Now that the sun has started to shine in the frozen north I can start thinking on using my espresso machine for some cold drinks. I was thinking of a banana iced coffee. But I would like a more natural flavour than most of the syrups I can buy in my local coffee shop. Do I was wondering if it would be possible to extract the flavour from a banana, without getting the texture/consistency from the fruit?
Consider making a banana consomme via gelatin filtration. As described in Harold McGee's 2007 NYT article, this technique produces liquids with the taste of all sorts of foodstuffs that are otherwise full of texture and color. A tomato consomme, for example, is clear and watery, rather than red and pulpy, while retaining the flavor of a tomato juice. The same technique can be (and has been) done with breads, meats, vegetables, fish, etc.
The process is to make a loose banana gel by mixing a banana with water, adding gelatin, and heating until the gelatin is dissolved. The mixture is then frozen until solid, then suspended in a strainer over a bowl in the refrigerator for a couple of days until the liquid essence distills out and the solids are left in the strainer trapped in a gelatin net.
The particular advantage I believe gelatin filtration will give you in this case is that it will give your coffee a full banana flavor without making it thick, pulpy, or cloudy.
Although isoamyl acetate is the most commonly used bannana flavoring commercially, this is usually produced artificially (like banana milkshake powder) and so isn't what you want. I would make a 'natural' bannana syrup to flavour the coffee. I would try layering halved bannanas with sugar and leave it for a few day to let the water be drawn out. Then I would discard the bananas and add enough water to make a syrup and heat it to dissolve. Having not tried it I don't know if it would produce a strong enough flavour; I don't know if anyone else has any other ideas...
Let the bananas turn as brown and spotty as you dare. The banana flavor improves as they ripen and over-ripen. Then freeze them and thaw them again. This will bust up the cell walls and turn them into a gloppy mess. Use a colander or something to separate the liquid.
I would try steeping the banana peels in alcohol. The peels actually contain quite a bit of flavor but aren't generally usable, and they should impart minimal texture to the final infusion.