Commentary for Avodah Zarah 78:21

The fear with milk, meat and cheese seems to be that kosher ones were switched with non-kosher ones. The fear with blue wool is that the non-Jew switched the wool with wool dyed with dye not kosher for tzitzit.
The hiltit may have been cut with an unkosher knife. The murias (fish hash) may have been switched with murias that has non-kosher fish in it. The wine may have been touched by the non-Jew, which would render it undrinkable. The issue with the bread is that it may have been switched with bread made by non-Jews, which is forbidden.
Rav requires two seals for some of these things, but only one seal for the others

With bread, if there is motivation for the non-Jew to switch a good loaf for a bad one, the Jew receiving it would know what had been done (the assumption is that the Jew knows what the other Jew sent him). And if they are the same, then why would the non-Jew go through the trouble of faking a seal. Thus one seal is clearly enough.

Rav says that cheese only needs one seal but milk requires two seals. Why the difference? R. Kahana affirms that there really should be no difference between the two and therefore allows milk with only one seal. He does not allow pieces of fish with one seal less the non-Jew switch the kosher fish with non-kosher one. Two seals will make this impossible.

Shmuel does not include milk or bread on his list. He seems to just not be worried at all that non-Jews would switch the bread, as we said above. And he removes milk, as did R. Kahana. But he does not need to add “a piece of fish” because the fish is the same as the meat.

The baraita here refers to buying food from Jews in Syria, the area which lies to the north of Israel. This is an area that rabbis usually consider “semi-observant.” According to the baraita, merchants there are not so careful not to sell to Jews products that were meant for non-Jews. All of these items may be forbidden to Jews. However, he may buy them from a reliable person. He may also eat with Jews there because the Jews there do not themselves eat these foods. Furthermore if a Jew sends him some food as a gift he will send from the food he himself is eating. We can assume that this food is kosher.

The rabbis likely are making a pun here on a Greek word, connecting it with salt eaten at Roman parties. We should note that the meaning of both the original word and the word for “Roman guests” is not entirely clear.

This baraita discusses the various reasons for why this type of salt was prohibited—the question is whether forbidden fish was mixed in with it. The other reason may be that it was smeared with pig fat.

The mishnah says that these foods are prohibited from eating but one may derive benefit from them. The rabbis often read these types of statements as excluding something—as if to say “these items belong on this list, but others do not.” The question is—what is being excluded from the list. Earlier on daf 38b, Hezekiah held that if we know there was wine used to preserve certain foods then the food is prohibited in benefit and not just for consumption. Thus the words “behold these are prohibited” exclude such foods. But R. Yohanan said that if the wine was only used as a preservative, a Jew may derive benefit from the food. To him the words exclude murias and Bithynian cheese, both of which R. Meir prohibits in benefit.

As we explained in the previous mishnah, the concern with milk is that the non-Jew might mix milk which comes from a kosher animal with milk that comes from a non-kosher animal (such as a camel). If the Jew is watching over the non-Jew we have no such concern and the milk is therefore permitted. Mixing foreign substances with honey would spoil the honey. Therefore we can assume that the non-Jew did not put anything into the honey and it is permitted. Even though some grape juice may be dripping from the cluster of grapes, we are not concerned that the non-Jew used this juice in idol worship and it would be forbidden. The mishnah also notes that the liquid that comes out of grapes is not the type of liquid which makes a food susceptible to impurities. As we have learned before (Eduyoth 4:6) food cannot become impure until it is made wet by seven types of liquids. Grape juice is not one of them. Preserved foods into which it is not customary to put wine or vinegar are permitted. If one can see that the fish in the brine is actually herring, it is permitted, unlike the minced fish which was discussed above. In the previous mishnah we learned that it was forbidden to eat pieces of the asa foetida, since the same knife used to cut this plant might have been used to cut non-kosher food. In this mishnah we learn that since they don’t cut the leaves of the as foetida with this type of knife, it is permitted. Olives that have been rolled out into cakes are permitted, since no wine is used in them. According to Rabbi Yose, if the olives have become so soft that the pits fall out, wine might have been put on them to soften them. Therefore, they are forbidden. Some types of locusts are kosher. When a seller sells them, he brings them from his storehouse and puts them in a small basket, onto which he mixes a little wine. Due to the addition of this wine, locusts that come from this basket are forbidden. The locusts that come from the storehouse are permitted since the wine has not yet been put upon them.
The mishnah points out that the same is true with regards to terumah. If a kohen sells locusts from the little basket, we must suspect that he has dripped wine on them, and that the wine might be terumah, which is strictly forbidden to non-priests. If, however, the locusts come from storage, we can be sure that there is no terumah-wine mixed in with them.

This baraita teaches something very similar to our mishnah.

The Talmud tries to figure out what exactly the scenario is in the baraita. If there are no forbidden animals in the flock, then obviously one can drink the milk. And if there are forbidden animals, then why is the milk kosher—how do we know where the milk comes from?
The answer is that there are indeed forbidden animals in the flock, but the Jew can see the non-Jew at least when he is standing. Although he cannot see him while sitting, the fact that he can see him some time, will cause the non-Jew, who knows that the Jew will not drink from milk from forbidden animals, will not take the milk from there.

Honey bought from a non-Jew is permitted. The Talmud lists three problems that we might have been concerned about, but are, in the end, not. The first is that there is something really not kosher in it (like pig fat). But this would be bad for the honey and cause it to go rancid. The second is that it was cooked by a non-Jew. But this prohibition does not apply to honey since honey can be eaten raw. Third is the possibility that the non-Jew cooked it in his own vessels, which had been used with forbidden food. But this is not prohibited because the taste from the non-Jew’s vessel is considered “a worsening flavor” and in such cases the substance is permitted. Thus there is no reason to be concerned about honey bought from a non-Jew.

For a food to be susceptible to impurity it must come into contact with liquid. The liquid that comes out of grape-clusters does not render it susceptible to impurity. But this is contrasted with a mishnah in which Shammai and Hillel (!) argue over whether grapes that are harvested to be brought to the wine-press are susceptible to impurity through the liquid that oozes out of them. While originally Hillel said that they were not susceptible, he eventually agreed with Shammai that they are.

The Talmud solves the difficulty by distinguishing between grapes harvested for food and grapes harvested for wine. If they are harvested for wine, then the person wants the liquid to come out of them. This makes the grapes susceptible to impurity.

The mishnah allows a Jew to buy a tarit fish which has not been minced from a non-Jew and eat it. The Talmud explains that the fish’s head and backbone must be recognizable. This is how we can be sure that a non-kosher fish did not get in there. The mishnah allowed “brine containing fish” as long as that fish was a specific fish, called the kalbit fish. This fish is a sign that the brine is kosher. In open barrels two of these fishes are necessary, but in closed barrels, one is sufficient.