As I understand it, this is the chemistry (biology?) involved with basic kombucha, oversimplified. I’m not an expert, but I’m presenting this as my effort to work though the process of what is needed, why it is needed, and how it ends up producing something desirable.
I want to start out by mentioning that the culture, which combines multiple varieties of yeast with multiple varieties of bacteria, is a complex symbiont in which some species will dominate the sugary beginning and other species will dominate the acidic ending, with yet more involved throughout the process, so it won’t be as simple as I describe. Additionally, the precise species and proportions of those species will depend on the culture’s historical and current conditions. It is affected by geography, climate, other encountered organisms (such as local bacteria and yeasts), and how we physically change the culture’s environment.
Not only does each ingredient carry potential “contaminants” (whether a dangerous bacteria or an innocuous wild yeast), but each one also changes the nutrients available to support the yeast and bacteria. Maybe are weakened by the fluorine in your tap water. Maybe some love the fructose in the agave syrup and thrive. Maybe your exclusive use of green tea, which contains higher levels of anti-microbial tannins, cause some bacterial to die. Maybe your environment is too warm for some or too cold for others.
If you always make kombucha the same way, your culture has probably reached some level of equilibrium. If you change something, you may not notice a difference. Or maybe it will have a significant effect, and your kombucha will need a while to to reach equilibrium with a new microbiotic makeup. If you persist in the change, you’ll probably affect a permanent change in your kombucha culture.
- You need a carbon source. (sugar)
- You need a nitrogen source. (tea)
- You need growth medium, (water)
- You need a culture of yeast and bacteria with which to inoculate. (scoby)
The yeast converts carbon to alcohol and CO2. Yeast is a living organism, and has basic nutritional needs, most significantly nitrogen.
The bacteria (especially acetobacter), converts alcohol to acid. Like yeast, bacteria is a living organism, and has basic nutritional needs, with nitrogen again being the most important.
However, this isn’t a simple process by which one organism performs step one, followed by another organism performing step two. It doesn’t strictly progress from sugar to alcohol to acid. The roles overlap. Acetobacter is responsible for producing a pellicle (scoby) on the surface. Some yeasts also produce a pellicle or yeast film. Some yeasts, especially those that produce a yeast film, produce significant amounts of acetic acid. Acetobacter also consumes sugar.
There are quite a few scientific studies available online, and they help demonstrate that while the processes I’ve described are common, each sample contains different proportions of it’s biological constituents, and contain many minor constituents that don’t exist in other samples. You can get a taste for that in the abstract and introduction of this study, but there are many other sources.
One takeaway is that without testing the contents of the culture you use, there’s no way to be certain exactly what yeast and bacteria are present. This means that I can’t simply apply the results of a study to my own experimentation. Even if I were to know that my dominant yeast and bacteria were the same as those in a study, the interactions with the minor players may be significant.
I foresee the accumulation of many quart-jar batches, so that I don’t risk spoiling a whole gallon, and don’t produce more than I can consume. I’m sure some future posts will explore more specific topics, like unorthodox nitrogen sources, sugars, or acids.