Juice Factory pH Issue

Osmosis control via a membrane will not in my view solve a pH issue regarding (assumed) acidic juice.

PH control is a no brainer, use a buffer.

"What is involved in a mixed juice/ph control system"...I am doubtful you are indeed in a position within this factory of implementing a control for solving your problem, especially if you ask this one question I highlighted. If you are asking out of curiosity, sensors combined with electronic control and solenoid injectors will maintain desired pH.

Even if you are able to set pH 7.0 neutral that of water without ions, it will not last longer in atmospheric exposure and will also change even if sealed with different rates.

First thing you have to work out the time duration for which pH is to be maintained and its acceptable level. If pH is changing from 7.0 towards 6.0 then it may be better to raise the pH to 7.5 or even more to balance it out in time.

You can also use a neutralizer agent which may become active only when pH is altered else remains neutral. I can not provide any information in this area and you need to hire an expert. However, such chemical agents are always added and require a perfect knowledge about it.

If you already have idea of chemical agents that can neutralize the pH change, then use that knowledge for monitoring and control.

You can even place a sensor which can warn about good / bad condition of the drink right on top of its packing. Such low cost sensor can be designed for high volume use at low cost.