A nice article over at the Guardian.co.uk about how to brew your own soda.
Brew it yourself
One batch of 7x formula will produce three batches cola syrup, or approximately 54 litres of cola.
Step 1: 7x formula:
Using food-grade essential oils, assemble 3.75ml orange oil; 3ml lime oil; 1ml lemon oil; 1 ml cassia oil (reduce cassia content for next production); 0.75ml nutmeg oil; 0.25ml coriander oil (6 drops); 0.25ml lavender oil (6 drops); 0.25ml neroli oil (optional/removed due to high cost).
Using a measuring syringe, measure out the oils into a glass or ceramic container. Keep covered to avoid volatile oil fumes escaping. Then dissolve 10g instant gum arabic (equivalent to 22ml) in 20ml water (low calcium/low magnesium, Volvic is good) with one drop vodka – Cube uses Zubrowka. (Be aware that total quantity of vodka will be 0.0007ml per litre of Cube-cola).
Place the gum/water/vodka mix in a high-sided beaker – stainless steel or glass are best. Using a high-power hammer drill with kitchen whisk attachment, whisk the gum mixture at high speed while your assistant droppers the oils. Mix in steadily with the measuring syringe. Continue to whisk at high speed for 5-7 minutes, or until the oils and water emulsify.
The resulting mixture will be cloudy. Test for emulsification by adding a few drops of the mixture to one glass of water. No oils should be visible on the surface. You now have a successful flavour emulsion, which should hold for several months.
Step 2:The mixers
This makes two allied concentrates, Composition A and Composition B, which can be stored separately before being mixed into cold syrup with the addition of sugar and water.
Mix 30 ml double strength caramel colouring (DD Williamson Caramel 050) with 10 ml water. While stirring, add 10ml 7x flavour emulsion (oils/gum/water mix).
Mix 3 tsp (10ml) citric acid with 5-10ml water, then sieve in 0.75 tsp (2.75ml) caffeine. Mix thoroughly using a pestle and mortar until caffeine granules are no longer evident. The mixture may behave erratically, turning either white or clear for no apparent reason. If it goes white, add more water. Pass through muslin or jelly bag to remove any anomalies.
At this point, A+B can be packaged separately and later reconstituted into cola syrup.
Step 3: The cola syrup
2 litres water; 2kg sugar
Compositions A & B
Make a sugar syrup (mix in a cooking pot on low heat to dissolve quickly) using 1.5 litres of the water and all the sugar. Filter if unsure. Mix Composition A into the remaining 500ml water. Add Composition B, then the sugar syrup. You now have 3 litres Cube-Cola syrup or approx 18 litres cola.
Step 4: The cola
As required, make up your cola as a 5:1 mix, five parts fizzy water to one part cola syrup. Cube uses 350ml syrup in a 2l bottle of Tesco Ashford Mountain Spring.
This cola recipe is released under the GNU general public licence.