I recently brewed a golden strong Belgium ale for the second time. The first time it turned out amazing. The most recent, not so good. I followed my typical single infusion mash procedure, boiled, chilled, aerated and pitch from a yeast starter made the day before. I hit my gravity of 1081 and fermentation began a few hours later. I kegged it and noticed an old scent, very sour like an apple. I tasted it and it tasted like a rotten sour apple. I figured i got an infection although never having one before i asked the dudes at the beer store and they argued tooth and nail that it was over fermentation that caused that type of flavor despite my assertion that my final gravity was 1012. Any beer judges have an idea what i did wrong? Was it an infection?