It’s time to get a bit technical now. Most people outside brewing and baking circles probably think of yeast as a single substance, but that’s not the case. It’s a single-celled organism in the fungi family that has been around for hundreds of millions of years and has well over a thousand known species.

Yeast’s role in brewing is to eat up the sugar and convert it into alcohol. Carbon dioxide and other substances like esters and phenols (which offer aromas) are side-products, but they are such integral parts of brewing culture that they too are controlled and manipulated to give beers their distinctive flavours.

Normally, brewers use a type of yeast from the “Saccharomyces” group (there are many different types used, and they all bring different accents). As you might get from the name, its job is to gobble up sugars and produce the alcohol and side-products like gas and other chemicals.

By the way, the same process happens in bread – shout out to all the sourdough starters out there. The difference is that the gas is the desired product (to inflate the dough with air bubbles) and the side-product – alcohol – is driven off during baking. Never bake your beer, people. It’ll suck.

Now we come to a second type of yeast: Brettanomyces. It’s a very common type of yeast, and is often airborne, meaning it can get anywhere that isn’t kept hygienically clean, including breweries, which is one of the reasons they often look like surgeries. It’s sometimes called “wild yeast” or just “Brett”. Brettanomyces’ big difference is that it produces acetic acid (aka vinegar) when there’s alcohol and oxygen present. And if you recall the definition of sourness from above, it’s the taste of acidity.

So when a sour beer is brewed, there’s a blend of normal Saccharomyces yeast and a very tightly controlled amount of Brettanomyces. Because the Brett is absorbing some of the alcohol to turn into acetic acid, sour beers typically aren’t very strong, but there are exceptions.

Of course, this process is merely the science bit – biology, to be specific. Armed with this knowledge, it’s down to the brewers to work their magic by using specific blends, ingredients and techniques to produce the sour beers they’re aiming for.

Sourness isn’t necessarily a flavour people associate with beer, or any other food for that matter. The more popular chocolates are flavoured with milk and sugar, and coffee undergoes the same treatment. It’s all to mask the bitterness while making the more immediately enjoyable parts palatable.

But still, people do cultivate a taste for darker coffee and chocolate with higher cocoa concentrations. You’ll know who these people are because they will 100% tell you.

Beer is a little different, though. Beer is itself an acquired taste – it can taste very wrong at first, but once you’re used to it ), it’s a taste you can become accustomed to and learn to appreciate for its vast complexities and depth. Then a whole world of flavours – stouts, IPAs, lagers and so on – is opened up. You’d think that sour beers would be part of that journey.

Unfortunately, we go back to the Brettanomyces. Because they’re such an undesirable thing to have in a brewery, and can absolutely ruin a batch if left to their own devices, so their use is kept to a minimum, and only then under controlled conditions.

So while the world developed a taste for bitter hoppy beers, sour beer brewing was left to a distinct set of brewing artisans. The craft tended to be concentrated in the north west of mainland Europe – lambic and gueuze (Belgium) and gose (rhymes with poser) from the Goslar region of Germany.

Lambic and gueuze (which is actually a type of lambic) have an interesting manner of preparation. The wort is exposed overnight to the atmosphere, where it accumulates natural microorganisms and yeasts (Hi, Brett! – amongst countless other strains) which end up making it sour. For any other style, this would be classed as an infection, but in a controlled environment, brewers are able to fine-tune certain desirable characteristics of the beer, which is eventually blended together. Many treat this process with the same respect as wine in which the beer’s blender/brewer is using their attuned palate to mix barrels of lambic which complement one another. These beers are then almost always aged in oak where time works its magic. A gueuze is a blend of 1, 2 and 3 year old lambic, so unlike most beer styles, this requires an immense amount of patience.

Wheat beer gose is slightly different in that it gets its sourness from lactobacillus bacteria. The dominant acid here is lactic rather than acetic, but coriander and saltiness make the blend more complex. The salt originally came from the local water but it’s usually added nowadays. It also goes through a boiling process to kill the latent bacteria.

BrewDog’s spirit of adventure has brought us a handful of sour beers, which we urge you to acquire a taste for. Try Cosmic Crush Raspberry, a rich and juicy raspberry delicately balanced amidst Brett funk, tartness and tannin. Or go the extra mile with Liberated Libertine, a bouquet of roasted malts and dank hops which pours black as a Libertine's soul.