In this series of short teshuvot, Rav Moshe Feinstein helps to define some of the parameters of when one must say a ברכה אחרונה. He begins by noting the view of the Pri Megadim that if a person eats a כזית, but does so slowly, that person needs to make a ברכה אחרונה. This runs counter to the normative view that the כזית must be eaten within a כדי אכילת פרס (somewhere between 2.5 and 9 minutes). Thus, Rav Moshe advises against relying on this view, and instead trying at some point in one's eating to have a כזית within that prescribed time frame.
Rav Moshe next deals with a case of a person who begins eating, eats less than a כזית, leaves the room, returns to the room, and continues eating. He rules that all of the eating can combine to create one כזית for the purpose of making a ברכה אחרונה.
In the case of a whole fruit which is less than a כזית, although there is a view that its wholeness gives it a certain status and importance, Rav Moshe nevertheless feels that one should endeavor to eat at least a כזית of the fruit.
If one is tasting something while cooking, even if he swallows it Rav Moshe rules that no bracha is needed. However, if one's intention is both to taste it and to benefit from and enjoy it, then a bracha is required.