My understanding is that after making a bracha, one should eat the food without a "hefsek" (interruption). Is there a minimum amount of food that one is required to eat after the bracha and before doing anything else? For example, if I say the bracha "boreh pri ha'etz" then eat one molecule of apple, can I speak for a minute, then resume eating the apple?
After a Bracha is recited it is best not to speak while chewing the first bite, until after a bit was swallowed. There is no need for a Kzayis to be swallowed before talking. [...]
If one spoke before any of the food was swallowed, B’dieved it is acceptable as long as a little of the taste of the food was enjoyed, and there is no need to repeat the Bracha. (Mishna Berura Siman 167:35 quoting the Chayei Adam)
Regarding bread, the Mishna Brura writes (end of 167:35) that it is good to swallow a kazayit before talking if not in a pressing situation to speak.
Note that these halachot are very relevant on Shabbat evening when many tend to speak soon after taking the first bite (e.g., regarding distributing the challa).
For more, see also here under "Interruptions Before Eating"