Chapter 10, Mishnah 3. The previous pair of mishnayos dealt with taste: if the taste of the forbidden item is detectable in the mixture, the mixture is forbidden, unless it imparts a taste that spoils, or unless it is a case of a substance that gives its own status, where even though the taste is not detectable the mixture remains forbidden.
In Mishnah 3 and Mishnah 4, however, a different question stands before us: not a question of taste, but of smell. Taste is what is sensed in the mouth, while smell is that same flavor picked up through the nose.
The difference between taste and smell:
It is hard to pin down exactly how Chazal understood what we today call chemistry, but the best definition of the distinction seems to be this: if what passes from one item to the other passes through direct contact, that is taste, since there is substance here, the material itself mixes in. Smell, on the other hand, means passage through the air, without direct contact, and therefore there is no physical presence at all: the forbidden item never touched and never mixed with the permitted ingredient. Even so, as we will see at the end of the mishnah, this very distinction leaves room for further thought.
The case in the mishnah:
A person "rodeh" - removes bread from the oven. The word "rodeh" is the verb for taking bread out of the oven, and removing the bread was considered a fairly complicated task, one that only a professional knew how to do without the loaf falling into the oven. The oven was a sort of cave with the fire burning in its middle, and the dough was thrown against the wall and stuck to it; to remove it they used a kind of flat wooden implement mounted on a stick, like the one familiar from pizzerias.
The bread comes out of the oven and it is "chamah" - hot, and it is placed on the mouth of a barrel of wine, a barrel that is an amphora of wine, with the wine not touching the bread at all, and a gap of air between the surface of the wine and the bottom of the loaf. But since the bread is hot and giving off steam, it is highly susceptible to absorbing flavors, and the result is that the aroma of the wine is absorbed into it, so that when the bread is later tasted one senses the flavor of wine in it, even though the bread never touched the wine directly.
The question, then, is this: if the wine was wine of Terumah, has the bread too become restricted as Terumah? There is no taste here, since no wine was spilled on the bread and none touched it, rather the bread absorbed the aromas of the wine because of its heat.
Three opinions in the mishnah:
The word "milsa" means a thing, as is familiar from the expression 'simana milsa' - said at the Rosh Hashanah meals, when we eat the pomegranate and the date and dip the apple in honey and say that through this we should have a good and sweet year: the very performance of the symbolic act is milsa, a real thing. So too here: 'reicha milsa' means that the presence of the aroma is a real thing.
Rabbi Meir - 'reicha milsa': if the aroma can be sensed, then it is something and not nothing, similar to the rule of 'taste is like substance.' Once the flavor of the wine that traveled through the air is found in the bread, its presence cannot be denied, and therefore the bread becomes restricted, meaning that only a kohen may eat it.
Rabbi Yehudah - 'reicha lav milsa': the aroma in itself is not a thing, and it is not something to be taken into account. Therefore, even though one can still sense a taste of wine in the bread, since it arrived through the air merely as a smell and an aroma, the bread is not restricted, and a non-kohen may still eat it, despite its having absorbed a bit of the aroma of the Terumah wine.
Rabbi Yosei - a middle position: "Rabbi Yosei matir b'shel chitim" - Rabbi Yosei permits in the case of wheat bread, and in this he agrees with Rabbi Yehudah that reicha lav milsa, and the bread is not restricted because of the aroma of the Terumah. In his view, however, barley is the exception: "v'oser b'shel se'orim" - and he forbids in the case of barley bread, "mipnei shehase'orim sho'avos" - because barley draws in. Unlike other types of bread, barley soaks up and pulls the flavor into itself. It does not receive the aroma passively alone, but tends to draw the flavor inward, and the result is that the bread received actual taste from the wine in the barrel beneath it.
It emerges that according to Rabbi Yosei, this is the rule generally, but with barley bread it is not aroma but taste that mixes into the bread, and for that reason barley bread is forbidden to non-kohanim. And the halachah follows Rabbi Yosei.
In summary: This Mishnah deals with smell rather than taste - with hot bread that was placed over the mouth of a barrel of Terumah wine without any direct contact, and absorbed its aroma. They disagreed about it: Rabbi Meir holds reicha milsa (aroma is significant) and forbids it, Rabbi Yehudah holds reicha lav milsa (aroma is not significant) and permits it, and Rabbi Yose distinguishes between wheat bread, which is permitted, and barley bread, which is forbidden, because barley draws the taste into itself. The halachah follows Rabbi Yose.
The next Mishnah will also continue to deal with the law of aroma.