OG was 1.055, yeast was US04 fermented at 18C (raised to 21 after 10 days), recipe had around 20% Munich & no sugar addition & mashed at 66C.Mmm. Not sure. Perhaps if your OG was quite high, you used an agressive yeast, used some simple sugars, mashed low or fermented near the top end of the yeasts temp range (or a combination of some of these), the exothermic reaction of the fermentation could have maybe pushed the fermantation temp into fusil territory?
I feel like I've checked every last nook & cranny for a mistake in my process but I do hate chalking one up to an unknown "infection" as I can't prove it (without an expensive lab report)!
Oh well, I'll just have to brew it again just to make sure! Along with the aforementioned deep clean of course.
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