These are all a matter of personal preference depending on what you want to achieve with your system and will likely require some experimentation on your part as things like coffee blend, strength, type of gas, pressure used, temperature served at, length of shaking and resting time will all alter the final product.
In general I personally use the following method:
- Make a batch of cold brew coffee using 60g of course ground coffee per litre of water, infusing for 12 hours in the fridge before straining.
- Pour the chilled, carefully strained cold brew into a keg leaving 1L of free space (4L of coffee in a 5L keg for example).
- Screw on lid and inject one bulb of N2O before shaking well. I listen for the sound to change as the drink becomes frothy in the keg.
- Inject another bulb to provide plenty of pouring pressure
- Attach tap and pour as needed, you can detach the tap to store the system when not needed.
- If the flow slows down, inject further bulbs as needed (I find that 2-3 bulbs total is plenty for the 5L keg, 3-4 bulbs for the 10L keg).
If you are using pure nitrogen (N2) bulbs you will need to use more bulbs than this however in the same manner, inject one or two, pour, add more as needed. I personally have not had enough experience to say how many, but on the packet it says one bulb per 500ml of coffee (I think that is an over estimation though).
If you are using a bottle of N2 gas then I attach the gas disconnect to the keg and turn the pressure up to 45psi. Shake well and then leave to sit for a couple of hours with the pressure still set to 45psi. You will also pour at 45psi so just check how the brew looks after a few hours by pouring a small amount. You can always speed up absorption by shaking the keg for longer or more vigorously.