I MADE MY OWN FONDANT, A WHILE AGO. SEEMS TO SMELL LIKE IT HAS GONE SOUR? SMELLS A LITTLE SOUR AND HAS SOME AIR IN IT. I BELIEVE IT HAS FERMENTED? IS THAT POSSIBLE? DON'T THINK i AM GROSS BUT IT IS FOR A SHOWPIECE THAT NOBODY IS EVER GOING TO EAT...WOULD IT MAKE A DIFFENCE IF I JUST KNEEDED IN A LITTLE SUGAR? OR MAYBE THE PIECE WOULD BE TO BREAKABLE??? HAS THIS EVER HAPPENED TO ANYBODY???
How long ago did you make it? What recipe did you use and what did you use for the liquid? Fondant lasts a long time, but it can definitely go bad. If it smells bad, I wouldn't take a chance on it.
Also, is this fondant or gum paste? Your subject line says gum paste ... if you are using it for flowers or something that will dry completely, it might still be okay. No one ever eats my gumpaste flowers, so I wouldn't be quite as concerned. I would probably still start over though.