Lactobacillus up close. The critter that makes your beer sour.

Most of the time, when consumers stumble across a sour beer – be it a lambic, a Flanders or some other interpretation – the brewer intended it that way. They wanted to sour the beer so used one of the multiple methods available to them to do so – adding lactobacillus or acetobacter or adopting sour mashing or spontaneous fermentation. But sometimes, most unfortunately, an infection can happen to sour a beer unintentionally. Most often this happens to homebrewers, who have less control over sanitation and wild invasion.

But it can happen to professional brewers as well. It is never a good thing, and often indicates a bigger problem with the brewery, but it can happen.

The reason I am telling you this is because about two years ago, I bought a six pack of craft-brewed beer and noticed just the tiniest hint of souring. At the time it mostly just sharpened the beer, but I knew that was not an intended flavour component of the beer. So, I decided to set a bottle aside, just to see what would happen. I put it in my cellar, beside my Trappist Tripels and barley wines, and let it sit.

There is absolutely NO WAY I am telling you the name of the brewer, as that would be too unfair. But I will say it was a stout made by a Canadian brewery. At any rate, I decided to open the beer the other day and see what had happened over the past two years.

The aroma gives my first hint that the beer had morphed. Rather than a rich, roasty malt I get a sharp note of tartness. The first sip confirms my suspicions – this beer is now a Flanders Stout. The dominant feature is a clean lactic sourness – not vinegary or medicinal, but as a good Flanders should be. I don’t get much roast any more, but do pick up some chocolate, a bit of raisin and plum, and touches of nuttiness. The finish is much drier than the original beer. The sharp, smack-inducing tartness is what is this beer is about now.

I say it is like a Flanders Stout for a reason. It strikes me as a darker version of Duchess du Bourgogne or an Oud Bruin. The same tart profile and refreshing finish. In a way I think it is a better beer now than when I first tried it (at the time I found it rather uninspiring). It has an enticing complexity and an angular presentation. The key to making it palatable is that the sourness is clean – suggesting lactobacillus rather than some other bacteria that create a more vinegar impression.

I am well aware that this character was not designed into the beer. The brewer would be embarrassed to know this is what has happened to their beer.

But as accidental as it might have been, it turned out pretty well, in my mind. As a homebrewer I have that experience all the time. I intend to brew one style and it ends up being closer to a different style. I have the luxury of just relaxing, having a homebrew, and calling it something different. No one is wiser. A commercial brewer has no option; their beer is out in the world with all its flaws.

This is a good reminder to all of us that beer is a living thing. Sometimes it just decides to go its own direction, and there is nothing we can do about it except accept it and decide whether we like the outcome. That is why it is such a fascinating experiment for me. I am in completely intrigued by how a micro-organism took control of the beer – but not in a bad way. It just made something no one expected. Don’t you just love beer chemistry?

In this case, I like the 2-year-old sour beer better than the original. Maybe they should consider taking a page from homebrewers and just calling it a Flanders Stout. They will likely be able to charge more for it if nothing else.