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I've pretty much settled the protocols for mashing, sanitising, yeast maintenance, and bottle conditioning. Generally my beers come out decent. With one exception, anything that uses roasted barley.

If I use roasted barley, a ready beer is usually quite good in the first 1-2 weeks after usual period of bottle conditioning (7-12 days). When age of that beer approaches 1.5 months, bottles turn to gushers. I don't taste any off-flavours that would hint of infection, however, overall beer tastes more acrid/sharp/bitter than when it was fresh.

Here's what I tried to fix it: 1. Use fresh yeast, or use a different strain. So far I tried WY London Ale (few varieties), WY West Yorkshire, WY San Diego Super, Mangrove Jacks M44. 2. Mash longer. Temperatures tried were int the range 66-69C. 3. Ferment longer. Raise the yeast from sediment by rocking the fermenter. Raise fermentation temperature after 1-2 weeks, to help the yeast to finish. 4. Mash without roasted barley, and add a "cold brew" of roasted barley at the end of boil. 5. Adding 10-20% less priming sugar.

None of those fixes actually helped. Oh, and btw, I did't dry hop any, so hop creep is not a factor, too.

I start to think that roasted barley may provide sugars that are fermentable on a few months timeline, rather than few weeks, but was so far unable to read anything about it.

Any commends on what else I might be missing?

  • It could be an infection, but it's strange that it would only be happening in your dark beers. Do you hop your pale beers? Have you tried hopping the dark beers? Have you tried making the same beer, but either omitting the roasted barley, or subbing it out for something different? – Frazbro Mar 22 '19 at 4:31
  • And when I say infection, it could be very hard to taste, and it could be just another yeast, not necessarily bacteria. It could be a diastatic strain like 3711, for instance. Does the gravity change over the time between when the beer tastes good and when it becomes a gusher? – Frazbro Mar 22 '19 at 4:33
  • @Frazbro yes sometimes I hop my pale beers to death, and tried doing hopped and low-hop versions of dark ones. I didn't try brewing side-by-side beer with and without roast grain as the only variable, but I tend to use the same vial of yeast many times regrowing with starter, so infection would've impacted all my beers - but no. – Roman Mar 26 '19 at 9:56
  • Infection can come from sources other than the yeast pitch, so it would be worth checking that out. Also, hops have antibacterial properties, and many infectors are not hop resistant. I would recommended hopping your next dark beer to rule it out. I usually hop my stouts quite aggressively early in the boil, which I think is reasonably standard – Frazbro Mar 26 '19 at 10:13
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I'd say you have lager yeast hanging out in your system. Roasted grains will have more maltotriose, a sugar ale yeast can't consume, but lager can.

This would explain why they're fine until later on, lack of off flavors and cold storage doesn't hold off the process. The acid bite, is probably the natural acid in dark grains without the maltotriose to balance It out.

Use an iodine based sanitizer on everything cool wort touches on your system.

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