Why do I get suck back at a stable temperature

[wsmith1625]

I'm just curious if anyone out there can explain to me why I get suck back at a stable temperature when I cold crash. Let me explain. I ferment and cold crash in a converted wine fridge that is controlled by an Inkbird temperature controller. My fermenter is a converted Fermonster with ball lock posts connected to the lid.

When I initially start cold crashing my beer, I like to drop it down in 5 degree increments. Each time I check the blowoff tube to see if I'm getting suck back. When I see Starsan in the tube, I disconnect the gas post and apply a little co2 to pressurize the fermenter. When I reconnect the blowoff tube, the pressure clears the tube and then equalizes. I continue this process until I get to my target temperature.

The suck back when dropping the temperature is expected, but why do I continue to get suck back after the target temperature is reached. My understanding is that as the beer cools, it contracts creating negative pressure in the fermenter. If I'm at my target temp, the beer should already be contracted, so I wouldn't expect additional suck back. The temperature controller has a 1 degree differential so it's not warming up very much before calling for another cooling cycle.

I'm sure it's all normal and expected behavior, but I'm hoping someone with a little more understanding could explain this to me.

NOTE: I'm not looking to change my cold crash method, it works fine for me. Just want to get educated on what's happening. Thanks!

 

[doug293cz]

As beer (or just water) gets colder, it will absorb more CO2 at the same CO2 partial pressure (14.7 psi absolute in your case, but since a gauge reads the difference between atmospheric pressure and vessel pressure, your gauge reads 0 psi.) So, when you cold crash, first the pressure drops just due to the temp change of gas in the headspace, but more CO2 starts absorbing into the beer due to the colder temperature. This will continue until the CO2 partial pressure is in equilibrium with the level of CO2 dissolved in the beer. It is possible to calculate just how much CO2 will be absorbed due to any temperature change, and it turns out that for a typical cold crash temp, the amount of CO2 that can be absorbed by the colder beer is way more than the volume change just due to the temp change.

Brew on

 

[wsmith1625]

That makes perfect sense! CO2 absorption never occurred to me. Thanks for that explanation.