Can Kombucha stall out?

[Wwillco]

Hi there- new to the forums, and to any kind of home brewing. I am on my third round of Kombucha, having purchased a SCOBY online and jumped in with a one gallon brew.
The first gallon was perfect, achieving a PH of ~2.9 at 7 days. After secondary ferment for 3 days, along with various flavor experiments, the brew was top notch.

I transferred the SCOBY and its offspring to a 1.6 gallon bottle with 2 cups of starter liquid, and began the process again. As I had to travel, I bottled at 7 days, drawing off 96 fluid oz and leaving the remainder as starter. This was premature, I found, at a PH of ~3.4. I added back in the tea/sugar solution, maintaining the ratio of 1 cup of sugar to 1 gallon of tea. (Organic Oolong)

PH on the third batch was initially ~3.7. It has since dropped, fluctuating between 3.4 and 3.5, after 14 days of ferment. I have no sign of mold- I'm just not seeing the PH drop I expected after this duration.

Anyone have any suggestions? Should I add more sugar, tea, etc, or retrieve the SCOBY and its spawn, drop PH with apple cider vinegar, and use some of the fluid to make a new batch?

Below is a pic of my setup, which will be replaced with a 5 gallon pickling crock as soon as I have time to build a cabinet to support it. I maintain ~ 84F for the brew.

 

[Timpy]

I'm jealous by how sparkling clear your kombucha is. Just curious, but what are you using to check the pH levels? Has it been calibrated? I was thinking getting one of those cheaper meters you can find on eBay to measure my kombucha, but from what I hear they're fairly unreliable.

As for what's wrong with your batches lately, I'm not an expert but this page seems to have the best troubleshooting writeup I've seen on the web. That being said, the author Len Porzio, doesn't seem to cite any sources of his knowledge so I'm not sure what it's all based off of.

Also, what made you decide to keep your brew at ~84°F? I've always heard that 80°F is the max it should be, but again, I'm not sure what that is based off of.

 

[Wwillco]

Thank you! I think the clarity may come from the water source (reverse osmosis) or the tea (organic Oolong), not sure. The pH is being checked with a pH2 ECO Testr, calibrated weekly to 7.0 using a test solution. I think the purchase was well worth it- $47- it arrived calibrated, and has passed its weekly calibration checks since I got it. The weekly cal checks aren't required, but I did chemistry for years in the Navy, and I'm surprised I don't do it daily prior to use....

Temperature: I wasn't clear at all about the temp, haha- I keep my temperature controller at 84- this puts the brew at around 75F. I don't have a good setup to monitor temperature, so I'm using a seed mat temperature controller with the probe sandwiched between the seed mat and the glass jug. The heat transfer is far from perfect, but I do know it is maintaining the brew well above house ambient (70F)

Thank you for shooting me to that reference/troubleshooting guide! I'm leaning on too weak a starter for my main cause. If that is the case, I guess my best bet is to wait and watch, heh. pH finally did drop .1 yesterday, so there is some progress.

 

[Timpy]

So regarding temperature, I've been corresponding with Len Porzio, the author of that guide I linked you to, "Kombucha, the Balancing Act". I told him about my Arduino controlled kombucha incubator, which keeps it at 80°F. Based on his experiments, he stated that a temperature that high (I'm assuming yours is sitting around the same) would basically throw off the balance of the yeast and bacteria by giving the yeast a big head start. I hope he won't mind me republishing his answer on here, but he puts it like this:

The warmer the temperature, the more excited the yeast get. That means they reproduce faster and consume glucose faster. What happens is they overpower the bacteria by consuming the glucose as fast as they make it, leaving just scraps to the bacteria. Also, in this heightened state (respiration/reproduction), they tend to produce no alcohol which is the other source of energy used by the bacteria. Brewing in this manner will result in a very thin culture, very little gluconic acid and a ferment that tastes like vinegar with a touch of rotten citrus.

Now I personally haven't had rotten tasting kombucha from my incubated ferment, but his statement here is definitely something to consider. Perhaps in your case, the bacteria are being outpaced by the yeast due to the extra heat? Again, this is purely speculation though. Has your pH dropped any more since?

EDIT: I missed the part where you said your brew was around 75°F - disregard what I said about your brew being at 80°F. I also found a reference from the Yahoo Kombucha Group about a guy's pH readings throughout the brew:

I have been brewing for a couple of months and the best brew that I ever
made had the following ph profile:

1) Sweet tea = 5.8

2) After adding starter: 4.8

3) After day 5: 3.5

4) After day 10: 3.3; flavored and bottled at this point