Gemara

(Rav Yehudah): If one put food in his mouth without blessing first, he should move it to one side and bless.

(Beraisa #1): He should swallow it.

Contradiction (Beraisa #2): He should spit it out.

Contradiction (Beraisa #3): He should move it to one side and bless.

Resolution: If it is a liquid, he swallows it. If it is a (solid) food and it will not be Ma'us (repulsive) if he spits it out, he does so. If it would become Ma'us, he moves it to one side and blesses.

Question: If someone ate and drank without blessing (beforehand), should he bless (now)?

Answer (Rav Chisda): If one ate garlic, and gives off an offensive odor, should he eat more garlic, to increase the odor?! (One who transgressed should not transgress more!)

(Ravina): Therefore, even if he finished his meal, he should bless;

(Beraisa): After immersion, he blesses, "... Asher Kidshanu b'Mitzvosav v'Tzivanu Al ha'Tevilah."

Rejection: That is not a proof. There, he could not bless before immersing. Here, he could have blessed. Since he did not, perhaps he is disqualified from blessing afterwards!

Rishonim

Rambam (Hilchos Berachos 8:12): If one put food in his mouth without blessing first, if it is a drink, he swallows it and blesses afterwards. If it is a Pri that would become Ma'us if he spits it out, e.g. pomegranates or grapes, he moves it to one side, blesses, and then swallows it. If it would not become Ma'us, e.g. beans or peas, he spits it out, and when his mouth is empty he blesses, and then eats.

Ra'avad: This is if he intends to eat more. If not, he is like one who finished eating. (He does not bless). Further, I say that if he has more drinks, he does not swallow what is in his mouth. Rather, he spits it out. We are not concerned for loss of a mouth's full of drinks. He should not benefit from them without a Berachah. However, if he has no more drinks, and he needs what is in his mouth, he may swallow it to avoid losing it, even though he loses the Berachah. Also if he spits it out he loses the Berachah, for he does not have more (to bless on).

Rosh (Berachos 7:33): R. Chananel says that if one put drinks in his mouth without a Berachah, he swallows them without a Berachah. They lost the status of drinks, for no one else would drink them. Therefore, we are not concerned to bless on them, even if he could do so with difficulty. The Gemara connotes unlike this. It discusses swallowing, spitting out, and moving it to one side and blessing. In all cases, one blesses on it. If it will not become Ma'us, he spits it out and blesses. If it would become Ma'us, he moves it to one side and blesses. One cannot do so for drinks, therefore he swallows and blesses. This is unlike one who ate and forgot to bless until he finished eating. He does not bless. Here is different, for while the food was still in his mouth, he remembered that he did not bless. This is a little like blessing before (a Mitzvah), since he had not yet swallowed it, just he could not bless at the time. The Ra'avad says so.

Poskim

Shulchan Aruch (OC 172:1): If one put drinks in his mouth without blessing first, if it is drinks, he swallows them and does not say a Berachah Rishonah on them.

Beis Yosef (DH veha'Rambam): One could explain the Rambam like R. Chananel, that he does not say a Berachah Rishonah. He says that one blesses on them at the end, i.e. only a Berachah Acharonah. However, it is better to say that he means that he blesses a Berachah Rishonah afterwards. If not, he should have said 'he blesses only a Berachah Acharonah.' Also, what is the Chidush to teach that he blesses a Berachah Acharonah? This is obvious! The Ra'avad understands that the Rambam holds that one says a Berachah Rishonah, like the Rosh. The Ra'avad holds that if he will not eat more, it is as if he finished, and he does not bless.

Beis Yosef (DH veha'Rashba): The Rashba says that the Ra'avad holds that if one finished his meal and did not bless, he blesses retroactively. The Gemara says that he swallows and blesses when he has no other drinks. If he has others, he spits out what is in his mouth and blesses on the others. He brought a proof for this from the Yerushalmi. Also Ohel Mo'ed says so. This is unlike the Ra'avad in his Hasagos (comments on the Rambam).

