Thanks guys. I'll decontaminate the boiler but there's no sign of foaming anywhere and never heard of this being a problem for tiny boilers, but it's worth a try. Boiled water also comes out looking very clean. The fitting for the steam line has two internal diameters forming a socket for the line to be soldered into, so it's impossible for the steam line to extend beyond the fitting into the boiler. Sounds unlikely, but maybe the fitting itself is too long due to a factory screw-up, who knows.