The Second FermentationFor ferments utilizing a mother culture such as kombucha or water kefir, it is during the second fermentation that the carbonation happens. During the first fermentation the culture is imparting probiotics, enzymes, and acids to the sugar water and is generally not sealed off from the air. This is then strained from the mother culture and bottled into airtight bottles for the second fermentation.
Enough SugarThe culture then consumes the sugar and produces carbon dioxide – which is how it “carbonates” the drink. In order to produce this gas, the beverage needs enough sugar – either left over after the first fermentation or added – to produce the bubbly fizz we love. Adding a teaspoon of sugar, fruit juice, or slices of fruit can aid in the carbonation process. One more thing to keep in mind, in regards to sugar, is that overdoing the first fermentation makes it difficult to achieve carbonation, even with added sugar during the second fermentation. I hypothesize that this is due to an overstressed culture and a too acidic/alcoholic beverage going into the second fermentation.