Kombucha + Fizz (Updated May 27, 2010)

Yes, kombucha can be fizzy.

I've been receiving numerous questions about fizz, so here is what I understand thus far on the matter!

To begin, fizz does not indicate the health of your SCOBY or your ferment. Fizz is more a matter of personal preference, and may indicate the balance of yeasts and bacteria in your brew (1, 2).

For the scientific side on the matter, here are the chemical reactions that occur during kombucha fermentation (3, p. 40):

The yeasts convert sugar to alcohol + carbon dioxide.

C6H1206 -> 2C2H5OH + 2CO2
sugar -> alcohol + carbon dioxide

The bacteria convert the alcohol to organic acids, such as acetic acid, and water.

C2H5OH + O2 -> CH3COOH + H20
alcohol + oxygen -> acetic acid + water

The CO2 (a gas) that forms reacts with the water in the tea to produce carbonic acid (think of carbonic acid as dissolved CO2). Carbonic acid is a weak acid that readily decomposes back into water and carbon dioxide. When the carbon dioxide bubbles out of the water, you get fizz (4).

CO2 + H2O -> H2CO3
carbon dioxide + water -> carbonic acid

Carbonation and the primary fermentation:
As the new kombucha mushroom forms at the surface during fermentation, the ability for the carbon dioxide to escape decreases, helping to create some of kombucha's natural fizz. The more quickly and the more thick your mushroom forms, the less the CO2 will be able to escape, which can increase the carbonation of your drink (3, p. 33, and 5).

A lower fizz may indicate a lower ratio of yeast to bacteria in your culture. The ratio of yeasts in your brew may be increased by (1):

• Using the bottom SCOBY from your previous batch
• Using more of the kombucha at the bottom of your brew, which tends to be more cloudy/yeasty, for your starter tea.

Carbonation and the second fermentation:

• Fizz can be significantly increased by bottling. My posts related to bottling can be found here.
  
• Consider experimenting with leaving and not leaving airspace. Read my post Bottling and Headroom.
  
• Adding flavors (such as jams, citrus peels, ginger, etc.) during the bottling process can also help to create more fizz. (Similar to adding a primer in beer making).
  
• Beer bottles, which have a very tight seal, are ideal for creating fizz in kombucha. In my experience, when I've used resealable bottles (such as GT's or Katalyst kombucha bottles), I've lost more fizz.
• -The longer you leave your bottles out for a secondary fermentation, the more carbonation will build up. HOWEVER, if you leave your bottles out for too long, you run the risk of them exploding! Some people recommend 1-2 days for a secondary fermentation, others 3-4 days, and some recommend even longer. It all depends on various factors, including the sugar content of your kombucha and the temperature (higher temperatures = faster fermentation).
  -My advice: If you're fermenting a batch, try a bottle after 1 or 2 days (more or less days depending on previous experiences, etc.). Then decide whether your other bottles are ready to refrigerate, or if they could use some more time fermenting.

Some more reading:

• Kombucha Balance on No fizz, Too much fizz, Increasing the ratio of yeasts, and Increasing carbonation
  
• getkombucha: The Fizz Factor #1
• A great thread that discusses fizziness and beverages
  
• Although there is limited information on kombucha and fizz, there is extensive information about carbonation and beer making on the web. Although beer and kombucha are clearly different drinks, they are similar in that they are both fermented beverages. Perhaps looking up beer carbonation will provide more clues as to factors that affect kombucha fizz? Try here and here for starters.

Happy brewing!

Sources:
1. http://users.bestweb.net/~om/kombucha_balance
2. http://www.getkombucha.com/spreisfifa1.html
3. Frank, Gunther W. Kombucha - Healthy beverage and natural remedy from the Far East. 4th ed. Austria: Wilhelm Ennsthaler, 1994.
4. http://uk.answers.yahoo.com/question/index?qid=1006051206579
5. http://kombuchakamp.blogspot.com/2009/04/fizzy-brew.html