Demai, Chapter 7, Mishnah 4. Our Mishnah discusses someone who buys wine from the Kutim. It is worth noting at the outset that this is not something that could be done in our times, since in the era of the Gemara it was determined that the Kutim were idol worshippers, and therefore they were given the status of full-fledged non-Jews in every respect, and one may not drink their wine.
The status of the Kutim in the time of the Mishnah:
In the time of the Mishnah the Kutim were still considered trustworthy regarding certain halachos, and therefore they were believed that their wine was kosher, and it was permitted to drink it and to buy it. However, they were not believed regarding the separation of Terumah and tithes, because the Kutim were not concerned about the prohibition of "You shall not place a stumbling block before the blind": they had no problem feeding someone else something that was tevel, even though they themselves would not eat it. Therefore, wine bought from a Kuti requires the separation of all the Terumah and tithes.
The question: separating on Shabbos:
Our Mishnah deals with a case in which the wine was bought on Friday, close to the onset of Shabbos, and there was not enough time left to separate Terumah and tithes from it before Shabbos. How, then, can one separate on Shabbos so that it will be permitted to drink the wine?
The Mishnah says: "HaLokeiach yayin mibein haKutim omer" - one who buys wine from among the Kutim says a certain formula. He cannot actually separate the Terumah and tithes on Shabbos, and he also cannot identify their precise location within the wine, since the very act of identifying the location is considered the separation itself, and that is a problem on Shabbos.
The stringency with definite tevel as opposed to demai:
Here we are more stringent, because we are dealing with definite tevel and not with demai. With demai we are lenient about identifying the location of the Terumah and tithes, as we have seen in earlier Mishnayos - that one may separate and state that this portion is not to be eaten and it will be terumas maaser. With definite tevel this approach is not available.
The wording of the declaration:
"Shnei lugin she'ani asid l'hafrish harei hein terumah" - suppose he has a hundred log of wine before him: the two log that he will separate in the future are considered Terumah.
"Asarah maaser" - another ten log are considered maaser. In practice it is nine log and eight tenths, since after separating the two log only ninety-eight log remain, but the Mishnah says "ten" as a round number.
"Tishah maaser sheni" - after separating the ten, about ninety log remain, or slightly less than that, and therefore nine log will be maaser sheni.
"Meichel v'shoseh" - from this point on he may begin drinking the wine on Shabbos.
The principle of 'yesh bereirah':
This Mishnah operates according to the principle of "yesh bereirah" (there is retroactive determination) - based on the assumption that one can say that what he is drinking now is from the portion that will ultimately be ordinary produce, and not from the Terumah and tithes. For since he did not identify their location, he cannot say, "This I am drinking and this I am not drinking"; rather, he needs bereirah, saying that it will be clarified retroactively that when he later separates the Terumah and tithes, they will not be from the wine he has already drunk.
It emerges, then, that this Mishnah follows the opinion that we do say bereirah. But according to the opinion that we do not say bereirah - and especially since regarding a matter of Torah law we do not say bereirah - one cannot proceed in this manner.
In summary: We learned that in the time of the Mishnah the wine of the Kutim was fit to drink, but because they were not concerned about the prohibition of "You shall not place a stumbling block before the blind" they were not trusted regarding the separation of Terumah and tithes. One who buys wine on Friday without time to separate cannot identify the location of the Terumah and tithes on Shabbos - a special stringency with definite tevel as opposed to demai - and therefore he recites a formula of declaration: two log Terumah, ten maaser, nine maaser sheni, and he immediately begins and drinks. The basis of this halachah is the principle of "yesh bereirah," and according to the opinion that we do not say bereirah this approach is not possible.