Open a bottle of soda and you hear the pressure let go. Inside, CO₂ that was forced into the liquid starts escaping as bubbles — slowly if the drink is cold, all at once if it’s warm. Getting good fizz at home is mostly about controlling those two things: how much gas goes in, and how well the liquid holds on to it.
1. Forced carbonation
The quick way. A soda siphon, a countertop carbonator or a keg pushes CO₂ from a cylinder into cold liquid. Two rules make or break it: go cold, because cold water dissolves far more gas; and carbonate plain water, then flavour it — sugary liquid foams up violently and wastes the gas.
2. Natural carbonation
The old way, and our favourite. Yeast eats a little sugar and gives off CO₂. In a sealed bottle the gas has nowhere to go but into the drink. A ginger bug, water kefir or a pinch of bread yeast will all do it. It’s slower and less precise, but it builds a depth of flavour a cylinder can’t.
Why cold matters
How much CO₂ water can hold drops quickly as it warms. At normal atmospheric pressure:
| Water temperature | CO₂ it can hold | Compared with 0 °C |
|---|---|---|
| 0 °C | 3.3 g per litre | 100% |
| 10 °C | 2.3 g per litre | 69% |
| 20 °C | 1.7 g per litre | 50% |
| 30 °C | 1.3 g per litre | 38% |
That’s why a warm bottle gushes and a fridge-cold one pours quietly: the same bottle, but half the capacity to hold its gas.
Pressure is real
Fermentation doesn’t stop by itself. Use bottles made for carbonated drinks — strong swing-tops or PET soda bottles — leave headroom, release a little gas each day, and move bottles to the fridge once they’re fizzy. A PET bottle doubles as a pressure gauge: when it’s rock-hard, it’s done.
Getting the sweetness right
Carbonation makes drinks taste less sweet and a little sharper — dissolved CO₂ is mildly acidic. A syrup that tastes perfect flat will seem thin once fizzy. Start at one part syrup to four parts soda water and adjust in the glass.