Somehow made my first "diacetyl bomb" beer. It's a recipe I've made at least 8 or 9 times before and has always been great, but this latest one turned out nasty -- a big load of butterscotch in what was supposed to be a nice, amber ale. I don't recall there being any "oh ****" moments on brew-day. What are the usual causes of this? Just bad temperature control during fermentation? Was brewed back in mid Sept, bottled 2 weeks later, and just now tasted a bottle -- couldn't do more than a couple of sips it was so buttery. 
recipe Amber Ale (og: 1055, fg: 1008, ibu: 33)
85% 2-row brewer's malt
5% munich 10L
10% caramel 60L
and brown sugar (at same weight as the 5% munich)
Cascade hops @60 and @10, no dry-hop this time
US-05
Fermented like usual in a swamp cooler down in a cool basement (just like every other time)

recipe Amber Ale (og: 1055, fg: 1008, ibu: 33)
85% 2-row brewer's malt
5% munich 10L
10% caramel 60L
and brown sugar (at same weight as the 5% munich)
Cascade hops @60 and @10, no dry-hop this time
US-05
Fermented like usual in a swamp cooler down in a cool basement (just like every other time)
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