Adjusting sugar and acid
Updated
The final tuning, and the two controls that are not independent.
Two dials that interact
Acid raises perceived sourness and lowers pH. Sugar raises sweetness and masks acidity.
So adding sugar to a drink that tastes too sharp changes the perception without changing the pH, and adding acid to a drink that tastes flat lowers the pH as a side effect.
Which means one of them is a flavor control and the other is a flavor and safety control at once, and they should be adjusted with that in mind.
Adjusting acid
With a measured addition of a dilute solution, mixed fully, then measured.
pH moves non-linearly, so the last small addition can move it further than the first large one, and a buffered product resists change until it suddenly does not.
Check pH after the batch is complete and at a standard temperature, because pH reads differently warm.
And remember that the pH which matters is the finished, equilibrated product rather than the reading thirty seconds after an addition.
Adjusting sugar
By weight, as a percentage, confirmed by Brix.
In a live product this is not only a flavor decision: sugar added at the end is fuel for a culture that is still active, and it is the most direct way to push a kombucha towards both thresholds.
Which is why sweetening a live drink after fermentation deserves a deliberate answer, a non-fermentable sweetener, or stabilizing first, or accepting a drier product.
Order and overshoot
Acid first, to the pH target, then sugar to the Brix target, then re-check pH.
Approach both from below. Over-acidifying and correcting back with alkali adds salts and changes mouthfeel, and over-sweetening cannot be corrected at all without dilution, which moves everything else.
Record what you added and what it moved, because a batch that needed noticeably more acid than usual is telling you a raw material lot has changed.