pH down to 3.5-3.0 in a few hours; is it a superSCOBY?

[Scientific hippie]

I've been posting a lot today; I have the day off and am delaying the bottling of some mead because the sediment got disturbed. I figured I'd make some kombucha. I am not wearing my watch, but I'm pretty sure I made it after lunch. It's now a few minutes after 5 PM EST and I thought, "Let's taste it now, just for the heck of it." Really sour. I stirred the jars and tasted again; no change. I broke out the pH tape; one container is 3.5 and the other is 3.0; I made sure the tape was OK by testing it with tap water: 7.0. Is this because I have been using the same scobies pretty regularly since August and they are huge and ugly? They had been sitting in their starter tea (I use a 1 gal and a 1.5 gal jar for them) for a few days. Has anybody else had this experience? I'll probably bottle it now.

 

[RPh_Guy]

PH is a tool to measure the progress of bacterial fermentation, but that's only half the picture. The yeast also need to ferment the remaining sugar and they probably aren't done in a matter of hours.

 

[Scientific hippie]

PH is a tool to measure the progress of bacterial fermentation, but that's only half the picture. The yeast also need to ferment the remaining sugar and they probably aren't done in a matter of hours.

So do you think that it would have mellowed some if I had just left it for the customary 3-4 days? I did happen to notice that the scobies are looking whiter, with less brown stringy yeast, than usual. Could the bacteria just have dominated? I'm still secondarily fermenting two half-gallons; I backsweetened everything. I'll see what happens to the room temp bottles. Happy Thanksgiving!

 

[RPh_Guy]

Happy Thanksgiving to you also!

The lactic acid bacteria (LAB) only metabolize a few gravity points of sugar into lactic acid -- near 100% of the LAB fermentation carbon utilization produces lactic acid. Lactic acid production will stop once the pH drops to whatever inhibitory level the LAB strains in your SCOBY can tolerate, typically around 3.0-3.5. This lactic acid will remain indefinitely; it won't mellow. Lactic acid is a smooth sourness.

Yeast metabolize all the remaining sugar into ethanol and carbon dioxide. You can monitor the yeast fermentation in kombucha the same way you do with other fermentation, by measuring change in specific gravity. It should probably finish around 1.000-1.005.

At room temp, acetic acid bacteria will metabolize ethanol into acetic acid as long as there is oxygen present, increasing the acidity. Acetic acid is more sharp, with the distinct pungent vinegar aroma and flavor.

I'm not sure what your process is, but generally the issue with bottling too early is that there may be too much sugar left for the yeast to ferment. Excessive fermentation in the bottle generates excessive carbon dioxide. With ~4 volumes of CO2 or greater (over 50psi at room temp), the bottle may gush when opened, or may explode causing a "bottle bomb". Each gravity point generates approximately 0.5 vol CO2 plus there is some CO2 already dissolved in the kombucha.

Hope this makes sense.
I just don't want to hear about explosions.

It's normal for repitched cultures to adapt to your brewing conditions, especially mixed cultures. Optimal strains will dominate and genetic drift occurs favoring the speedy and more acid-tolerant.

 

[Scientific hippie]

Thanks for getting back to me. Nah, no bottle bombs. I just tasted some leftover starter tea and it tastes better today, so I think I just should have left it alone for the usual 3-4 days. Another lesson learned!