Lesson 6: All Mixed Up

Prior to technological advancements in the mid-nineteenth century, all beer was to some degree, sour. Poor sanitation and naturally occurring yeasts and bacteria were to blame for infecting beer, giving it an undesirable sour quality. The advent of pasteurization and refrigeration really changed the beer game, allowing for clean, crisp beers to take the main stage.

Although it’s a time-honoured tradition in Belgium, the demand for sour beer is a relatively new concept in North America. In craft brewing today, sours are generally made through a process called mixed-fermentation. As opposed to single-culture fermentation, where a single strain of yeast is used, mixed fermentation involves a blend of yeast and bacteria strains that work symbiotically to convert sugars into alcohol. Most often, a yeast strain called Brettanomyces is combined with two types of bacteria, Lactobacillus and Pediococcus.

Brettanomyces, or Brett, is a sub-category of yeast, considered to be “wild” because it grows naturally on fruit skins. Brett works to convert available sugars into alcohol, as well as producing fruit esters such as pineapple, stone fruit, honey etc. or funky phenols that can include barnyard, horse blanket, hay, or medicinal flavours.

There is a common misconception that Brett makes beer sour, but it’s actually the addition of the bacteria that creates the tart and acidic quality in beer. Lactobacillus or “Lacto” works to increase its acidity, leaving a clean, sour taste. Pediococcus “Pedio” works in much the same way as Lacto, but can also add unwanted funky phenols to the beer.

Off-flavours may present after primary fermentation. The symbiosis of yeast and bacteria is particularly evident in secondary fermentation, when the Brett continues to clean up beer, consuming dead yeast cells and metabolizing any undesirable by-products.

Brewing a sour beer is a complex process and working with wild yeast strains can yield some unexpected results. The good news is that with dedication of many talented craft brewers, there is no shortage of amazing and innovative mouth-puckering brews for us to choose.