Beis Yosef (DH ul'Inyan): Since R. Chananel is explicit (says not to bless), and we can explain the Rambam like him, presumably he holds like him, so we hold like them, for they are outstanding Rabanan and they are lenient about Berachos mid'Rabanan.

Bach (1): We can say that R. Chananel compares drinks that (he put in his mouth without a Berachah and) he swallowed, to one who ate and drank and did not bless. The Gemara concludes that he does not bless. The Rashba says so in the name of the Ge'onim. The Rosh showed that here is different. The Beis Yosef rules like R. Chananel, and the Rema rules like the Rosh. I say that the Ra'avad's opinion is primary. If he will eat more, he did not finish his meal. Therefore, he swallows them and says a Berachah Rishonah. If he will not eat more, it is as if he finished. Therefore, he swallows them and does not bless, like the Tur wrote (167). The Ra'avad says so in his Hasagos.

Bach (DH ul'Inyan): If he finished eating and he has more drinks, he spits them out, blesses on the others and drinks them. If he has no others, he spits them out, does not benefit from them and loses the Berachah. If he needs what is in his mouth, he swallows them and does not bless, for Safek Berachos Lehakel. We hold like the Ra'avad's Hasagos, for all Ge'onim hold like this. The Gemara concludes like this; it says 'this is wrong.' Perhaps these words were not in the Ra'avad's text, therefore he retracted in the Hasagos. The Tur rules like this in 167.

Magen Avraham (1): It is proper to do like the Ra'avad and not enter a Machlokes (different opinions).

Magen Avraham (2): He says a Berachah Acharonah.

Mishnah Berurah (1): Levushei Serad says that if one put a small amount of drinks in his mouth and he can bless with difficulty (while it is in his mouth), he should bless.

Bi'ur Halachah (DH v'Eino): L'Chatchilah it is proper to be concerned for the Ra'avad's opinion and spit them out if he has other drinks. The other Rishonim did not bring his opinion. Therefore, we do not protest against one who is lenient.

Kaf ha'Chayim (1): I say that it is better to bless in one's heart than to spit them out. The Rambam holds that one is Yotzei b'Di'eved in this way. Even though we do not rule like the Rambam, if the drinks are already in his mouth and if he spits them out they will be wasted, and if (he swallows them) there is a Machlokes whether or not he blesses, one should rely on the Rambam. If he cannot bear to keep the drinks in his mouth until he blesses in his heart, he should spit them out.

Rema: Some say that he blesses on them. It seems that this is primary.

Magen Avraham (3): It is not clear whether (according to the Rema) he says also a Berachah Acharonah. We never find two Berachos (Rishonah and Acharonah) next to the other. I say that on the seven species, he should bless (also a Berachah Acharonah), for some say that it is mid'Oraisa. For water and other species, some argue. Perhaps then, he should not bless. This requires investigation. Therefore, he should spit them out, like I said above.

Mishnah Berurah (5): Most Poskim hold like the first opinion, that he blesses only a Berachah Acharonah if he drank a Shi'ur. Eliyahu Rabah agreed to them.

Shulchan Aruch (2): If one forgot and put food in his mouth without blessing, if it would not become Ma'us if he spits it out, he spits it out and blesses on it. If it would become Ma'us if he spits it out, he moves it to one side and blesses.

Bach (1): If a food will not become Ma'us if he spits it out, l'Chatchilah he may not move it to one side and bless, for it says "Yimalei Pi Tehilasecha." If a food would become Ma'us, he moves it to one side and blesses. Even for a food that would not become Ma'us, if he moves it to one side and blessed, he was Yotzei.

Mishnah Berurah (6): Beans and similar things, which are hard, do not become Ma'us if he spit them out.

Mishnah Berurah (7): He must spit them out before blessing, for it says Yimalei Pi Tehilasecha. The Eshkol learns from here that one who blesses must bless with a full mouth, and not stutter.