Lagering and diacetyl rest

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Oct 7, 2008
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I am making a Kellerbier OG 1053 fermented with S-23. This is only the second traditional lagering that I have carried out and the first was years ago. I carried out a primary fermentation for 10 days at 12°C, dropped to 4°C at a rate of 2 degrees per day and held it there for 41 days. The gravity seemed stable for several days (I have an iSpindle in the fermenter) and during the last couple of weeks of lagering I didn't manage to catch a bubble pass through the air lock.

So I decided to increase the temperature to 20°C for a diacetyl rest before packaging. The temperature had increased to just 6.7°C when I noticed a fairly regular bubbling through the airlock and activity evident across the surface of the beer.

So what's happening? Is the airlock bubbling because:
1. The beer and CO2 above it are expanding.
2. Dissolved CO2 is coming out of solution.
3. The yeast has decided to come back to life.
4. Something else.

All help gratefully received.