My Own Theory for the Kombucha Symbiosis

or more reasons to live together cooperatively
 

Yeast, like most other eukaryotic cells, is considered a facultative anaerob which is able to metabolize glucose with or without oxygen. Since we prepare the tea with the more complex white sugar (sucrose), the yeast most likely sends an enzyme into the tea to break down the sucrose into the simpler glucose and fructose. If oxygen is available, all the sugar is converted into energy + CO2 + water and no alcohol is produced. This is actually the more efficient way to meet its own energy needs for growth and maintenance of internal functions. If there is no oxygen, yeast switches to fermentation which utilizes only about 5% of the energy contained in glucose. It goes through several steps with ethanol alcohol as the end product which still contains a large portion of the original energy (686 kcal per 180g = 1 mole of glucose). The internal machinery of the yeast cell (glycolysis) has to work a lot harder and use up a lot more sugar to get the same amount of energy it would get if it had oxygen available. This is exactly the reason why commercial fermentation is done anaerobically, it produces the maximum amount of alcohol.

It is a well known fact that fermentation in an open container still provides alcohol. Compared to closed fermentation containers (with an airlock) this process occurs at a slower rate since some of the yeast will use oxygen to get more energy. In other words, in an open container yeast operates in both modes, anaerobic as well as aerobic.

And here comes the rest of the K. microorganism population, the bacteria - and my theory. The acetobacter xylinum bacteria - the ones making vinegar - get their energy from alcohol by converting it to acetic acid. If the yeast would operate completely aerobic, it would not provide any alcohol to the bacteria. But by producing a zooglea on top of the liquid, two objectives are being met: 1. The bacteria have a solid surface on which to settle giving the easy access to oxygen which they got to have (see my previous post). 2. Oxygen supply to the yeast in the liquid below is reduced causing this symbiotic partner to produce more of the needed alcohol.

My guess is that the bacteria can also metabolize the sucrose in K. tea directly after breaking it down into simpler molecules. But perhaps they prefer the alcohol because it does not require an initial investment of energy before getting anything back. In glycolysis of glucose (anaerobic or aerobic), 2 molecules of ATP (Adenosine Triphosphate, the universal energy molecule found throughout the living world) have to be burned up first to get 4 molecules back, a net result of 2.