Hello yoon and all, I have put plain white vinegar in my syrup for several years, a quarter cup for every 10 pounds of sugar and it stops the black slime. I have about 4 gallons of syrup left over from fall that is still just like it was made. I do add my sugar to boiling water and instantly turn off the heat and stir to dissolve. I have sometimes wondered if the ickey stuff is in the sugar and the boiling water also has helped to kill off the cause. Al Boehm Columbus N C :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: -- Visit www.honeybeeworld.com/BEE-L for rules, FAQ and other info --- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::