Fermenting Soda

Packaged soda contains preservatives intended to prevent microbial activity such as Sodium Benzoate and Phosphoric acid. I would guess this is interfering with the yeast. Also it is quite acidic overall, some google links say PH around 3.3. This would also cause problems with the yeast.

As others are saying just add vodka (you can distil your own if you really want to ferment something and it's legal where you live) to the soda.

To be honest fermenting soda is probably not a good idea. This is because alcohol made from synthetic sugars always comes out weird. Usually it's not even recommended to add sugar to a mixture to produce alcohol, because using any kind of sugar that isn't completely natural just makes the result bad. If you want a alcoholic soda I'd recommend just mixing in some sort of high proof spirit, like vodka or something, rather than trying to ferment the soda itself. Soda flavoring also pretty much 100% comes from the sugar, so when the yeast eats that you're not left with anything, which is why your end result didn't taste anything like Mtn Dew.

Mix cane sugar and water in proportions to get the abv you desire, ferment like you normally would. Obtain a box of fountain soda syrup, I suggest a local restaurant as a source, mix syrup in after fermentation and carbonate.

First, the Dew taste was gone because without sweetness, MD tastes like unripened grapefruit juice. Any corn syrup in the MD was converted to alcohol, so you did not get any sweetness.

Second, the preservatives, as stated above are going to affect your yeast. That's why the preservatives are ' n there, to keep it from being affected by spoilers, such as yeast.

So, here's what you can do...as stated above, add Everclear to MD. But, I suspect you want a Mountain Dew Brew. So, make it like you did, by the way, adding sugar was a good move if the MD refused to let the yeast convert its syrup. But, after brewing it, mix it with new Dew.

One more thing. If you can get imported Mexican Mountain Dew, use it. It is made with real sugar instead of corn syrup.

Why not boil down the mountain dew to reduce it to a thickened state and add it in late in the fermentation (or even afterwards)

Your best bet is going to be start with a simple beer/wine/mead recipe, depending on what you prefer, to get your alcohol. Then, you can throw in a liter or two of Mt. Dew if you want, for flavoring, but keep in mind that sodas are generally only about 10% - 20% fermentable, so actual soda will only water down your brew. The key, however, is finding a concentrated flavoring to add, like this mountain dew candy flavoring:

http://www.amazon.com/Mountain-Dew-Flavor-Candy-Flavoring/dp/B0000DK64P

You could also throw in some lemon or lime zest, maybe a pinch of anise, and other things to replicate the Dew taste.

I don't recommend using much Mt. Dew though, for the following reason:

If you were to pitch perfectly healthy yeast, like that harvested from the krausen of an actively fermenting beer, into pure Mt. Dew, I think you'd find that you wouldn't get more than about 1% ABV from fermentation. It's just not very fermentable, and it doesn't usually taste much like the original soda after the yeast have their way with it.

Going back to recipe formulation, I might start with something like 11 lb of pale 2-row malt (or 7.5 lb LME), an ounce of a noble hop like Saaz, and the aforementioned flavorings.

If you brew it, let us know how it turns out!

Chris Colby, the editor of BYO magazine has what appears to be a mild obsession with using Mountain Dew in beers. There are a couple of recipes that have appear in the magazine: Beelzeboss (Oct 2007 issue) and Mt. Brew (Mar/Apr 2005 issue).

His experience seems to be that the preservatives do slow fermentation somewhat, but they are at a level designed to deal with normal spoilage encountered on the shelf; they are no match for a large, healthy colony of aggressive yeast.

I think the key is to give the yeast every chance to thrive in an environment which is somewhat sub-optimal: make a good sized starter, aerate well before pitching and add some yeast nutrient. White Labs makes Servomyces, which I have not used myself, but which seems to work well for those who have tried it. Also, be patient and let the fermentation have time to complete.

Now I think of it, it sounds very similar to brewing mead. Honey is a mild preservative, after all.

Why don't you just add vodka or something? Fermenting seems a bit elaborate when all you want is basically Mountain Dew with some alcohol in it.

I think your biggest problem might be the preservatives (sodium benzoate and others) used by Pepsi when making Mt. Dew. Some of those are sure to give the yeast some trouble growing and fermenting your drink, or at the very least will lead to additional stress on the yeas that produce off flavors.

Wikipedia has a good list of ingredients in Mt. Dew, along with links to individual pages with explanations about many of them. Perhaps you could try making your own Mt. Dew syrup from the list of ingredients, but leave out some of the preservatives and more obscure industrial chemicals to help the yeast ferment.