Force-carbed soda bottles getting infections - preservatives?

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?

What was your recipe process? Did you heat the honey and sweetners in some fashion? It sounds like it started fermenting from some natural yeasts which either weren't killed off during the pasturizing process, or had gotten a hold somewhere from something that wasn't sanitized. If you sanitized, then I'd guess it was the pasturizing process.

I don't know a lot about this, but there are specific bugs you have to watch out for. Some are killed by low pH, and some are not. To eliminate those that are not, you have to boil or use preservatives. FDA has strict limits on how much preservative you use— bugs aren't the only things they kill! And the pH in your mix sounds pretty high, which is why some makers add citric acid as well.