Dave,
Basicaly you get 2 types of bean/seed oil, 1/ first press , 2/ extreme pressed , were all remaining oil is extracted. Now first press oils useually contain no gums, medicinal castor is 1st press as it also contains no/low levels of the toxin makeing it suitable for internal use, this also means its suitable for our model motors. You'll comonly see Olive oil sold as virgin, this is 1st press and is also low in gums. The remaining oils extracted by extreme pressures are the type needing degumming.
Back to castor, some castor is heated to cause a level of polimeriseation which modifies the molecules allowing it to mix with petrolium based fuels. A good read on castor can be found here :-
http://modelenginenews.org/faq/index.html#qa5
Stewart