I use 1 cup of citric acid to 150 gallons of syrup. I wouldn't worry too much about HMF in sugar syrup unless you get it too hot or if you reheat the syrup to excess. HMF is basically a byproduct of what chemists call a browning reaction (carmelization) and the main sugar of concern is fructose. HFCS is especially high in fructose so its especially vulnerable to HMF production when overheated. I always like to err on the safe side so I heat my water to nearly scalding temperatures in a large diary tank that has an agitator paddle. I then remove the heat source, add sugar and keep the paddle spinning until it's all dissolved. After adding the sugar, bag at a time, the solution cools so the syrup is warm to the touch but never hot when it's mixed at 2:1 by weight. That's when I add the citric acid and I let it spin (usually overnight) to let the acid do it's thing. The ratio I use was passed along from another commercial beekeeper, not a chemist, so whether it's the optimal amount is a question for which I have no answer. Works for me. They tell me that bees don't gain weight on 2:1 syrup in fall but mine do get heavier so I suspect I am getting a pretty good inversion. You could add more citric acid and probably be ok too. I buy organic food-grade citric powder on ebay and it's inexpensive. Other acids will invert sucrose but citric is the one I use. Sucrose is a disaccharide (di=2, and saccharide= sugar). In sucrose, those 2 sugars are glucose and fructose; the acid splits (or inverts) the sucrose to glucose and fructose. Bees do that too but they expend lots of energy in the process (hence, why hives tend to gain little, if any weight, when fed straight sucrose--they burn so many calories to break the molecule apart that weight gain can be a wash). If you do it for them, they will store more and the hive will get heavier. More important in fall than in spring but I always invert the syrup--they still draw comb like crazy in spring, just like straight sucrose.
