Honey-B-Healthy helps to inhibit mold. But some of my syrup will ferment, especially if I stir in any of the protein supplements like Mega-Bee.
Randy Oliver gave me the best formula. 1 tsp of household bleach per gallon. It's not that much that you even notice it and within 24 hours, the bleach breaks down but still protects the syrup.
I had a hard time accepting the addition of bleach. All I could think about was the paranoia that we had as young parents that our toddlers would drink bleach.