Why less priming sugar in a mini keg?

Question

I am used to bottling my beer and do bottle conditioning. However, I just bought a few 5l mini kegs for the next batch which is currently fermenting. From what I have read from different sources, you need less sugar when conditioning in a mini keg than in a bottle (it varies from source to source, but it ranges from 75% to 50%, or even only 33% of the amount of sugar per liter that you would use for bottle conditioning.)

My problem, is that I would like to understand the reason behind this. I wasn't able to find any serious explanation, only things along the line "The amount of head space is different". I normally fill my bottles so that the liquid raises midway to the bottle neck (only a few cm^3 of air), and the filling instructions for the mini keg says to fill up to about 2.5cm below the rim. This means that the ratio of free space over beer volume is larger in the minikeg than in the bottles.

And now this is where I think things are getting counter-intuitive. In my understanding of the situation, a given amount of sugar will produce a given number of moles of CO2. Then the amount dissolved in the beer will depend on the pressure that builds up in the dead space above the beer. The pressure in this part, given the perfect gas approximation will be proportional to the number of moles of CO2 that escape from the beer divided by the volume of the dead space (p=nRT/V). Which means that the larger the dead volume is, the larger number of moles you need to get to the same equilibrium pressure, and therefore the same amount of CO2 dissolved in the beer. Consequently, given the larger dead space in the mini keg, I would expect to use MORE sugar per liter for the mini keg compared to bottle conditioning. This line of reasoning is in agreement with this answer, and the general observation that soft drinks tend to loose their carbonation when you only drink part of the bottle.

However, this in in contradiction with the general wisdom that you need about half as much priming sugar for mini keg than for bottles. This link even specifies, when mentioning filling of the mini keg that "Over filling the keg will result in reduced carbonation levels". For me this is completely counterintuitive, as less dead space means less moles of CO2 necessary to build up pressure in the dead space to reach equilibrium

I would be very happy if someone can give me a sound explanation as to why less priming sugar per volume of liquid is required with a mini keg compared to bottle conditioning.

Answer 1

Your general understanding is pretty much spot-on. I think the thing to consider here is that your reasoning assumes that half or a third of the priming sugar is meant to yield the same amount of carbonation as it would in the bottle. I'd argue this isn't the case. Notice how recommendations like this keg-underpriming 'common wisdom' usually don't go so far as to tell you what the carbonation level is that you're aiming for?

Here's the way I see it: pouring a beer at 2.7 volumes out of a bottle is pretty easy to do without foaming too much, but maintaining beer in a keg at 2.7 volumes requires enough pressure (14.5 psig @ 40°F) that unless you have a lot of resistance from the tubing between your keg and your tap (from a long or skinny line), the dispense velocity will be high enough that you can't pour at that carbonation level without foaming over. Obviously with a mini-keg, where you have little or no tubing to provide resistance, this problem is exacerbated to the extreme. Therefore, when kegging, a balance has to be struck to achieve acceptable carbonation while not going overkill on dispense velocity. It's a problem with a definite solution (a properly balanced draft setup) but I think the common-wisdom solution largely assumes a poorly balanced beginners' draft setup.

What you're trying to wrap your head around makes a lot more sense when you realize that no one's saying 1/3 the amount of sugar will yield the same carbonation level (at least not that I've ever seen), just that it will yield a more appropriate level for the typical keg-service setup.

Answer 2

There's a lot of debate about if you really do need to use less sugar to prime a keg. From my point of view, it's an unsettled question. I'd advise you to experiment and decide for yourself.

Answer 3

It's a matter of volume. I don't know the PSI of a typically charged beer bottle, but less just talk about the math, without doing any math. :)

5 psi in a beer bottle in much less pressure than 5 PSI in a keg. The PSI is the same, but there are a lot more square inches in a keg than in a bottle. Cubic inches really, but the measurement is how much force it pushes against a square inch. Anyway, the amount of pressurized gas increases cubically, and so you don't want nor need the same PSI (therefore need less sugar) to charge a larger volume of beer.

Answer 4

Please note that these mini kegs are rated for a maximum pressure of 4 Bar (58 psi). They are designed for counter pressure filling (where you can keep the pressure under control) and not for keg conditioning (where the pressure can get out of hand).

That doesn't mean that we can't use them for keg conditioning, of course. But be careful. These mini kegs have been known to rupture due to over-pressure.