Keg priming with sugar

[Tess Tickle's Brewery]

Good morning everybody!

Can't find a thread on this so asking here. Sorry admin if it's in the wrong section.

My question is:-

If you keg prime with sugar with a half-size batch (around 10L) do you need to put extra sugar in the keg to account for the large headspace, and if so, how much extra?

The problem is, I have a half-size batch of a style (can't say what it is) that I'm currently fermenting in my Fermenter King Jr. When I was shaking it up to oxygenate though, the dip tube fell off so now I'm going to have to syphon it and I hate doing that. Bottle conditioning not an option for this one either (don't ask why ). I thought doing a secondary fermentation would be safer.

Any wisdom please? Brewfather calculator is what I normally use but there's no option for keg headspace!

 

[Caramel Ox]

I think the answer is no you don’t need extra as you will purge and pressurise the headspace from the co2, the priming sugar is just for protection against oxygen dissolved in the beer.

But is it worth taking the lid off the fermenter at kegging time and reattaching or bodging a dip tube, re pressure and continue as you would have? Then you won’t have the priming sediment in the keg.

 

[Erik The Anglophile]

I keg condition with sugar all the time, to get a more cask-like pour.
Treat the keg like a large bottle.

 

[Hazelwood Brewery]

Hi Tess, yes you need extra priming sugar or your beer will be under-carbonated.

What temperature do you store your beer? This affects gas solubility and pressure so in turn affects the maths to calculate the priming sugar.

How many volumes are you intending to carbonate to?

 

[Hazelwood Brewery]

I found a relevant web page for this…

Headspace calculations – Beer and Brewing

They also provide a calculator but the link to it is broken so we have to do this the hard way. Spoiler alert, the answer is at the end.

It would be helpful for any other brewing science/ physics/ maths geeks (you know who you are) to check my approach and maths so I don’t lead Tess astray!

In principle it’s quite straightforward:
1. work out the gas needed to carbonate the beer,
2. work out the gas required to pressurise the head space and maintain equilibrium,
3. work out how much sugar is need to produce the total amount of gas.

Starting with the beer…

To calculate the number of moles of CO2 required per litre of beer we just divide the volumes of CO2 required by 22.4. Assuming you’re looking for 2 vols, this gives us approx 0.1

The molecular weight of CO2 (grams in a mole) is 44g so the mass of gas required to carbonate the beer is 44g x moles of CO2 per litre (worked out above) x no of litres of beer so:

44 x 0.1 x 10 (assuming 10 litres of beer) = 44g

First bit done - we need 44g of CO2 to carbonate 10 litres of beer to 2 volumes.

Now the head space…

This starts with our old friend Henry’s law which is concerned with the amount of gas dissolved in a liquid at a given pressure but it’s not the same for all temperatures or for all gasses so we have to adjust Henry’s constant (0.034) for the gas used and for the temperature you store your beer (in Kelvin just to add interest! - you just add 273 to degrees C). This works out, if my maths is correct, at 0.052 for CO2 assuming 10C is your storage temperature.

The required head pressure in Pascals is 101325 x moles of CO2 required per litre (worked out at the top) / the adjusted Henry constant -1):

101325 x (0.1 / 0.052 - 1) = 93,530

The mass of CO2 required to pressurise the head space is the head pressure in Pascals x the volume of the head space in litres / (188 x the storage temperature in kelvin):

93530 x 10 / (188 x 283) = 17.58g

Second bit done - we need 18g of CO2 to fill 10 litres of head space with enough pressure to maintain 2 vols of CO2 in solution in 10 litres of beer.

In total we need 44 + 18 = 62g of CO2 but we need to account for any CO2 already dissolved into the beer through fermentation. Assuming fermentation was at 20C and you haven’t used pressure fermenting or performed a cold crash while feeding in more CO2, this works out at about 14g for 10 litres of beer. Accounting for this the new total is 48g of CO2.

The last bit is working out how much sugar is needed to produce 48g of CO2…

The molar weight of sugar is 180g and one mole of sugar produces 2 moles of CO2. The mass of sugar needed is therefore 180 x (total mass of CO2 / 88):

180 x (48 / 88) = 98g sugar

 

[Sadfield]

Unless I've missed it, the above doesn't consider that the beer will already contain CO2 from fermentation. Around 2g/L at around 18°C.

 

[Hazelwood Brewery]

Unless I've missed it, the above doesn't consider that the beer will already contain CO2 from fermentation. Around 2g/L at around 18°C.

Thanks Sadfield. Yeah, you missed it, I subtracted 14g assuming a 20C fermentation.

 

[Clint]

All sounds barely legal.

 

[Leon103]

Why don't you carb it with co2?

 

[Sadfield]

So I did, I think I expected it to be calculated first.

