Chapter 1, Mishnah 6. The Mishnah continues to discuss the definition of the completion of processing (gemar melachah) with respect to various types of produce, listing for each type the stage at which the processing ends and casual eating becomes forbidden.
The types of produce and the definition of completion of processing for each:
"HaPered, HaTzimukim v'HeCharuvim" - the pered is a dried pomegranate; the plain meaning of the word is a piece that has broken off, and it refers to a piece of pomegranate that broke off and was set out to dry. The tzimukim are dried grapes (raisins), and the charuvim are carobs, in their ordinary sense. Once he has gathered them into a pile - that is the completion of processing, and from that moment casual eating is forbidden.
Onions: once he has removed from them everything he does not intend to bring to market - cutting off the roots and the base, shaking off the dirt, and the like. And if he does not intend to shake these things off, then once he has gathered them into a pile - that is the completion of processing.
Grain: once he has smoothed the pile, that is, after he has threshed and winnowed and gathered all the kernels into one large heap, and then comes and smooths off the edges with a pitchfork - this smoothing is the final stage in the processing of grain, and it constitutes the completion of processing. And if he does not intend to smooth it, then once he has set it up as a large heap that has not been smoothed - that is the final stage of the process, and it is considered the completion of processing.
Legumes: beans, peas and the like - once he has sifted them in a sieve, which is a straining utensil. Sifting is required because legumes are pulled out of the ground with dirt clinging to them, and the dirt must be separated from the mixture. And if he does not intend to sift them, then once he has smoothed the pile of legumes - that is the final stage of their processing.
The portions that never reached full processing:
Even though he has already smoothed the pile of wheat, he may still take from the "kotea". The word comes from the sense of severing, that is, a portion that is separate and set apart, and it refers to the bits of wheat that remained behind in the chaff, still inside the stalks, or that were left at the sides of the pile, or that got mixed in with the straw at the time of winnowing. From all of these it is still permitted to eat a casual meal.
Explanation of the principle:
Until a person finishes processing his food, before the completion of processing, it is permitted to eat from any of the food in the manner of casual eating. This principle holds true even when he intends to make five piles of wheat and he is in the middle of the work, having made three of them so far. Even though he has completed three piles, since two still remain he has not reached the completion of processing, and therefore he may still eat from the wheat, and even from the wheat that has already been gathered into piles, since the completion of processing has not yet occurred.
Only when the entire crop has finished the processing procedure does the whole crop become restricted from casual eating before the separation of tithes. However, even after the processing of the entire crop has been completed, and the food products have become forbidden to eat until after the tithes are separated, this applies only to the crop that was processed and is located in the main pile.
In summary: whatever missed the processing - and this is the case our Mishnah is dealing with - the small pieces that were left off to the side, or that never made it to the threshing floor, or that never properly came out of the threshing floor, those are still permitted to be eaten. Even after the completion of processing, these pieces remain permitted for casual eating, and this is precisely the ruling of the Mishnah here.