Maasrot, Chapter One, Mishnah One
Our mishnah teaches two general principles with regard to what foods cannot be eaten before they are tithed.
1) They said a general principle concerning tithes: whatever is food, and is looked after, and grows from the land, is liable for tithes.
2) And they have further stated another general principle [concerning tithes]: whatever is considered food both at the beginning and at the conclusion [of its growth] even though he holds on to it in order to increase the quantity of food, is liable [to tithe] whether [it is harvested] in its earlier or later stages.
3) But whatever is not considered food in the earlier stages [of its growth] but only in its later stages, is not liable [to tithe] until it can be considered food.
Section one: This is the same principle we saw with regard to peah in Peah 1:4. Only plants that are food for humans, and are worthy of storing (looking after) and grow from the land (according to the rabbis this excludes mushrooms) are liable for tithes. Foods that dont fit all three of these categories need not be tithed before they are eaten.
Section two: If there is a plant that is generally eaten in its early stage of ripening and in a later stage, it must be tithed no matter when it was harvested. Even though he might generally leave it in the ground in order to give it time to grow bigger, since it is also eaten at the earlier stage, it must be tithed when it is picked earlier. We shall see examples of this below in mishnah four.
Section three: However, if the produce is not considered edible at its earlier stage of growth, if it is nevertheless harvested at this earlier stage, it can be eaten without being tithed. An example might be a banana. A banana harvested at an earlier stage is not considered food and therefore one who does eat such a banana need not tithe it. It would only need to be tithed if harvested when ripe.