Why did my SCOBY sink?
There are several reasons why your kombucha SCOBY may have sunk. The sinking of a SCOBY has nothing to do with the health of your brew, but rather the forces of physics. Dropping a SCOBY into a new batch of kombucha will most likely result in a sunken SCOBY rather than a floating one. This is simply due to the weight of the tea above the immersed SCOBY. In time, the CO2 created by the fermentation may push the SCOBY back up towards the surface of the brew. Again, not that this really matters. All that matters is that the tea gets inoculated with the bacteria and yeast cultures, and fed sugar. This allows the small population of bacteria and yeast from the starter tea and SCOBY mushroom or pellicle enough fuel and enough room to reproduce and create a healthy population of microbes.
If your SCOBY starts off floating but sinks in the middle of the fermentation period, it could be due to a sudden temperature drop. The ideal temperature for fermenting kombucha is around 75-85F, that is when the yeast thrives and is most proactive. If the temperature of your kitchen drops below productivity temperatures, it will cause the yeast to become less active. Less productive yeast produces less CO2, thus one less force keeping the SCOBY afloat. Now, this still doesn’t affect the health of your kombucha; it’s just less productive, meaning the fermentation will take a little bit longer.
As temperatures become colder, yeast slows down as a means of energy conservation. In your kombucha brew, however, they have plenty of sugar to feed on. They will eat the sugar; it’ll just take them longer to do so then at, say, 80F. If your kitchen is naturally colder, I would maybe recommend buying a kombucha warmer to snug around your kombucha to help it along a little bit.
Also, not all SCOBYs are made equal. One SCOBY may be denser than another, causing it to sink perpetually. This density inequality may come from a spike in the population of one bacteria or yeast to another during the fermentation period. Some of the microbes in your kombucha are directly responsible for building the SCOBY, while others are not. If there is an imbalance of these microbes during a specific time during the fermentation, your SCOBY maybe a little more or a little less dense than usual, causing it to sink or float.