We are learning the fifth Mishnah of the fifth chapter of Maseches Demai. This Mishnah too deals with the concern of demai - the doubt that maaseros may have been separated from part of the produce a person has and not from the rest.
"HaLokeach min he'ani, v'chein he'ani shenasnu lo prusos pas o filchei develah":
The Mishnah is speaking about two cases:
"HaLokeach min he'ani" - one who buys food from a poor man who collected his food by going begging from door to door.
"V'chein he'ani" - the poor man himself, when he is a chaver and wishes to separate from the demai in his possession.
And what did they give him? "Prusos pas" - pieces of bread, or "filchei develah" - develah is a pressed cake of figs that has been cut into segments, and they gave him various segments of it.
"Me'aser mikol echad v'echad":
He must separate maaser from each piece of bread and from each segment of develah on its own, since we must be concerned that he received them from different people - some of whom separated maaser and some of whom did not. Therefore each and every one has to be tithed separately, both in the case of one who buys from the poor man and in the case of the poor man himself who received the food from different donors.
"U'vitmarim u'vigrogaros - bolel v'notel":
With dates and dried figs he may mix them all together and separate from the mixture, for our Mishnah holds that with demai we say "belilah" - that is, that the items become evenly mixed, even in the case of dried fruit. Since he mixed the figs and dates together, the separation takes at one and the same time from what is obligated in maaser and from what is exempt from it, and it turns out that he is separating from the obligated on behalf of the obligated, since the fruits are spread throughout the mixture in equal proportion.
The concern that he might separate from what is exempt from maaser on behalf of what is obligated in maaser exists specifically when the items are not distributed evenly; but here, in a mixture, they are evenly distributed.
"Eimasai? Bizman shehamatan merubeh":
The Tanna qualifies the law of belilah and distinguishes between two situations:
"Bizman shehamatan merubeh" - when the gift given to the poor man, those dates and dried figs, is a large quantity. Then we can say that the fruits became properly mixed, and when he takes from the large quantity his separation contains the correct proportion of obligated and exempt fruits.
"Aval b'matan mu'at - me'aser mikol echad v'echad" - when it is a small quantity of fruit, he does not mix them but separates from each and every fruit on its own, like the law at the beginning of the Mishnah. The reason: with only four or five fruits there is no way to mix them properly, such that he could say that with each and every separation he is taking the correct proportion of obligated and exempt.
In summary: In this Mishnah we learned that one who buys from a poor man, and likewise the poor man himself, who received pieces of bread or segments of develah from different donors, separates maaser from each and every one, out of concern that some of them were tithed and some were not. But with dates and dried figs he mixes and takes, because with them we say "belilah," provided that the gift is a large one; but with a small gift, where there is no possibility of even mixing, the law reverts and he tithes from each and every one.