Post #743998 by Swanky on Mon, Jun 1, 2015 8:52 AM

If you are making it according to standard recipes with a high proof alcohol and sugar syrup, you have 2 of the key elements: alcohol and sugar. Both are important to keeping mold at bay. Do not use a liquor that is not a solid 120+ proof or your results will be less favorable. I use PGA often. It has no flavor and I prefer not to use rum since it adds flavor.

You start with a mix that is 50/50 sugar at least which will prevent mold growth ON THE MIX but it may still grow on the bottle or elsewhere. It is helpful to boil the bottle before hand to kill problems. If you had mold before in the bottle, especially clean it. If it gets mold inside and you can clean it off, so long as it is not on your syrup, it is still okay. If you get mold on or in the syrup, toss it. The high alcohol content of 50/50 mix of high proof and your syrup will also help.

Refrigeration is a third helper in this task. Cooler temps will slow the growth all the way around.

One more tip is to strain your lime juice before adding it. Any time I store lime (or any) juice for any time I strain out the extra pulp and any other solids to keep it more stable.