Maasrot, Chapter One, Mishnah Six
1) Dried pomegranate seeds, raisins and carobs, [are liable for tithes] after he has made a pile.
2) Onions, once he removes the onion seeds.
a) If he does not remove the onion seeds, after he makes a pile.
3) Grain, once he smoothes out the pile.
a) If he does not smooth the pile, after he makes a pile.
4) Pulse, after he has sifted it.
a) If he does not sift, after he smoothes out a pile.
5) Even after he has smoothed out the pile, he may [without tithing] take from the broken ears, from the sides of the piles, and from that which is mixed in with the chaff, and eat.
Section one: These dried fruits are liable for tithes once he has made them into a pile in order to bring them to market. [As an aside, I am eating a delicious pomegranate, fresh not dried, while I write thisyummy and healthy! It doesnt get any better than this.]
Section two: Albeck explains that the mishnah refers to removing the mothers of the onion, the parts of the onion used to grow new onions. Other interpreters make a slight modification in the spelling of the Hebrew, and interpret the mishnah to read once he has peeled the onion, which would refer to the very outer skin.
The default point for onions and for other types of produce is when he has made a pile which is the first point when it comes to the dried fruits in the previous section.
Section three: Grain is liable for tithes once he smoothes out the pile, but if he doesnt smooth out the pile, then it is liable from the time he makes a pile.
Section four: Pulse, or legumes, is liable for tithes once it has been sifted in order to remove the shmutz, the dirt and pebbles. But if he does not intend to remove the shmutz, then pulse too is liable for tithes once he has smoothed out a pile.
Section five: This section refers to the grain mentioned in section three. Although grain is liable for tithes once he has smoothed out a pile, he may nevertheless continue to nibble from certain parts of the grain, such as ears of grain that have not been successfully threshed, without tithing them. He can also eat ears of grain that are on the sides of the pile and not in the pile itself, and he can eat ears of grain that are still mixed in with straw. In other words, he cant eat from the ears of grain that have been successfully processed and placed in the pile, but he can continue to eat, without tithing, from ears of grain that were not successfully processed.