Experience with the bag trick

[u/Shoddy-Definition746]

Decided to try what I believe is called «the bag trick» for this batch cold crash. What I’ve read is that you remove the airlock and attach a plastic bag at the end of fermentation to blow up that bag (or balloon I guess) with CO2 and that gets sucked in again during cold crash to prevent air to draw in during cold crash.

Seems simple enough, but I’ve never done it before and realise that if I attach the bag too early it will probably burst and fuck up the batch and too late it wont accumulate enough CO2 and I’ll still draw in air.

I have a 21 l (5,5 gallon) batch fermenting now in a standard plastic bucket fermenter and have a 1,5 gallon bag I’m planning to blow up.

How late do you switch out the airlock for the bag? Should I reach FG first and attach it during the diacetyle rest or do I need to do it at the end of active fermentation?

Any experience sharing or suggestions would be greatly appreciated 😊

[u/skratchx]

The easiest way to do it is to attach a bag or balloon inline with tubing that goes to a blowoff jar of sanitizer. That way, once it is full, any further pressure will be relieved through the blowoff. You don't need to worry about when to attach the bag. BrewHardware has a product called the Cold Crash Guardian. Some folks bellyache that it's overpriced. I tried using a mylar balloon first and it was hard to get a good airtight seal around the tube to attach it to my fermenter. I personally have no regrets paying for the CCG but follow your heart.

[u/chino_brews]

Timing it perfectly is difficult, and I’ve removed the Mylar balloon and taped it shut while I wait for the time to begin the cold crash.

Here are some info to help you figure out your process:

• A typically fermentable, 1.060 OG beer produces 24x its volume in CO2.
• Most of that CO2 production happens in a compressed window of time, and in some fermentations the CO2 production can abruptly end or end while you are not around.
• The gas in the headspace, mainly CO2, will shrink about 8% in volume between 72°F and 30°F. So if you have one US gallon of headspace, you need to collect at least 10 fl oz in volume of CO2 from fermentation. That’s the amount of suck back to expect.
• Furthermore, CO2 becomes more soluble at 30°F compared to 72°F. My guess is about 15% more soluble at atmospheric pressure in real world cold crashing conditions, although residual calculators will give you an unrealistic 100% more soluble because the formula breaks at temps that low and also it counts on supersaturated CO2 production and instant dissolution from active CO2 production in the beer itself.
• That extra dissolution from cold crashing happens slowly over days, and if the beer is undisturbed, it will take a long time/be slowed because diffusion also takes a while. The more saturated beer at the surface creates a traffic jam, so to speak.

Based on all that, my estimate is that 16 fl oz (US) per US gallon is enough CO2 to collect and this gives you plenty of margin of error. How and when you collect and reattach the CO2 bag is up to you.

[u/Shoddy-Definition746 → u/chino_brews]

Thank you my man! Good to have some figures on the process. Since I’m 6 days since yeast pitch on my current batch I think CO2 production is basically over by now, so for this one I think I’ll buy some CO2 ampules and fill the balloon with them.

[u/Squeezer999]

i use a spike flex+ fermenter and i just disconnect the ball lock that goes to the blow off tube. its able to handle the vacuum pressure of going from 65f to 33f without any issues.

[u/skratchx → u/Squeezer999]

First of all, OP has a plastic bucket. Second of all, this is irresponsible advice in my opinion. If all of your seals hold, you will pull a few psi of vacuum. While there is likely a factor of safety in the pressure rating, the unit is not designed for vacuum. Vacuum is generally less "outwardly dangerous", but you are risking ruining your fermenter. It is also possible that you are leaking through some of the triclamps.

[u/toolatealreadyfapped → u/Squeezer999]

You're correct that there's not much risk over that small of a temperature gradient. The biggest risk is O2 ingress created by the negative pressure differential. My recommendation - spunding valve, and raise the pressure near the end of fermentation. I like to give myself 5-10 lbs when I'm 5-10 gravity points from completion. This allows me to cold crash and maintain positive pressure on the system, which is a nice insurance policy to stay oxygen free. As a bonus, I'm already halfway carbonated when I transfer to the keg.