Physical concerns regarding cold crashing

[ajdelange]

One aspect of diffusion that I keep forgetting to comment on is that the rate of diffusion depends on what the gasses are diffusing through. In the video demo they are diffusing through the equally wide jar mouths and it takes half an our. Later in the video the bromine is in a small tube attached to a larger tube, full of air, through a stopcock. When the valve is open, if the smaller container is at the same pressure as the larger, the diffusion will still take place but it takes much longer. Similarly in a fermenter. If we are talking Yorkshire squares the diffusion will be relatively fast in comparison to a carboy in which the CO2 blanket is perhaps 50 cm in diameter and the mouth of the carboy 5. The same level of diffusion should take about 10^2 = 100 times longer. Rate of diffusion also depends on sqrt(T/m). The hotter the gas the quicker the diffusion and the heavier it is, the slower.

 

[PlinyTheMiddleAged]

If we are talking Yorkshire squares the diffusion will be relatively fast in comparison to a carboy in which the CO2 blanket is perhaps 50 cm in diameter and the mouth of the carboy 5. The same level of diffusion should take about 10^2 = 100 times longer. Rate of diffusion also depends on sqrt(T/m). The hotter the gas the quicker the diffusion and the heavier it is, the slower.

Correct. However, this was not really the OP's posed issue. The issue concerned the air introduced to the headspace as a result of cold crashing.

Assuming a 30°F drop in temperature during a cold crash, you'd get about 6% air mixed with 94% CO2 in the headspace. Volume2/Volume1=Temp2/Temp1 if I assume the pressures at the beginning and end of cold crashing are the same. I've also ignored the contraction of the fluid volume as being small relative to the change in the gas volume. And, of course, the temperatures are measured on an absolute scale (K or °R). Once that 6% of air is drawn in to the headspace, it will not take long to diffuse in to the CO2.

If the issue were simply removing the airlock, then your point would be absolutely relevant - it would take a long time for the air to mix completely with the CO2 in the headspace.

So, I agree - removing the airlock will not introduce a significant amount of air into a carboy. But, cold crashing will (depending on the amount if headspace). And the air will mix with the CO2.

 

[ajdelange]

It is understood that you don't want to draw any air in. That's why the rubber glove suggestion is so brilliant. It not only prevents air from coming in but indicates that there is still positive pressure even after the crash so that you know it is safe to remove the rubber glove. Ideally you would replace the rubber glove with the airlock thus excluding oxygen incursion altogether.

Better still, of course, is to do this in a sealed cylindroconical and crash only if you have enough positive pressure to insure at least 0 psig at the conclusion of the crash. This is, in fact, identical to what we would be doing with the glove and carboy and only better in the sense that it is easier to manage.

 

[johngaltsmotor]

Or just ferment in kegs with a pressure relief valve on the gas fitting. You'll keep the CO2 dissolved from the beginning and ensure O2 is eliminated. Then when you cold crash you don't have to worry. And as long as you transfer cold the CO2 will stay dissolved and not foam when transferring to the keg or bottle. Plus you can pressure transfer without having to open the fermenter and letting in a gulp of air.

 

[ajdelange]

And as long as you transfer cold the CO2 will stay dissolved and not foam when transferring to the keg or bottle.

A bit of topic drift here but you don't even have to transfer cold if you transfer against CO2 counter pressure. Presumably you would transfer cold, though, as you have just crashed. For minimum O2 pickup you still want to counter pressure transfer.

 

[MikeInMKE]

One of these may be in my future:
Build Your Own Spunding Valve to Carbonate in the Keg

 

[Bobby_M]

Put a tee in the blowoff line that leads to a deflated mylar balloon. The balloon will inflate full with CO2 during ferment and the excess will vent to the blowoff basin. When negative pressure is introduced, the CO2 sucks out of the balloon. This solution can be implemented at any time during active primary and doesn't need special timing.

 

[PlinyTheMiddleAged]

This person had an interesting idea. They used a Mylar balloon to capture CO2 and then used the harvested CO2 to carbonate their beer. Same concept can be used to protect the headspace during cold crashing.

http://www.angelfire.com/cantina/carbonation/

Bobby_M's idea is pretty slick. The balloon acts as a CO2 reservoir until it fills, then the excess goes out through the airlock. Then during cold crashing, the reservoir is used to refill the headspace. Nothing to adjust - fully automatic.

 

[grathan]

That is a neat idea with the bicycle pump. I wonder if they can be used to vacuum a keg.

 

[trentm]

Thanks for all the discussion and tips. I decided to try the balloon trick using a Nitrile glove (powder free), and per A.J.'s suggestion use one finger of the glove as a fill tube. The wrist of the glove was secured with several laps of electrical tape stretched tight. The very tip of the index finger was clipped with scissors and the glove was inflated and released 5 times to purge O2. After final inflation, the index finger was tied tightly closed. Seems to have worked great. The first image below shows the gloves inflated at 72F. The second is 48 hours later at 34F (sorry for the dark image). While the gloves deflated significantly there is still apparently CO2 present.

Question: Would it be best to replace the gloves with an airlock after the desired temperature is achieved?

 

[Psylocide]

[snip]
Question: Would it be best to replace the gloves with an airlock after the desired temperature is achieved?

I did something similar but just used a Ziploc bag that I starsan'd and attached with a rubber band.

Once the temp was reached, I put the airlock back on. No suck back and it helped me sleep better at night.

My .02 fwiw.

 

[orangehero]

I wonder if one of these regulators would work during a cold crash? Connect to CO2 line and send output to sealed off stopper or carboy cap.

Camco 59013 Single Stage Propane Low Press Regulator

11" WC is ~0.4 psi. I believe they are used as cask breathers as well.

 

[bigdaddybrew]

The air drawn in as the headspace contracts will be asorbed into the cooling beer causing some oxidation. This is why wine makers use a larger primary and smaller secondary. Wine frequently needs to be aged for months so they rack the wine off the lees into a secondary that is smaller to minimize the headspace because they know that the air in the headspace will be absorbed. Also changes in temperature will cause air to move in and out of the airlock so having minimal headspace minimizes this volume and they cellar the wine at a stable temp to minimize this effect.

I think the amount of oxidation occurring during cold crashing will be most evident in a beer that needs to be aged. I wouldn't cold crash these beers. For beers you drink quickly I don't think it matters much. I have noticed some oxidation after saving the last couple of bottles of a favorite batch when drinking them months later.

If you really wanted to prevent this a cask aspirator valve might do the trick. This is the valve used to prevent oxidation in beers being pumped from a firkin.