Second Ferment
How you flavor your water kefir is up to you. You can certainly drink as is, but most people will put their first ferment water kefir through a secondary fermentation process. A second ferment is quite easy.
We like to use 16 ounce bottles with about 1/4 cup juice or puree. Juice or fruit will give a lightly flavored beverage. Puree or smoothies will give a more robust flavor.
Pour water kefir into thick glass bottles and add juice, fruit, purees, or even vegan smoothies. Cover with a tight fitting lid and let ferment for an additional 12 -24 hours. Once complete, place in the refrigerator to halt the fermentation process. Once chilled, carefully open (it’s aliiiiiive! No, but seriously, it’s dormant, not dead, so you want to carefully open just in case it’s pressurized), and enjoy!
Now, a word to the wise. This process can end up with explosions if you’re not using the correct equipment. We like to use thick glass bottles like the ones below. They have a swing top cap and are very airtight. You can buy these at any homebrew supply store, online, or can probably talk your homebrew friends out of a couple. If you feel uneasy, you can place the bottles to second ferment into an ice cooler to contain any messes that may occur. So far, most of us have been lucky. Although we did open a bottle and the water kefir shot out like champagne! It was quite exciting to watch but a pain to clean up the kitchen floor.
The easiest way to mitigate explosions when you open the swing top is to hold one hand firmly on the lid while you open the bailing wire with the other. If you see mass carbonation happening, close the wire again and refrigerate 15-30 minutes. Keep doing this until you can open it without the kefired water shooting all over the place.