I hate when I can’t answer a kid’s question. I have a glass filled halfway with rootbeer, I plop a scoop of ice cream into it, immediately the surface begins to bubble and it foams up to the top of the glass. I take a spoonful of the foam and it tastes like rootbeer. Why did the rootbeer/ice cream mixture foam up? Is the foam a liquid, solid, gas or a combination of them all?
After doing a little research, it seems that somehow the sugar in the ice cream interacts with the gasses in the rootbeer to make it foam up, but I am not sure how.
Well, in the dictionary, the definition of ‘foam’ starts off…
“a light frothy mass of fine bubbles formed in or on the surface of a liquid”
Which seems like a good place to start. It’s tiny bubbles… which are envelopes of gas enclosed with a thin film of liquid. In the specific case of rootbeer foam, there might well be tiny solid particles (ice crystals from the ice cream?) in the more liquid parts of the foam, though not in the ‘foamier’ parts probably.
As far as why the mixture foamed up, I’m not too sure about that. You have the root beer, which is a supersaturation of carbon dioxide gas inside a water solution of sugar and other flavorings. (The distinction is that the carbon dioxide is at the point of precipitating out of solution, which is how bubbles form in root beer and other soda, while the other solutes inside root beer don’t tend to do that.)
Ice cream is… I’m not quite sure what, technically. It does have tiny air bubbles suspended in it, and ice crystals, and a liquid sugar-cream base (along with any other flavorings in it.)
This is a wild guess… I’m not sure that sugars have that much to do with the process. The addition of the ice cream will chill the root beer, and also provide a bunch of tiny solid particles, all of which IIRC will aid in the precipitation of CO2. The cream ingredients from the ice cream aid in the formation of highly stable bubbles. That seems to give us all the requirements for some foam.
I’m gonna guess it’s the protein in the ice cream that causes the bubbles to persist. Normally, in just plain rootbeer, the CO2 bubbles reach the surface of the drink and explode, releasing the CO2 to the environment. Now that you’ve added ice cream, the protein in it acts as a surfactant? (is that the right word?) causing the bubbles to be much stronger. Consequently, the bubbles which would just explode in the plain old rootbeer stay in bubble form in the float and add up.
One of God’s perfect foods. Some people overanalyze.
For those that do I’d lean toward milk protiens rather than the fat content as you can make perfectly good and reasonably durable foam by frothing skim milk.
