Carbonation is created when yeast converts sugar into C02 and alcohol. If you want to increase the C02 (carbonation), there are several things you can do to increase the fizz during both the first and second fermentation.
First, let the first fermentation brew for longer. Allowing your initial brew to go for at least 14 days will strengthen all of the activity in your current brew and create stronger starter tea moving forward (making your future batches better). At the end of your first fermentation, there should be at least a small amount of natural carbonation that has built up.
If you want serious levels of carbonation, then bottling for a second fermentation is a must!
Try these second fermentation steps to get your kombucha bubbly:
1) Fill your kombucha bottles closer to the top, leaving only an inch or so of space between the liquid and the cap. By reducing the amount of oxygen present in the bottle, more C02 is dissolved into the kombucha.
2) Increase the sugar content in your second fermentation. Either add fruit or fruit juice to each of your bottles (if you've already tried this and it didn't make it more carbonated, try adding 1 tsp sugar to each bottle).
3) Allow the second fermentation to go longer, try 4-7 days. Do note, you will need to start burping the bottles once a day or so after about 4 days of fermenting to ensure they don't explode (it's quite rare, but it happens). Simply open each bottle slowly to release the C02 and recap.
4) Your second fermentation should always be at room temperature just like your first fermentation. Make sure you are storing your bottles someplace warm (68 degrees or above). If you can’t find a warm place, place a heating wrap underneath or around the bottles. And remember, as soon as you place a bottle in the refrigerator, fermentation, and therefore, C02 creation, will mostly cease. So don’t put the bottles in the fridge until they have had an ample second fermentation.
5) If you are using old kombucha bottles from the store, try placing a small square of parchment paper underneath the cap to get a tighter seal. Bottles with a long neck that use swing top or poly cone tops are the best bottle type for building and holding carbonation.
6) Know that your kombucha will change from brew to brew based on a variety of factors including temperature, the length of brewing, the tea used, etc.. Additionally, each of your brews may not always reach the highly carbonated levels of commercial kombucha because many breweries add artificial CO2 during bottling.
7) Speaking of simply adding CO2, if your kombucha seems flat and you want to add some fizz in a hurry, you can always add sparkling (soda) water to your kombucha. Or, use an at-home carbonator, such as a Soda Stream, to inject CO2 directly into your kombucha.