Measure out 1000 mL double deionized H2O and pour in large flask. Add a good-sized stir bar. Place on magnetic stir plate.
Set temp to 400C and start stirring water. Pour entire contents of the GF bag slowly.
Use glass stir to rod if necessary to get all the GF powder in solution or to get rid of big clumps. I'm usually not too worried about clumps because after I add all the GF, I stir at a higher speed.
Add thermometer to the GF. I usually tape it on so that it doesn't touch the bottom since the bottom gets to 100C before the GF does.
Let stir at 400C for ~20 min. After this time, the GF should be at 100C and boiling. If your hot plate doesn't have temps, just keep an eye on it so it doesn't boil over/get burnt.
Once boiling, quickly move over to a cool stir plate (oven mitts/pot holders/something to protect from heat) and let GF cool to 40-60C (~20-25 minutes) while stirring at a medium speed. I prefer 50C.
Use gloves. Add 6 mL of Propinoic Acid (aka propionic acid) and let stir for a couple of minutes.
Remove thermometer and start pouring to desired amount. Important: I put the GF to stir for a few seconds after every 5 bottles or 15 vials have been poured so that the GF does not separate.
Pour before it gets lower than 40C because it gets too viscous to pour. One bag here made 20 bottles and 42 vials. It all depends on whether you need bottles or vials and how much you pour.
Poured food. Sometimes it has bubbles in it from stirring.
Cover food (so that curious/hungry fruit flies do not take residence inside) and let cool overnight.
Flug bottles and vials once food is cooled and there is no condensation visible inside. If you see separation of GF, you poured while the GF food was too hot or not stirred well while cooling.
We store the vials in a Tupperware bin since it takes us about a 3-7 days to use them. I would recommend storing bottles and vials at 4C but the condensation process at RT could give rise to bacteria.