How to increase the carbonation for water Kefir fermentation?
The “fizz” is created by adding the CO2 under pressure, so that it dissolves (native speakers are welcome to add a better term) in the liquid, then fizzes when opened.
For artificially carbonated drinks, either small tablets of frozen CO2 are added when bottling, or the CO2 is pumped into the liquid (think Soda Steam).
For natural CO2, e.g. champagne and kefir, the yeasts must work at least some of the fermentation time in closed containers, the typical process is that for the first, very vigorous fermentation, an airlock prevents pressure buildup, and in a second step, the almost ready drink is bottled and stoppered or corked, so that the remaining yeasts create enough pressure to carbonate the liquid, yet not enough to burst the container, then die back as planned from lack of food or alcohol level.
It is a bit of a fine line, though. If too little sugar (= “yeast food”) is left at the time of bottling, the drink stays mostly flat, if there’s too much, you may be in for a nasty (and potentially dangerous) surprise, like that one year when my darling MIL used “just a few extra flower heads” in her elderflower champagne. If you know your kefir strain, you’ll soon know how much sugar in the second fermentation is just right. For starters, I would simply bottle up according to the recipe’s second step and see what happens. Just make sure you use a sturdy container, that can handle the pressure. And plastic can be safer than glass.