One of the Dr. Oetker products is a fruit glaze, which sets to a clear gelatin, and which the ingredients list indicates to be principally tapioca starch. Can I just use an equal quantity of cornstarch to do the same job instead? Do the two starches behave similarly enough to be a straight substitute, or are there differences which will make for a noticeably different result?
Unfortunately no. Cornstarch will cook up white while tapioca starch cooks up remarkably clear.
You may be able to find tapioca starch at your local Asian market or if your grocery store has a decent organic or gluten-free section.
That said, if you speifically need to avoid tapioca for some reason, you could try arrowroot powder. I haven't seen it myself, but I've read that it also cooks up clear, but maybe not as thick as tapioca.