This one is for my brother-in-law. He spent some time in Honduras and he always misses their banana sodas that they apparently have down there.
If you recall, I had tried using an actual banana for flavor, but that just turned out to be such a mess, and had more of a cooked banana flavor once it was finished. Maybe I'll save that method for a banana-bread type soda flavor if I want to do further development there.
To get more of the fresh fruit flavor, we're going to need to go the extract route. I used Watkins Banana Flavor because it's a decent banana flavoring, and it's readily available at most Walmart Supercenters. You can also order it online direct from Watkins or through Amazon. Another key part to getting a fresh fruit flavor is some added tartness. You can get that using citric acid or you can add some lemon juice. If you use lemon juice, try not to use too much because it will carry the lemon flavor with it.
1 tsp Watkins Banana Flavor
1 lb sugar + 1/2 to 1 cup of water for syrup
1 tsp citric acid or 2 tbsp Lemon Juice
top up to 1 gallon with carbonated water
Start by making your syrup, you can add some cream of tarter and boil to 240° to invert the sugar if you would like, or you can use your sugar as is. Allow your syrup to cool somewhat before you add the flavor and the citric acid. One important thing to remember with flavorings is that they are usually mixed with an alcohol base which boils quickly and at a lower temperature than water. It's not uncommon to pull your syrup off the stove, throw in your flavor and *POOF* it boils right off. Of course that smells great right away, but it doesn't leave as much to linger in your finished beverage. You also need to wait to add citric acid because it can burn easily and leave behind a bitter taste. If you've ever used citric acid but it comes out bitter and you have no tart left, you've let it get too hot at some point.
Once the syrup has cooled, you can add it to the carbonated water by the glass, or in a larger bottle. You can color it if you wish. I find that about 4 drops of yellow and 1 drop of neon green looks great for banana.