Does passing from diacetyl rest to lagering necessitate no bubbles at all through the air lock?

[Meaulnes]

I brew my first lager and I have just started the diacetyl stage. I monitor the gravity with a Float device as well as the temperature. I fermented at about 12°C knowing that the yeast manufacturer recommends between 12 and 18°C.
Three days and a half ago, for the diacetyl rest I moved the fermentor in an atmosphere of about 19°C and let the temperature rose gradually for a little more than three days to reach 18.5 °C. The gravity curve has been flat for about 2 days now.

I notice that the air lock lets out a bubble every two minutes or so.

I’m not a chemist. My questions are:

• Does the transformation of diacetyl or other byproduct produce gas (carbonic or other)?
• Before I start the lagering rest, do I have to check that there are no more bubbles escaping from the air lock?
• Do the bubbles show that there is still unfermented sugar ? In other words does the gas come only from sugar fermentation ?

 

[Zambi]

I can't give you a precise answer, but things like change of air pressure, movement etc can cause bubbles to escape via the air lock, so I would put more emphasis on gravity readings than airlock activity

 

[Minbari]

Doing diacytle rest usually means warm temps, then you crash to lagering temp. I would worry more about suckback than bubbles. The cold will cause a slight vacuum

 

[HighVoltageMan!]

Does the transformation of diacetyl or other byproduct produce gas (carbonic or other)?

The D-rest is performed when there is still some fermentation activity. So just before the beer hits final gravity, the temperature is raised to speed the process up. Raising the temperature is not necessary for a D-rest, it just speeds it up. Some have opined that a D-rest isn't really needed if the fermentation went well, but most will at least let the beer set a little longer at fermentation temperatures to ensure the yeast "cleans up" the diacetyl. The air lock will be active even though the beer hit it's final gravity due to de-gassing. As the beer warms, the CO2 comes out of solution. There is no carbon dioxide produce, or at least enough to see airlock activity, when yeast metabolize diacetyl.

Before I start the lagering rest, do I have to check that there are no more bubbles escaping from the air lock?

As long as the beer hit final gravity and the D-rest is complete the lagering can begin. Again, airlock activity doesn't always indicate the yeast is still metabolizing sugar. Degassing will produce air lock activity. I make mostly lager beer and I just wait 2 weeks or so after fermentation to crash cooling the beer to 1C.

Do the bubbles show that there is still unfermented sugar ? In other words does the gas come only from sugar fermentation ?

Again this is the beer degassing, this is accelerated by warming the beer. Although it could be the yeast just finishing up. If you suspect the beer hasn't hit final gravity, either wait a few days more or take a gravity reading to see if it close to expected FG. Typical lager fermentation can take from 6-10 days depending on pitch rate, temperature, gravity, etc.

 

[Meaulnes]

Ok. I get it. Thank you very much. I will just wait 4 days at stable gravity and then start decreasing temperature for lagering.

 

[Yooper]

Ok. I get it. Thank you very much. I will just wait 4 days at stable gravity and then start decreasing temperature for lagering.

No need for that, although no harm either. If it's done, it's done especially if you reduce the temperature slowly. If you're cold crashing, getting it to lagering temps really fast, there could be some benefit to waiting.

 

[Meaulnes]

No need for that, although no harm either. If it's done, it's done especially if you reduce the temperature slowly. If you're cold crashing, getting it to lagering temps really fast, there could be some benefit to waiting.

Agreed. I was starting to think the same. As I plan to decrease slowly I will start now.