When I compare commercial sodas to some basic homemade sodas, a couple of things I notice right off the bat are the difference in that frothy head on top and the difference in body or mouthfeel. So after doing some research, here is what I've come up with.Gum Arabic Powder
Also known acacia gum, this powder helps your homemade soda in a couple different ways. Acacia is sometimes used in beverages to emulsify flavoring oils. The original Orange Crush recipe used this to mix orange oil in water, and it still uses it today. I'm not sure if Crush was the first beverage to use it, but I do believe it was one of the early ones. Some species of acacia produce a gum that has better emulsification properties than some other species, however, you don't know what species it comes from specifically unless you go to the source. So it may not be the best way to mix your flavor oils with water.
One property you can count on with any species of acacia is it's ability to thicken. just a small amount will give your soda body that you didn't know you were missing. Along with that comes a decent head retention as well. Compare the two most recent pictures I've posted here. My photo for homemade cola, and my photo for homemade Fresca. The cola I added just a pinch of acacia gum to the syrup, whereas the fresca I added nothing. You can definitely see the difference.
Acacia gum can be difficult to get to dissolve, so you make sure you have a hand mixer or something with some force behind it to get it to mix in correctly.
There are also other options you can try as well. Maltodextrin
is also a thickener used in beverages, it's used by homebrewers regularly because it's not fermentable by yeasts. You can usually find it at a homebrew store if you want to source it locally. Another good foam stabilizer that I have not tried in a soda is a protein. Proteins develop matrices that trap air and hold foam quite well. Meringues and whipped cream are good examples of protein stabilized foams. For sodas, I've considered adding a small amount of whey protein powder, but I haven't had the opportunity to do so.