As spring turned into summer, I noticed that my scobies seemed to be very thin in the middle. At time I had a scoby 1.5″ thick at the edges and 1mm in the middle. A veritable donut scoby. At first I thought it was due to the formation of large bubbles of CO2 pushing the middle of the scoby out of the surface. I started checking the scoby weekly, also making sure that it wasn’t sticking to the side of the jar. I’ve now resolve the problem by reducing the temperature of the heating pad located under the jars.
I’m not sure why the higher temperature caused a circular scoby, but I have a hypothesis. The heat caused the kombucha to circulate up the middle, out to the sides, and down the edges of the bottle as the heat escaped and the kombucha cooled. My guess is that the temperature in the middle was simply a little too high. The thermometer stuck to the side of the glass indicated 86°, which is supposedly the upper range for kombucha culture comfort, and the middle would have been warmer.
I turned the heating pad from high to medium, and the thin area got smaller, but didn’t do away completely. The outside temperature was now between 82° and 84°. After a month or so of similar scobies, I turned the heating pad to low and have found the scoby production is again of relatively even thickness. The final temperature is 78°-80°.
So, if you get a scoby shaped like a donut, you may want to lower the temperature on your fermentation jar. Then again, the shape of the scoby doesn’t seem to have any bearing on the health or flavor of the kombucha, so maybe the higher temperature is just fine.