Post #749392 by Swanky on Tue, Aug 18, 2015 6:16 AM

Not sure how much a little alcohol will help, but a lot of sugar will. It is the high sugar content that keeps mold from growing. It may grow on the inside of the bottle, but rarely on the sugar syrup. Refrigeration will also slow the process as will alcohol, but that's usually at about 80 proof. i.e. 50/50 mix of sugar syrup and overproof.

I add a drop or two of lime juice to aid in the breakdown of the sugar in water and to help keep it from crystalizing later.