On August 8, I made two starters for London Ale yeast (Wyeast 1968). Both were in growlers with airlocks, unfortunately only filled with water as we're out of booze. The stoppers have been replaced semi recently (like 2 years ago), the airlocks not that recently.
The next day when we brewed an all-grain batch of Southern English Brown ale, we used one of the starters for that. We aerated with an aquarium airstone that had (we thought) been well sanitized. We put the beer in a glass 6 gallon fermenter, which we thought had been well cleaned and sanitized, and gave it an airlock (as we didn't think it would need a blowoff - and it didn't).
Two days later, when we brewed an English Mild ale, we went to get the other starter. It had gone sour (lactic - definitely not acetic). Also, it had blown off enough that the airlock was completely filled. We didn't use it, obviously; instead we siphoned some beer out of the Southern English Brown, which was by then at high krausen, and used that to inoculate the Mild.
Flash forward to last night. We bottled the Mild (which was fermented in a fairly new Better Bottle); it was delicious and we sincerely hope it continues to be so. We then went to bottle the Southern English Brown. It was sour - acetic! Vinegary smelling. We didn't think vinegary taste could develop so soon. We had to throw it out, although we saved a few bottles as People's Exhibit #1 for the next BJCP session.
So we have two sour beers in the same brewhouse, both weak English brown ales, one's gone lactic (luckily it's just a half gallon starter) and one's gone acetic. Any ideas as to why? We suspect either the glass 6-gallon carboy that the S. Eng. Brown fermented in wasn't thoroughly cleaned, or that the airlocks or maybe the stopper were contaminated.
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