Brewers can choose from a range of processes when souring a beer, but most techniques are based on the use of acid-producing bacteria. Lactic acid bacteria – a popular group of souring agents that include Lactobacillus and Pediococcus – are used to ferment the sugars in the beer, converting them into lactic acid. This simple process is similar to the method used for making yogurt or sauerkraut. Here are some souring methods that use these bacteria.

Acidifying the mash/wort

In this popular acidification technique, bacteria are added pre-boil to either the mash (a mix of grist and water) or the wort (a liquid solution extracted from the mash). In an air-tight tank, at the right temperature, the bacteria will acidify the sweet liquid in roughly 24 hours. The wort, once soured, goes through the normal brew process: boiling and fermentation.

This technique, generally known as "sour mash" or "sour wort,” works quickly and creates an up-front, simple acidity. Most summer sour beers – especially the fruit ones – are made this way. It’s also the traditional method used to brew two German beer styles: Berliner Weisse and Gose.

Simultaneous fermentation and acidification

In their ongoing quest to simplify brewing processes, various breweries and yeast laboratories have developed yeast and bacteria blends that can be used to ferment and acidify the beer at the same time. The process only takes a few days, and the degree of acidity varies for each blend. The up-front acidity produced is not very complex.

Post-fermentation acidification

The acid-producing bacteria can also be added post-alcoholic fermentation, in a stainless steel vat or a wooden barrel. This process can take weeks – even months – and usually yields a more complex result.

Barrel-aged beers are more likely to be exposed to oxygen, and to therefore produce acetic acid – the type of acid vinegars are made of. It’s nice in small quantities, but too much can be bad news. It’s the hallmark of oud bruin and Flanders red.

Spontaneous fermentation

This age-old technique combines two processes – acidification and fermentation – into one. What’s different is that the choices are left to nature, rather than made by the brewer. The surrounding yeasts and bacteria are deposited on the hot wort post-boil, as it is air-cooled.

Once the liquid reaches room temperature after several hours’ exposure, the mixture is transferred into a tank for fermentation – a process that takes one to three years. The tank also contains yeast, bacteria and micro-organisms in the thousands, and the final result is of unmatched complexity. This technique is used to make Belgian lambics, and is now being adopted by more and more North American – even Quebec – breweries that have decided to “go bold.”

Blending and acidification using fruit

A brewer looking to create a slightly complex beer with a hint of tartness may try a blending technique. For example, they may add 20% sour beer to a wheat beer or a saison to give it a little bite.

Acidic fruits, such as cranberries, sea berries or honeyberries, can also add an acid note. Once the sugar in the fruit has fermented, the acidity remains in the beer.

The world of acidic tasting beers is vast, and offers all sorts of potential. What’s more, the wide range of sour beer styles and flavours presents a new option for every occasion… and as we enter the warmer months, this brew is a must-try.

Written by Yan Lortie, Beer Sommelier