The pH of honey and the pH of sugar syrup are quite different. One effect of this is the bacteria in the gut of the bees (which protects against nosema, AFB, EFB and Chalkbrood) does not do well with sugar syrup. Ascorbic acid seems to do nothing to set off robbing. Vinegar sets off massive robbing in my experience. It's the smell, I sure, as when using vinegar the bees will come out to meet you. Inversion is a side effect of the acid, not an effect I am looking for. The syrup keeps much longer at a pH of 4.5 or so.