The ESB had a slight bacterial infection. It had lost all of its malt flavours and there were next to no hop aroma, making it monotonous, grassy and slightly dry. It also had an astringent yeast-like character, but I don't think that has got anything to do with the yeast at all. The infection started out as just a hint of altered flavours, but has over the last couple of weeks become much more evident. It is without a doubt an infection. At this rate I pick up an infection every 35 batches. This is the second time it has happened.
I judged a couple of flights at this year's Norwegian Homebrewing Championships and noticed that a couple of the entries had the same defect. At that time I suspected that it was a fermentation issue, most likely caused by autolysis or bad yeast health. I'm now confident that it is indeed a small bacterial infection, or the beginning of one.
There's nothing that one can do about a bacterial infection other that to discard the beer. I'm sure you could drink it, but life's too short. Unlike other defects there's actually no steps you can take to improve the beers drinkablity. So the ESB is going to make do as fertilizer in the garden.
Interestingly the three beers I have brewed after the infected one do not have any signs of infection. This has led me to suspect that it is the fermentation vessel that has caused the infection. It could of course also have been a mistake made by yours truly.
Plastic has the unfortunate characteristic that it easily get scratched over time, and those scratches can harbour bacteria. I have four plastic fermentation vessels and I do think I used all four on those last four beers, so it seems that none of the other beers touched the infected one.
Anyway, my plastic fermentation vessels are five years old. I should have replaced them earlier, but they have now for sure reached their end of duty. They were thrown out yesterday together with the plastic tubing.
New fermenters will have to be bought ASAP as I have more work to do before the summer.