crash cooling question

[Grant Abbott]

when I crash cool the fermenter it draws in air. I am worried this will oxygenate the brew. To avoid this I have filled a sanitised plastic bag full of co2 and secured this to the end of the blow off tubing. Is this a stupid idea? is it really necessary? 

[Sam DT]

Mate,

You're a machine!

Is it a stupid idea - no, is it necessary - possibly? Is it worth the effort - not for me but you seem to have done a good job.

I was also concerned about this too so actually stopped cold crashing for a while and just transferred direct to my kegs and did all the crashing there - could also do the same with bottles. The problem is that once you get up to around 100g the residual hop matter starts to block the poppet springs in your keg outlet - I nearly lost my shit with a keg of USIPA that i'd hit with 160g of dry hop - I emptied the poppet spring ten times and then just ended up putting some pressure on and fountaining out a couple of litres of beer - bastard still re-blocked once more.

More recently I've made the call to just add more dry hops and make sure I drink my beer withing a month or two - I usually go for 100g for a pale ale 150g for an IPA and 200+g for a double IPA. If you're bottling I'd be more worried about O2 exposure while you're bottling. Another option could be just to put your conical under CO2 pressure when you cool - basically achieving what you're already doing with your bag - could get a spunding valve or just hook directly up to your tank?

Cheers

Sam

[Tom]

I have been closing my blow off/outlet valve and adding Co2 intermittently to the head space as required. This also allows for pressure transfers.

[David R]

I've never bothered going that far, probably no harm in it though.

Is it necessary? Look at it this way; a commonly used figure is 4% shrinkage when chilling wort from boiling to fermenting temperature. So if we [generously] say 2% further shrinkage when going from ~20C to 1C, that will only draw in an additional 0.4L of air (not oxygen) into your fermenter, based on a 20L batch. Air is only ~21% oxygen, so you've actually only added 0.084L of oxygen to the space in the fermenter. CO2 is also heavier than air, so maybe that small amount of oxygen isn't even going to contact the beer unless you're planning on shaking the fermenter after cold crashing.

Disclaimer: I'm not a scientist and may be completely fucking wrong. But anyway, I can't imagine you're going to pick up much oxygen from this, certainly not enough for my uneducated palate to notice!