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

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