We learn about the requirement to bentch Birkas Hamazon after eating and being satiated from the posuk in Devarim, Perek Ches, posuk yud. The gemara in Brachos, daf chof alef, amud alef says that we learn that Birkas Hamazon is a mitzvah d’Oraysa from the above posuk. All other brachos achronos are mi’DeRabbanan.
The shiur for drinking in order to be required to say a bracha achrona is discussed in the Tur, Orach Chaim, siman kuf tzaddik which discusses that there is a safek whether the shiur for a bracha achrona is a kzayis or a reviis. The Tur therefore says that a person should be careful to either drink less than a kzayis and be exempt from a bracha achronaor more than a reviis thereby definitely necessitating a bracha achrona. Regarding a kos shel bracha, the Shulchan Aruch says that a person is required to drink a cheek full, which is the majority (roiv) of a reviis. If a person drinks from a kois shel bracha, he should drink a full reviis so as not to have any doubts of whether or not a bracha achrona is necessary.
The following question arises. We understanding that regarding a kos shel bracha we say a cheekful is enough because it is the majority of a reviis and the rule is rubo kekulo (majority is like all of it). Why don’t we say the same svara regarding a person who drank the majority of a reviis; shouldn’t he be required to say a bracha achrona since it is rubo kekulo?
The Ran in Psachim and the Bach in Orach Chaim, siman taf ayin bais say that the reason a person needs a cheekful of wine is not because of rubo kekulo, but rather there is a requirement of te’ima (tasting) and a person who has kos shel bracha needs a te’ima chashuva, which is a cheek full. According to the Bach and Ran there would be no question as to why a bracha achrona is different from a kos shel bracha.
The following question arises regarding the four cups of wine at the seder where both the Ran and Bach require the person to drink the majority of the cup of wine. The Bach explains that drinking roiv kos is like drinking the full cup. We need to explain why we don’t say the majority of a reviis is like all of the reviis regarding bracha achrona?
The concept of rubo kekulo can be analyzed in two ways. Is the fact that a person who does roiv of the mitzvah considered as if he did the entire mitzvah, and therefore the minority part isn’t considered relevant? For example, if one slaughters roiv of the simanim of the animal, is it as if all of the simanim are slaughtered; or do we say it is as if only majority is done, but the Torah says that a majority is enough and therefore the fact that the minority is not done is inconsequential?
If we say that the rubo kekulo is a din that the majority needs to be done and the minority is not considered done, but becomes inconsequential then we can understand why it is sufficient to drink the majority of the kos when it comes to daled kosos and the leftover would be inconsequential. However, when it comes to bracha achrona, where the shiur of bracha achrona is when a person’s stomach enjoyed the drink, we won’t say that majority is considered a totality since the stomach of the person still feels a lack. One would therefore not be mechuyav in bracha achrona unless he had the entire thing and we won’t say rubo kekulo.
It is important to thank Hashem for all the food and drink and look to say brachos of thanks.