I just made a 5gal batch of a honey cream soda, force carbonated in my keg and bottled into glass using a counter-pressure bottle filler. Most of the batch was drunk the day after carbonation, and it turned out great. Within 3-4 days, though, the remaining bottles all had a distinct layer of sediment at the bottom, increased carbonation levels, and a slight off-taste - obviously some nasties got in somehow.
Everything was well sanitized, but I'm thinking part of the problem was my recipe; most commercial sodas you see have preservatives and/or extra acid (phosphoric, ascorbic, citric, etc), but my recipe had none (honey + brown sugar + molasses + vanilla extract). I'm guessing the role of the acid is to help stave off infection. Does anyone have any tips for adding safe preservatives? How to formulate a recipe that involves a sufficiently low PH? Anything else to help stave off infection?