Well, one other thing to point out . . . besides bulk shipping food like you mentioned . . . there is a good chance the 'spice' trade would pick up again. Sure rice is great . . . but generally you will want something to give it some flavor so traders would probably also ship kegs of curry, soy sauce, ginger, cinnamon, etc which aside from soy sauce (can it be de-hydrated? less weight/volume) will take up less space but will have a higher value per milligram than the rice.
So rice would be high volume but low margin . . . and the spices/seasonings would flip that, low volume buy high margin.