I think we need a better vocabulary around the subject of sourness in beer. I hear a lot of stuff described as sour and I’m rarely quite sure what someone means. Usually it just means they’re a malty beer drinker and they don’t like a thin pale ale. (Such as Oakham Citra being described as “sharp”.)
Keep in mind that beer is always acidic… be it berlinerweisse at a pH of 3 or malty ale at a pH in the low fours… and palates vary… but some things are definitely detectable and definable.
Here’s a few forms of “sour” I come across:
Sharp – what I call the sharpness of thin pale ales, the Session IPAs of this world, I actually hear people take a sip and say “oh, that’s sour”… I’d not use the word sour personally, but I do use the term “sharp”. See also: being contacted by a bar and asked why a beer is so sour… worrying about infection… then hearing it is a 3% session pale and thinking: ah-hah… Some I know think of this as a lack of “balance”, especially in new-style Brit-IPAs that lack the meatier crystal malt character of some of their US counterparts.
Lactic – this is a properly biting lemon-juice sour which can be often found in mouth-puckeringly dry berlinerweisse, but also sometimes in bigger sweeter beers which take the edge off it. It is a “clean” sourness, not one that usually comes with a “sour” aroma, it can be described as enamel-stripping at times, but shouldn’t be too challenging to the white wine or cider drinker. Ice cold it is amongst my favourite type of summer refresher, especially at typical sub-4% strengths.
Acetic – always bad in my opinion, but accepted by some in some styles. Basically this is vinegar, sometimes complete with malt-vinegar pong. Usually a sign of badly kept cask ales. Sometimes mildly deliberate – notably in the (in)famous Duchess de Brogdoggognogwhateveritisyer...
Brett – I still don’t quite understand this one. I see bretted beers decribed as a “sour style” and I’ve had folk say they won’t have bretted beers because they “don’t like sours”. I’m still a bit confused by this. I’ve had some sour-ish bretted beers but I don’t think of brett as a giver of sourness. Usually it is more umami and woody spiciness… but hey ho, it is worth mentioning. The sense of “sourness” could come from them often being dry (little sweetness) and not hugely hopped (little bitterness). Perhaps this subject needs a bit of deeper exploration beyond Orval ;)
Ropey – this is sort of my own one. Is is that specific sourness you only seem to get in old fined cask ale. I suspect it is the finings going “off”… it’s a sort of tang, I call it a “twang”, a discord in the beer flavour. It is distinct from any of the above… think old pongy cheap port perhaps, not acetic but soured. Needs more analysis… unpleasantly.
There are probably others but these are what I some across most often in talking to folk who buy beer at various levels and drinking beer myself. Are their tannic sours, citric? Not sure. But the point is we need more precision as currently the word “sour” is doing me ‘ed in.