well i never really thought about it but if brewing tea and the foam created is anything like brewing beer and that foam then id have to say the foam ontop of the brew isnt showing there is material in the solution thats not been broken down yet, but showing that the microbes in the solution are doing their thing, when there is no more foam being produced it shows the microbes have eaten everything they can, if you add some more molasses for example the solution will begin to foam again shortly as the microbes start to break down the new food(molasses) you added, much in the same way as the foam ontop of brewing beer is basicaly 1 of 2 waste productes produced by the yeast, Co2 (causing bubbly foam) and Alcohol... so if the process going on with the microbes is similar to that of yeast and booze then id say the foam shows the herd of microbes is healthy and doing their thing breaking down whats needs to be broken down. as well as the simple comparison to beer brewing, my hydro store guys told me, when i asked about the brewing process of their root tea, that the best time to use it is after at least 12 hours of airation and if there is still a good head of foam as this shows the microbes are active, and if there is no more foam to add more tea mix or molasses, bubble for another few hours then feed the plants.
just my 2 cents, either way if you have a good formula for tea its gonna be ever so helpful and youll notice the difference.