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can I use "yeast oxygen scavenging" to purge an empty keg?

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twd000

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I recently read about yeast oxygen scavenging to de-aerate mash water. Was wondering if the same principle could be used to purge a receiving keg, rather than pushing out 5 gallons of sanitizer. If it worked it would also have the benefit of topping up my batch size - I could ferment 4 gallons of high-gravity wort in a corny keg, then push the finished beer into a receiving keg with 1 gallon of de-aerated water to reach my target diluted FG.

CO2 weighs 1.98 kg/m^3 at atmospheric pressure

a 5 -gallon keg holds 0.01893 m^3 of volume

to fill a keg once with CO2 requires 0.0375 kg of CO2

table sugar produces 1/2 its weight in CO2 during fermentation, so I need 75g of sugar to produce 0.0375 kg CO2

75g of sugar dissolved in 1 gallon of water is a 1.008 solution, and 100% fermentable.

If I wanted to dilute the air headspace with CO2 by a factor of 2x, I'd need 150g sugar, 3x would require 225g sugar, etc.

So my question is how many gas-dilution multiples do I need to get O2 down below __? ppb

If you were only interested in keg purging instead of high-gravity dilution, you could dissolve your 75g of table sugar in just 0.25 gallon of water to make a 1.030 solution


 

Vale71

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Keep in mind that if performing dilution you'll be adding yeast and all the off-flavors from sugar-only fermentation byproducts (including ethanol which will increase your ABV) to your beer together with the dilution water.

If you're just purging you'll need quite a lot of CO2 and unless you pay very high prices for bottled CO2 the sugar required to produce all that CO2 is probably going to end up costing you more.
 
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twd000

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Keep in mind that if performing dilution you'll be adding yeast and all the off-flavors from sugar-only fermentation byproducts (including ethanol which will increase your ABV) to your beer together with the dilution water.

If you're just purging you'll need quite a lot of CO2 and unless you pay very high prices for bottled CO2 the sugar required to produce all that CO2 is probably going to end up costing you more.
agreed about yeast and flavors

was not considering the cost, but let's make the comparison

I get my 5-lb CO2 bottle swapped for $20 at my LHBS, so $4/lb
a 10-lb bag of sugar costs $5, and produces 1/2 its weight as CO2 during fermentation, so you're making 5-lb of CO2 which is $1/lb.
So sugar is 1/4 the price of bottled CO2

regarding "quite a lot of CO2" - how much is "a lot"? How many volumes of CO2 would I need to produce, in order to dilute the headspace from 21% oxygen, down to say 100 ppb?
 

Vale71

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I pay a little more than half that for a 10-lb refill so as I said it all depends on local cost.

To get down to 100 ppb you'd really need way too much CO2 especially if you need to purge most of the headspace with little liquid (no-dilution scenario). The good news is you don't neet to have <100ppb O2 in the keg itself as that is the target value for TPO or total package oxygen once the vessel (keg or bottle) is filled with beer. Since ppb is a weight/weight relationship and beer notably weighs a lot more than gas you don't need <100 ppb in the gas to get to less than 100 ppb in the filled keg. How low you can go strongly depends on the liquid to headspace ratio of the filled container and also on how much oxygen the beer will pick up during the transfer itself. A keg is already at a strong advantage compared to say a 500 ml bottle as the amount of headspace is smaller than in a typical bottle and you can also fill through the dip tube wich reaches all the way to the bottom, thus minimizing O2 pickup.
 

jddevinn

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