The biocide is actually a lucky by-product of the FSII (fuel system icing inhibitor) which gets added to turbine fuel (I can't remember the % now but it's not much). Without it a bacteria called cladosporium resinae lives on the kero and eats its way through alloy fuel tanks.

As for castor, I've never had any go off and especially not when mixed with methanol. Raw castor slowly oxidises (that's why engines gum up over a long period) though so it's best kept in an air tight container same as you'd store fuel.