Adding vinegar to the sugar syrup is an attempt to prevent short term fermentation. However, a number of variables play a role like ambient temperature, cleanliness of tanks and containers, time the syrup is taken and what not. The outcome is not always as wished for.
 
Over the years I have experimented quite a bit to solve this problem and, without going into details of my experiments, I found a solution which completely satisfies me. Adding thymol did the trick.
 
I fill a barrel with 108 litre water and add 160 kg granulated sugar for a 60% sugar solution. A submersible sump pump is used to dissolve the sugar in the cold water and I add a measured amount of thymol stock solution. After the solution has cleared, it is filled off in 2 litre plastic pop bottles. I take care that I have always bottles ready to go. The left over full bottles from winter feeding can be used the next year.
 
I have used thymol for 6 years now. After emptying a bottle there is always a small  amount of syrup left, which I used to clean out before refilling but even that is not done anymore. I have one bottle from April 17, 2008 sitting around, which still doesn't show any sign of fermentation. The first sign of fermentation is when you shake the bottle and the solution turns milky. This is caused by small carbon dioxide bubbles. Is the fermentation more advanced, the bottle gets pressurized. Squeezing the bottle will tell you.
 
The following links have info about the thymol stock solution:
http://www.scientificbeekeeping.com//index.php?option=com_content&task=view&id=56 (at bottom of page)
http://www.dave-cushman.net/bee/thymolx1.html
 
 		 	   		  

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