Sour beer

In most beers bacteria are indeed unwanted, because they can cause spoilage. In sour beers, lactic acid bacteria, and sometimes acetic acid bacteria, are among the desired microbes. They make the organic acids that make the beer fresh and tart.

That happens in a controlled way. With kettle souring the wort sours in the brew kettle in one to three days, after which the souring is stopped by boiling or by adding strongly hopped wort. Flanders red matures for up to two years in large wooden vats and is then blended with young beer.

The BJCP style guidelines also describe what is a fault. Berliner Weisse must never be vinegary. In Flanders red a little acetic acid is acceptable, but a dominant vinegar character is not. And in lambic and gueuze a clear vinegar or cider aroma counts as a fault among Belgian brewers.

Sourness is therefore a deliberate flavour choice. A slide into vinegar is what brewers see as a fault.

Traditional sour beers do indeed mature for a long time. Flanders red matures for up to two years in large oak vats (foeders). Lambic ferments and matures for one to three years on wooden barrels.

But there are also fast methods. With kettle souring, lactic acid bacteria sour the wort in the brew kettle, usually within one to three days. After that the souring is stopped and the beer ferments in the normal way. According to the BJCP, many modern commercial sour beers are made this way: quickly, and as an alternative to the laborious production on barrels.

In addition there are yeasts that make lactic acid themselves. With them, beer turns sour during fermentation, without bacteria and without wooden barrels.

In short: wood maturation belongs to some of the sour beers, not to all of them.

The BJCP describes Flanders oud bruin as a malty, fruity, matured and lightly sour brown beer from East Flanders, with notes of caramel, toffee, chocolate or raisin and often a sweet-and-sour profile. The guideline explicitly adds that it has nothing to do with the dark, sweet Dutch beer of the same name. It is also something different from lambic: Flanders oud bruin is not fermented spontaneously and contains no wheat.

In the Netherlands 'oud bruin' is even a legally defined designation. Under the Warenwetbesluit Gereserveerde aanduidingen (art. 7e), that name is reserved for sweetened, dark-coloured beers with an original gravity extract of 7 to 11%.

A Dutch oud bruin is therefore something quite different from a Flanders oud bruin, even though they share a name.

The Voedingscentrum (Netherlands Nutrition Centre) explains that THT ('ten minste houdbaar tot', best before) indicates until when a product is guaranteed to keep its proper quality. The advice: also use your senses after that date to judge whether something is still good. That is different from TGT ('te gebruiken tot', use by), which is about safety. The European labelling rules (Regulation 1169/2011) prescribe the wording 'best before …' (in Dutch 'Ten minste houdbaar tot …'), and drinks with 10 percent by volume of alcohol or more, according to those same rules, need not carry a durability date at all.

That the date is not a spoilage limit does not mean that beer does not change. According to the Brewers Association, beer begins to age after packaging: the bitterness declines, fruity and floral aromas can disappear and notes of cardboard, honey or sherry come in. The vast majority of beer is meant to be drunk as fresh as possible. Of weizen, for example, the BJCP writes that you drink it best young and fresh.

At the same time, a guide from CraftBeer.com (Brewers Association) names three groups that lend themselves to being laid down, a classification by Patrick Dawson, author of the book Vintage Beer: strong (in general from 8% alcohol), sour and smoked. Of Flanders oud bruin, a tart brown beer (not to be confused with the sweet Dutch oud bruin), the BJCP writes that the style is traditionally meant to be kept, and that examples with some age are considered better.

In short: after the THT, beer is usually not spoiled, but it is different. With hoppy beers often less fresh, with some strong or sour beers sometimes actually better.