Oddly, in my experience, keg priming requires far less sugar than bottle priming, and Beersmiths calculator does to. Possibly due the different way foam is created at serving. There's a couple of interesting points in this thread that considers transfer off-gassing and headspace ratios when considering one keg vs multiple bottles.

https://beersmith.com/forum/index.p...n-tool-and-sugar-amount-keg-vs-bottles.11810/

 

[Hazelwood Brewery]

So I did, I think I expected it to be calculated first.

Oddly, in my experience, keg priming requires far less sugar than bottle priming, and Beersmiths calculator does to. Possibly due the different way foam is created at serving. There's a couple of interesting points in this thread that considers transfer off-gassing and headspace ratios when considering one keg vs multiple bottles.

https://beersmith.com/forum/index.p...n-tool-and-sugar-amount-keg-vs-bottles.11810/

Curious. I’m not convinced about head-space ratios other than maybe in the short-term, the differences are not significant enough. Similarly, off gassing couldn’t be more than all the gas escaping and this wouldn’t make the difference. A combination? Still seems unlikely so is this a question of perception rather than measured differences.

I’ve no idea if this is at all related and I have no rationale for this either but I do find carbonation in a keg from secondary fermentation (rather than forced carbonation) produces a more appealing foam and mouthfeel - more lacing and softer.

 

[Agentgonzo]

In principle it’s quite straightforward:
1. work out the gas needed to carbonate the beer,
2. work out the gas required to pressurise the head space and maintain equilibrium,
3. work out how much sugar is need to produce the total amount of gas.

This is what I thought at first. But then I remembered that in the calculators, if you double the carbonation it takes much more than double the amount of sugar.

In short, I'm not certain the above approach is right, but I don't understand the chemistry enough to be certain or answer the question properly

 

[Agentgonzo]

(either that, or the grainfather app lies to me - and it's been pretty reliable so far)

 

[Hazelwood Brewery]

This is what I thought at first. But then I remembered that in the calculators, if you double the carbonation it takes much more than double the amount of sugar.

In short, I'm not certain the above approach is right, but I don't understand the chemistry enough to be certain or answer the question properly

This may be because you need a disproportionate increase in head-space pressure to maintain carbonation?

In my example above you need 0.1 moles of CO2 to carbonate 10 litres to 2 vols. To carbonate to 4 vols would require twice as much, so 0.2 moles.

Now look at the head-space pressure needed to maintain it.

At 2 vols:
101325 x (0.1 / 0.052 - 1) = 93,530

At 4 vols:
101325 x (0.2 / 0.052 - 1) = 288,386

At 8 vols (don’t try this at home):
101325 x (0.4 / 0.053 - 1) = 678,098

 

[Agentgonzo]

This may be because you need a disproportionate increase in head-space pressure to maintain carbonation?

In my example above you need 0.1 moles of CO2 to carbonate 10 litres to 2 vols. To carbonate to 4 vols would require twice as much, so 0.2 moles.

Now look at the head-space pressure needed to maintain it.

At 2 vols:
101325 x (0.1 / 0.052 - 1) = 93,530

At 4 vols:
101325 x (0.2 / 0.052 - 1) = 288,386

At 8 vols (don’t try this at home):
101325 x (0.4 / 0.053 - 1) = 678,098

Wow. Ok, yeah. I figured that headspace is normally small, and the calculators don't take it as a parameter, so I figured it wasn't taking into account. Maybe all the calculators have a sensible assumption for that value

 

[Hazelwood Brewery]

Wow. Ok, yeah. I figured that headspace is normally small, and the calculators don't take it as a parameter, so I figured it wasn't taking into account. Maybe all the calculators have a sensible assumption for that value

That’s a good point! I may be barking up the wrong tree here.

On the other hand there has to be a build up of pressure to hold all these volumes of CO2 in

 

[Agentgonzo]

Who knows. It's something I'll definitely look into... Just not at bedtime after a Delerium Christmas beer!

That's definitely a job for Tuesday when I'm back at work

 

[Sadfield]

Is difficult as carbonation tools don't treat keg and bottle equally, but also don't assume someone would half fill a keg. Although, at the end of the day, over carbonation isn't a problem as corny kegs are easy to vent.

I do feel there's a difference between force carbing and priming a keg. Even if it's just the extra byproducts from another fermentation.

 

[foxy]

I think the answer is no you don’t need extra as you will purge and pressurise the headspace from the co2, the priming sugar is just for protection against oxygen dissolved in the beer.

But is it worth taking the lid off the fermenter at kegging time and reattaching or bodging a dip tube, re pressure and continue as you would have? Then you won’t have the priming sediment in the keg.

I don't understand how priming sugar is a protection against DO?

 

[foxy]

Is difficult as carbonation tools don't treat keg and bottle equally, but also don't assume someone would half fill a keg. Although, at the end of the day, over carbonation isn't a problem as corny kegs are easy to vent.

I do feel there's a difference between force carbing and priming a keg. Even if it's just the extra byproducts from another fermentation.

For some reason some don't feel the difference between natural and forced carbonation, for me there is a distinct difference.