At a previous brewery I encountered low levels of a similar gushing effect for beers in our library that had been stored warm for more than 6 months. Carbonation and microbiological testing did not suggest unstable bottles. Looking at which brands did or did not gush, I came to the conclusion that it was related to sedimentation and increasing the amount of nucleation sites within the bottles. Specifically, in batches of a brand that was prone to foaming/ gushing, if there was no evidence of sediment in the bottles prior to opening them, there was rarely any foaming events. I believe it was directly related to the effectiveness of our clarification process. Increased cold aging time, brightness of filtration, and the use of Clarex all seemed to diminish (or in the case of the Clarex, postpone until out of code) the likelihood of foaming. Similar to your experience, our hoppy, light-bodied, modest alcohol beers (<6% ABV) were most likely to have evidence of foaming or gushing. These beers were also some of the most sediment prone. Our higher strength IPA's showed less foaming, and our dark, malty brands were also less likely to foam. This could also have been directly related to carbonation, as our most foaming-prone brands were higher in carbonation by 0.1-0.3 volumes of CO
2.
Calcium oxalate is a huge concern, but we could not find evidence for it in this case. Checking your beer under a microscope as suggested is a simple check for that, and increasing calcium dosage as mentioned, or increasing your cold aging time may help precipitate the crystals. Said crystals are beautiful under a microscope, though.
Happy to follow up if it's useful
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Franklin Winslow
Brewer
Tarboro Brewing Co
Tarboro NC
franklin@TarboroBrewingCompany.com------------------------------