The transport system V0.1I have labeled this V0.1 because I can imagine it changing a couple of times in the future.
Since the weight of the grain and the size of the individual grains, a valve doesn't seem possible in my opinion. There would always be something stuck in between causing it not to open or close properly.
What should work is a wormwheel (don't know if that is the right term) but since I have absolutely no idea where to find that I'll have to make one myself using a large wood-drill. The idea is to drill an L-shape tube in a wooden block and let the drill rotate in reverse inside the tube so it transports the grain to the end of the tube where it will fall in the hole into the recepient.