Gemara

31b - Question: Why is beer of Nochrim forbidden?

Answer #1 (Rami bar Chama): This is a decree to distance from Nochrim, lest Yisrael come to intermarry with them.

Answer #2 (Rav Nachman): This is where they leave the water used for beer exposed to become clear. Perhaps a Sheretz drank, and left venom in it.

They would take beer out of a Nochri's house, and Rav Papa would drink on the porch. They would bring beer from a Nochri's house to Rav Achai's house, and he would drink in his house.

Both agree that it is forbidden (to drink in the Nochri's house) due to intermarriage. Rav Achai chose to distance himself further.

Rav Shmuel bar Bisna came to a city in which Yisraelim were not concerned if Nochrim touch wine. They brought to him beer; he did not drink it.

This was a personal stringency, lest he drink their wine.

(Rav): Beer of Nochrim is permitted. My son Chiya may not drink it.

Rav forbids due to exposure. The bitterness of the hops burns the venom. If one is already sick, this will aggravate his condition. His son was sick.

(Rav Yosef): Beer vinegar of Nochrim is forbidden, for they mix dregs of their wine with it.

(Rav Ashi): If one sees it taken straight from the (large storage) barrel, it is permitted. They would not mix in wine, for this makes it spoil quickly.

Rishonim

Rambam (Hilchos Ma'achalos Asuros 17:10): One may not drink beer of Nochrim that they make from dates, figs and similar things. This is only where it is sold. If he took it to his house, he may drink it there. The decree was primarily lest the Yisrael dine with the Nochri.

Rambam (25): Chachamim forbade beer vinegar of Nochrim because they mix dregs of wine with it. Therefore, what is taken from the storehouse is permitted.

Rosh (2:15): Chachamim were not as strict about beer as about bread, because bread is the staple of life and requires greater separation. It seems that the Isur is only when he is Kove'a (fixes) himself to drink in the Nochri's house the way people are Kove'a to drink. Chachamim did not decree about one who enters the Nochri's house and happens to drink Derech Arai (not Kav'ua). Gedolim of the island (England) conducted like this. Similar, if one lodges in the Nochri's house, the inn is considered like his own house, and he may drink. Beer was not forbidden in the days of the Tana'im. Later, in the days of the Amora'im they decreed about it. Bishul Akum (food cooked by Nochrim) does not apply to barley beer, for it does not go on kings' tables. Alternatively, just like the grain is not important regarding Borei Peri ha'Adamah (rather, we bless sheha'Kol on beer), it is not important regarding Bishul Akum.

Ran (11a DH Mipnei): Chachamim were concerned lest a Yisrael regularly make drinking parties in a Nochri's house, lest he put his eyes on the Nochri's daughter. When he does not drink in the Nochri's house, this does not promote comradery. Another reason why we are more stringent about bread is because women make it, so there is more concern for intermarriage.

Poskim

Shulchan Aruch (YD 114:1): All beer of Nochrim, whether from dates, figs, barley, grain or honey, is forbidden due to intermarriage. It is forbidden only where it is sold. If he took it to his house, he may drink it there. The decree was primarily lest the Yisrael dine with the Nochri. They forbade only when he is Kove'a himself to drink the way people do. If he enters the Nochri's house and happens to drink Derech Arai, it is permitted.

Beis Yosef (DH v'Chasvu): Tosfos (31b DH v'Travaihu) says that we may not drink in the Nochri's house, for Rav Papa and Rav Achai, who were Basrai, did not. The Rambam is stringent like Rav Achai, even though Rav Papa was Basra and he is cited in the Gemara more than Rav Achai, in order to distance from matters of Nochrim. Semag brings only Rav Papa. The Rashba says that Chachamim did not decree against beer, just Kedoshim distanced themselves from Nochri repasts. This is why the Gemara asked 'why is beer of Nochrim forbidden?' It did not ask 'why did Chachamim decree against beer of Nochrim?' Therefore, a Ba'al Nefesh (spiritual person) should distance himself more, and not drink it at the entrance of the store, until they bring it to his house, like Rav Achai. However, the Halachah follows Rav Papa, who permits drinking on the porch of the store. We follow the lenient opinion in all mid'Rabanan laws. The Mordechai (818-819) says in the name of Sefer ha'Yashar that there is no concern for intermarriage with our beer. It seems that he holds that since our only source to forbid is from Rav Papa and Rav Achai, we forbid only beer like theirs, which was from dates, but not from barley or other species. Tosfos and other Poskim disagree.

Shulchan Aruch (ibid): If one lodges in the Nochri's house, it is considered like his (own) house. One may send to the city to buy beer from Nochrim.

Beis Yosef (DH u'Mah she'Chosav b'Shem, citing Tosfos (ibid.): Perhaps he may drink even his host's beer, to avoid enmity. The Isur due to intermarriage is to drink in a store, or to regularly drink among Nochrim.

Rema: Some permit beer of honey or grain. The custom is to be lenient in these lands.

Shulchan Aruch (2): Where people are lenient about wine of Nochrim, even beer is forbidden.

Beis Yosef (DH Kasav): The Rashba learns from Rav Shmuel. Perhaps this is only for a Ba'al Nefesh, but for others it is like a decree for a decree. Or perhaps for all it is like one decree. It is good to be stringent. The Tur says that Stam that the Rashba forbids, like he says in Toras ha'Bayis ha'Katzar.

Bach (2): The Maharshal says that nowadays that it became like total Heter in some places, one may drink, just not in the Nochri's house. I do not understand. Since we forbid beer due to wine, we totally forbid! Rather, he decrees about places like the case of Rav Shmuel, that the entire city was Yisre'elim, who were suspected about Nochri wine, so they decreed not to buy beer from them, lest they buy wine. Nowadays that most of the city is Nochrim, one may buy beer from them, but not from the Yisre'elim.

Taz (3): It is unreasonable to be more stringent about Yisrael than Nochrim! Rather, the Maharshal means that only where people know that wine of Nochrim is forbidden, just they are lenient, there Chachamim made a Siman that people not drink beer of Nochrim, so they will realize that they may not be more lenient (to drink Nochri wine). However, where they do not know that there is any Isur of wine, there is no reason to decree about beer, and one may drink beer of Yisre'elim or of Nochrim.

Shulchan Aruch (4): One may not buy beer vinegar from Nochrim if it is more expensive than wine, for we are concerned lest they mixed in wine (Rema - to the extent that there are not 60 parts of beer to be Mevatel it).

Taz (4): Why do we need 60 parts to be Mevatel it? Wine is Batel in six parts of water (134:5)! The same applies to other liquids, for they are Mevatel the taste more than water is! The Mordechai requires 60 parts, like he requires regarding water, but we do not rule like him!

Beis Yosef (DH v'She'ar): We are not concerned lest they mix in wine without benefit, just to cause Yisrael to sin. Do not say that wine strengthens the vinegar, for the Gemara says that it causes it to spoil. We are concerned in a store for it is sold quickly before it will spoil. Tosfos and the Mordechai say that one must investigate beer that Nochrim make nowadays, for some put in dregs of wine.

Shach (6): One must be concerned even if wine is more expensive if it improves the vinegar.

Shulchan Aruch (ibid): This is when he sells in a store. If one sees them take it from the (large storage) barrel, it is permitted. They would not mix in wine, for this makes it spoil quickly.