Just like it is forbidden to benefit from anything that is Hekdesh (property of the Bet HaMikdash) without first redeeming it, so too it is forbidden to benefit from this world since the entire world belongs to Hashem without first redeeming it by making a bracha. 
- There is no minimum amount necessary for Bracha Rishona, even if one eats a very small amount of food one must make the appropriate Bracha. 
When is a new Bracha Rishona required?
- As long as one didn’t have a hesech hadaat (interruption of thought) one doesn’t need to make a new Bracha Rishona. However, if one made a Bracha Achrona, and one wants to eat again a new Bracha Rishona is needed. 
- If one sat down to learn and from time to time one has tea or coffee, even if one had long breaks between the drinks, one does not need to make another Bracha Rishona for the next cup and one bracha rishona covers everything. It's preferable to have in mind that one's Bracha Rishona will exempt all future cups. However, if one made a Shinui Makom by leaving the doorway of the shul or bet midrash, one should make a new Bracha Rishona. 
- If one sat down to learn and from time to time one has snacks, if one ate a Kezayit in Kedi Achilat Pras, and one is afraid that one will forget to make a Bracha Achrona or that the time for Bracha Achrona will expire , one may make a Bracha Achrona and then when one continues to eat, one does not need a new Bracha Rishona.
- One doesn’t make a Bracha Rishona if one intends to taste food even if one swallows. However, it’s preferable if one is going to swallow to have intent to eat it and make a Bracha. 
- One who tastes a food before it’s cooked to see if it needs spices and even if one tastes it after it’s cooked to see if it tastes good, it is considered tasting and one doesn’t make a Bracha. 
- Kitzur Shulchan Aruch 50:1, See also Maharsha Brachot 35b
- Gemara Brachot 38b records the story where Rabbi Yochanan made a Bracha Rishona and Bracha Achrona on a salted olive. The Gemara asks how Rabbi Yochanan could make a Bracha if after removing the pit there wouldn’t be a shiur (certain amount of volume). The Gemara answers that the shiur is an average olive and since he had a large olive even after removing the pit he still ate the shiur. Tosfot 39a s.v. Batzar writes that the Gemara is specifically speaking about Bracha Achrona, however, one should make a Bracha Rishona for any amount because one shouldn’t benefit from the world without a Bracha (Brachot 35a). Tur and S”A 210:1 rule that one must make a Bracha Rishona on any amount of food because one may not get benefit from the world without a Bracha. Kitzur Shulchan Aruch 50:1 agrees.
- Beiur Halacha 190 s.v. Acher, Vezot HaBracha (pg 53, chapter 5)
- Chazon Ovadyah (Brachot p. 173, Yom Tov p. 311)
- Chazon Ovadyah (Brachot p 178 and 539)
- S”A and Rama 210:2, Mishna Brurah 210:19. Tur 210:2 writes that the Rambam holds that even if one swallows one doesn't make a bracha for tasting food and the rabbenu chananel holds that only if one tastes and spits it out does one not make a bracha however if one swallows one should make a bracha. Shulchan Aruch and Rama 210:2 seem to rule like the Rambam but Mishna Brurah 210:19 adds that in order to avoid this dispute if one plans on swallowing one should intend to eat it and benefit from it and then make a bracha and swallow.
- Mishna Brurah 210:13