The Gemara in Masechet Berachot, Daf 35, says that one is obligated to make a Beracha Acharona after having consumed a certain minimal measure of food. From the Torah, one must make Birkat HaMazon at the end of a meal based on the Pasuk ‘ViAchalta ViSavta U’BeErachata Et Hashem Elokecha.’ The Gemara also discusses Beracha Rishona, which is a Rabbinical law that requires us to make a Beracha before we eat any amount of food.
Rabbi Yoseph ben Ephraim Caro (author of the Shulchan Aruch) asks a question on this topic in one of his books entitled ‘Kesef Mishne’ (a commentary on Rambam). He asks how can it be that we are stricter with a Rabbinical law than we are with a Torah law? Beracha Rishona is from the Rabbinate, and is required every time you eat, no matter how much is you eat. Beracha Acharona is from the Torah, and is only required after eating a certain minimal measure of food.
So Rav Yoseph Caro gives us a sound and logical answer to this question. The Rabbinate set this law of Beracha Rishona on any amount of food, in order to prevent a likely problem from arising. In general, people do not limit their food consumption to a predetermined minimal serving. One might plan to eat an ounce of cookies, but in reality eats 5 or 6 ounces before stopping. So if the Rabbinate based Beracha Rishona on a set amount of food, it would be very common to find someone who planned to eat less than the minimal amount, but in fact ate more. What would such a person do? The Rabbinate therefore decreed that Beracha Rishona should be made on any amount of food one eats.
The Teshuvot Bet Ephraim brings yet another answer to this question. The Gemara explains that the basis for making Beracha Rishona stems from the principal that one must make a Beracha when deriving any pleasure in this world. People derive pleasure when eating even from a morsel of food. But when it comes to Beracha Acharona, the Torah says ‘ViAchalta ViSavta’, meaning one makes Beracha Acharona only after he ate and was satiated. To become satiated, one must consume a minimal measure of food.
Halacha Lema'ase, one must make Beracha Rishona even if consuming just a tiny drop of food. This is according to the Rabbinate, for one derives pleasure from the food, and one can not really be sure how much he shall eat. But when it comes to Beracha Acharona, even though it’s from the Torah, one must have consumed certain minimal amounts.
