Plumbing – What flux should I use for copper that will be used for food prep

You might want to take a look at what micro-breweries use. The connections between the various tanks are fairly modular, allowing the wort to be pumped out of the mash tun, through a heat exchanger, and into the holding vessels. From what I've seen, the pipes have flanges that are held together by a circular clamp.

Yes, quite simple.

1. use a Scotchbrite pad to shine up the copper tube so the ferrule ring has a clean, gouge free seating area
2. slide the nut on
3. slide the ferrule ring on
4. push the copper tube into the fitting till it seats (fittings I've used have an internal stop at about 1/4" from ferrule seating area, double-check that the fitting is designed this way)
5. push the ferrule ring into place against the fitting
6. thread the nut on by hand (make sure the tube doesn't back out of the fitting and till nut is hand tight)
7. tighten the nut just till the fitting is leak-free (about 1/2 turn from hand tight)

No plumber's goop, no teflon tape on assembly, the seal is formed by compressing the ferrule ring.

Where most people go wrong is to over-tighten the nut for the ferrule ring. The ferrule ring is trapped between two conical surfaces (nut and fitting) that force the ring to bite into the soft copper which both seals and forms a mechanical connection to keep the tube from backing out. Over-tightening causes the ring to dent the tubing, causing leaks and the inability to disassemble and reassemble the fitting as all the give is taken out of the ferrule ring and it cannot compress any more to seal effectively on the next reassembly.