This question comes up from time to time and more so when it comes to ginger beer. If you are looking for a strong alcoholic drink, then brewing homemade cultures is probably not your best bet. But if you are looking sit back and enjoy a little extra kick, there are things you can do to increase that alcohol level. This is for responsible adults only.
1. Do a 2nd ferment with sugar or juice
During the first ferment, the water kefir grains or GBP will keep the alcohol in check as the bacteria will munch up excess alcohol. Add a little extra sugar or juice after the grains or GBP is out and the yeast (which is responsible for the alcohol level) will thank you.
A 2nd ferment with juice is by far the best option for a good alcohol kick. Add the juice and then let is sit at room temperature for 1-2 days. Try the delicious Kefir d'Uva which is made with grape juice.
Many times if you just leave kefir in the fridge, the alcohol level will naturally increase. The colder temperatures will cause the yeast to respire less and create more alcohol. Add a little sugar or juice and then store in the fridge to get that yeast extra active.
Try putting it in a swing top bottle and leaving it for a week or more in the fridge. You should notice a significant increase in alcohol. If its not sweet enough at that point, just add some extra sugar or juice and drink up.
3. Use an airlock system
This won't significantly increase the alcohol and its perfectly okay to ferment with an airlock system for children and those looking to keep the alcohol level low. But if you are looking for every possible advantage to have the most alcohol available, then an airlock system is the better option. In a closed lid anaerobic (airlock) environment, some of the bacteria struggle to utilize all of that alcohol available.
4. Ferment slightly warm
This is tricky because there's so many variables involved. Generally when you brew alcohol, people look for consistently cool temperatures. Fermentation of bacteria and yeast is a little different though. The best option is to do a slightly warm ferment to get the yeast very active. Make sure to put a good amount of minerals in there also.
A warm ferment may not make much alcohol as the yeast will respire more and create CO2 instead of alcohol. However a strong yeast presence will help build that alcohol during a 2nd ferment and storage which is the most important part of increasing that alcohol %.
For those who enjoy a little extra alcohol in their water kefir or ginger beer brew, there are a couple things you can do to significantly help the alcohol %. A second ferment with fruit juice and/or long term storage in the fridge is the best way to do. Using an airlock system and fermenting a little warm and mineral rich will get those yeasts strong and healthy which turn will help create a deliciously carbonated and alcohol rich drink.