פירוש על עבודה זרה 62:1

This source describes a Jew who sends a substance with either a Samaritan or a non-Jew. Rashi explains that with brine the fear is that the non-Jew may switch it with his own brine that has non-kosher fish in it. Murias is also made of fish, but Rashi explains that it might have wine in it. In any case, if the Jew can tell that the wine, brine or murias has not been opened then he may drink it. But simply closing the cask is not sufficient.

Zera solved the contraction by positing that in the town the Samaritan (or non-Jew) will not open his wine because the Jew might pop in at any moment. But on the open road, he would have time and therefore simply closing the cask is not sufficient. The Jew must be able to tell that it is has not been opened.

Yirmiyah notes that those in the town surely got there by the road—after all they were sent on their way from one place to another by the Jew. So how can we really be lenient with wine in the town?
Therefore, R. Yirmiyah offers an entirely different solution. The leniency that a simple closed cask is sufficient applies only to casks found near the wine presses. Since there are many people there, the Samaritan would be afraid to let the non-Jew touch the wine, lest the Jews see and refuse to buy from him. But elsewhere, where not that many people are around, Jews cannot drink Samaritan wine. Not because Samaritans libate, but because Samaritans do not observe the rabbinic stringencies surrounding the prohibition of non-Jews coming into contact with Jewish wine.

According to one opinion, drinking beer with non-Jews encourages fraternization with them, and that is why it is prohibited. In other words, not allowing people to drink together is a good way to prevent them from marrying each other. The other opinion holds that the problem is that non-Jews leave their beer uncovered, and it might be susceptible to snake poison. The problem with that opinion is that Jews also let their beer remain uncovered. The answer is that if the beer is left so that all the sediment falls to the bottom, then the water on top would be subject to the rules of uncovered water.

We can see here a variety of opinions on drinking non-Jewish beer. Some of the amoraim held that as long as the Jew did not drink with the non-Jew, particularly in the non-Jew’s house, he could drink the beer. But one amora was particularly cautious, and assumedly did not drink the beer at all.
I should note that by the Middle Ages the laws involving beer had been much more relaxed. Today religious Jews drink beer brewed by non-Jews. Indeed, on a personal note, this particular Jew very much enjoys the excellent beer crafted by non-Jews. And that’s an understatement.

According to Rashi, Marguan is a city in which non-observant Jews reside. R. Shmuel b. Bisna would neither drink theie wine there nor their beer. Theoretically, there is no reason for him to have been so strict with the beer. But the fear seems to have been that due to their lack of observance, he should act even more strictly than necessary and not even drink the beer.

In principle, Rav allows Jews to drink non-Jewish beer. It is not prohibited like wine. But he does not let his son drink it lest it have been left uncovered. Since his son already seems to have some sort of sickness, the little bit of venom in the beer that would not injure a healthy person, would injure his son.

And just like that, we’re back to the topic of uncovered liquids!

Shmuel cites a fascinating tradition he learned from Rav. It seems that non-Jews do not observe the prohibition of drinking uncovered liquids. If such liquids are truly dangerous, then we’d have to ask why? The answer is that they have a vaccine immunity to the poison. Since they eat snakes and other such creepy creatures, they are immune from their poison. Clever answer!

Beer vinegar may be prohibited because it may contain wine yeast (the dregs would be used to re-ferment the beer). But if taken from a storehouse, we can assume it has no wine in it.