I fermented 2 batches of soda....One batch of rootbeer turned out good....a seccond batch of birch beer and it turned out spoiled or sour.....any ideas....I used ale yeast on both kegs. I sanitized both kegs and left them ferment 4 days in 70=75 degree environment (finished off basement) I refrigerated the beer on friday and today tested it to my dismay.....