"slow the growth, as would the addition
of acid, bleach, some essential oils, or a preservative."
 
I make my syrup in 50 gallon batches with an electric rubberized heat belt at the bottom of the barrel.  I add 1/8 tsp regular, plain, household bleach per gallon of syrup, which amounts to 1-1/8 cup per batch after all the syrup dissolves.  I use a large paint paddle on an electric drill to agitate.  There's more chlorine in my drinking water than this bleach provides, and probably a whole lot more from my neighbor's swimming pool.  Don't freak out on me.
 
I generally feed the syrup the next day after it cools and the bees seem to lap it up prior to spoilage.  I've never emptied a spoiled or soured bucket.
 
I've added thymol, from time to time, at 1/2 gram per gallon weighed on a postal scale, pre-mixed with vodka...never touch the stuff, myself, and don't get too concerned about others who question serving alcohol to underage nurse bees. 
 
Honey-B-Healthy works to preserve (though my experience is how often it incites robbing in times of dearth), at 1 tsp per quart (if I'm feeding quarts, like in my nucs) or 2 tablespoons in my normal 2 gallon feeders.  I generally add HBH per container and not to the large batch.
 
Grant
Jackson, MO

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