I have noticed that when brewing a variety of teas that the first 1-2
brews will sometimes generate a foam. I have been taught that if this
happens when brewing in a gaiwan to just scrape the foam off with the
lid and discard.
My question is, what causes the foam? Is it debris/dust in the leaf?
Is it chemical residue? Is it a plant constituent?
I have noticed this foam on both oolongs and sheng puerhs (the teas
that I primarily drink), and in the case of puerhs, I notice it very
often on Xia Guan produced teas.
Any info or advice?

Thanks
David L.