It's frustrating how few descriptions there are of vat ageing. This gives us at least a glimpse into the process:
"FLAVOURS DUE TO YEAST ACTIVITY
As regards acidity and flavours we knew of an Essex brewery many years ago which was noted for the pine-apple flavour of its old strong beer. This beer, of about 30 lb. O.G., was made in three successive brewings which after primary fermentation were pumped into a 500-barrel vat. These vats were very acid ; getting into them to see if they were properly cleaned (!) made one’s eyes water, and the beer went through a strong bacterial fermentation in from six to nine months after filling. The beer was then full of long rods, often called vibrios in those days but actually a butyricus bacillus, and the taste was offensively acid. But on further storage the acidity mellowed greatly, the titratable acidity dropped at least 30%, a pine-apple flavour developed and the beer was ready for consumption at the end of two years. There were one or two breweries in the East Anglian counties which brewed these old acid beers which incidentally were much favoured by jockeys for keeping their weight down !"
The Brewing Trade Review, September 1943, page 278.
30 lbs per barrel is 1082.4º. Strong, but not stupidly strong. who knows what bugs there were in that vat, All sorts of stuff, as in a Lambic barrel would be my guess.
Fascinating that a very specific flavour is mentioned: pineapple. And that it took a couple of years for it to develop and the beer to mellow out.
Not sure why this beer would have been so useful for jockeys. I'm baffled.