Walk into any self-respecting bar these days you’ll be sure to find a sour beer on tap or in the fridge. While sour beer is arguably one of the oldest styles around, it has come full circle and is now the newest trend to hit the beer scene. Many breweries have been producing world class sours for years, such as Boon, Cantillon & Rodenbach. But due to the rise in popularity of sours in recent years, breweries have seen a gap in the market and opened up shop as exclusive sour only breweries, combining old word and new world techniques, The Rare Barrel and Inoculum are an example of this. But what exactly are sour beers? How are they made? And what makes them sour in taste?

The purpose of this two part article is to hopefully give you some insight into how sour beer is made. The most practical way to approach the process of how these beers are made is to understand the yeast and bacteria behind them, they are the stars of the show. Knowing a brief bit about them will help you get a grasp of why breweries are picking certain yeasts or bacteria and how they put them to work on their beer to give them their signature taste. There are four main players in sour beer anatomy and they are as follows:

Saccharomyces: Most commonly known as brewers yeast, this is a spore forming yeast from the Saccharomycetaceae family, that can be broken down into two main strains for brewing:

Saccharomyces Cerevisiae (ale)

Saccharomyces Pastorianus (lager)

This yeast grows aerobically on glucose and maltose and produces Co2 and ethanol as a byproduct. In beer making, this yeast feeds off the sugars extracted from the malt during the mash phase and is responsible for most beer styles available today, such as IPA, lager, bock and wheat beers. Strains of saccharomyces separate themselves by producing different esters and polyphenols, for example, hefeweizen strains produce more isoamyl acetate (banana aroma) whereas Belgian pale ale strains produce more 4-vinylguaiacol (clove aroma). Unless it is branded as a wild ale or otherwise, every beer you come across will have been primary fermented from a strain of saccharomyces.

Brettanmoyces: This is a non-spore forming yeast also from the Saccharomycetaceae family that is most commonly found living on the skin of fruit and is commonly referred to as brett. It is a slow fermenter and can break down complex sugar chains that saccharomyces cannot, making it ideal for secondary fermentation or bottle conditioning. The combination of slow fermentation and being able to eat sugars saccharomyces cannot, means brett can ferment beer in the bottle and change its character, making the beer more complex if cellared for a few years. Brettanmoyces can be broken down into the following strains for brewing:

B. bruxellensis – Used by Orval for bottle conditioning

B. lambicus – Used in lambic style beers and Flanders red

B. anomalus – A lesser used strain, that delivers subtle brett character and fruitiness.

Over the years brett has become more popular in brewing for its unpredictability in flavours while fermenting. Depending on the strain and conditions, brett can deliver earthy, barnyard, fruit and tart flavours to beer, but unless it is intended to be added, it can infect and ruin a batch of beer. It is notoriously known for this in the wine industry, by living on the skin of grapes and infecting barrels of wine. Whats worse for wineries is that brett can feed off the sugars in wood, thus being able to live dormant in oak barrels leading to the wine and the barrel being ruined. While the wine industry sees brett in oak as undesirable, the brewing industry sees this as a desirable technique, ageing many different beers on the dormant brett in the barrel can lead to some unique and individual flavours from that barrel.

Lactobacillus: Is a bacteria from the lactobacillaceae family most know for its lactic acid production in sour beers and also referred to as lacto. It is an anaerobic microbe that gets to work fast and can sour beers within 24-48 hours and is know for delivering a very clean sour flavour. It does this by eating up the sugars and dextrin’s in the wort and produces lactic acid as a by product, this lowers the pH of the wort making it more acidic, thus, sour. Despite this, only two styles traditionally rely on lactobacillus as their primary souring microbe and they are Berliner Weisse and Gose. However, in todays brewing industry, kettle sours / quick sours are becoming more popular. These type of sours are made by transferring the wort from the mash tun to the kettle, adding lactobacillus in the kettle and keeping the temperature between 38ºC and 49ºC ensuring optimum health for the bacteria. Once the wort has reached the magic number of 3.5pH you are ready to boil off the lactobacillus and continue your brew as normal. As mentioned earlier, with lactobacillus, these can take only 24-48 hours to sour, and because of this they have been received with some controversy. Purists deem these as cheap sours made by breweries who do not want to commit months or years to make a complex and more traditional sour, but still want to jump on the sour trend and sell beer.

Pediococcus: Or pedio, is another anerobic bacteria from the lactobacillaceae family, that is known for its lactic acid production in beers, but unlike lactobacillus, it is a slow producer of lactic acid, taking months to get its desired flavour into beer. So why choose pediococcus over lactobacillus if they both essentially do the same thing? Well, pediococcus offers a more complex and harsher sourness than lactobacillus, creating more funky flavours that complement brettanmoyces. You will find in a lot of sours, brettanmoyces and pediococcus are used a lot together to create a more complex beer because they work so well together. But this is not the only reason you find pediococcus and brettanmoyces together. Pediococcus works very aggressively to create lactic acid, but as a result, also creates large amounts of diacetyl in beer, resulting in a buttermilk taste as well as making the beer ropey (mucus like consistency). With time brettanmoyces will clean up an remaining diacetyl, not only making the beer palatable again, but that much more complex.

Acetobacter: Not desirable in most sours as it produces acetic acid, this eats up the ethanol in beer and when it is finished, you are essentially left with vinegar. There are very few breweries that have a desire for this in their beers and I can only think of one, Brouwerij Verhaeghe in their Duchesse De Bourgogne beer. While I have not tasted it, a lot of reviewers have noted it having heavy acidic presence, vinegar, cider and sourness in the flavour.

Inside Sour Beers – Part 2 will follow up on this article talking about sour beer styles, techniques that make these beers what they are by style and how they use the yeast and bacteria that we talked about in this article, to control what they are making.