasts), cleanliness is next to godliness. Brewers need to clean in regimented fashion, and occasionally will double up on equipment, in the apt words of Brett, “much like a chef uses different chopping boards for vegetables vs raw chicken.”

How is this different for home-brewers, who cannot use the industrial cleaners that breweries have access to? Dan Pixley proposes that “mechanically speaking, there isn't a big difference in using bacteria at home versus in a commercial brewery to make sour beer. All it takes is a microscope, some agar plates, and knowledge of some simple aseptic techniques. Home-brewers can use the same methods of extensive cleaning and sanitizing, boiling any stainless equipment, and using separate hosing to handle beers that contain living lactic acid bacteria. There is a process called ‘kettle souring’, where LAB is added to the wort in a kettle and left to ferment. The soured wort is then boiled to pasteurize it, killing the lactic acid bacteria. This method protects the downstream equipment from contamination, and is a popular process for both homebrewers and commercial brewers.”

“It is impossible to say exactly how many species of lactic acid bacteria are being used in commercial breweries,” says Dan, “but it’s over a dozen. In the US, Carolina Bauernhaus Ales in Anderson, South Carolina cultures wild bacteria from fruit trees or flowers. Blue Owl Brewing in Austin, Texas uses bacteria found on the same malted barley that is used to produce their beer.” Beers less familiar to Western palates include tchoukoutou of Benin, tchapalo of Côte d'Ivoire, pito in Ghana, Togo, and Nigeria, and dolo in Burkina Faso.

For 99% of the beers on this planet, Saccharomyces is the sole microbial component, and any deviation is considered a flaw. But with a record number of sour beers being entered into the Great American Beer Festival; more and more international beers appearing in bottle shops (and Beer52 boxes); and the likes of Wild Beer Co. and Little Earth Project in the UK, the results of the occasionally face-imploding sourness of bacteria are worth trying - if you haven’t already.

Did you know?

Lactic acidification of the mash can improve the extraction, fermentability, and nitrogen yield of wort and the foam stability, color, and flavor of beer.

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