Brachot 14 - Greeting a Fellow Before Morning Prayer
If one accepts on himself a fast (besides those mandated by the Torah or by the Sages), can he taste food, if he does not swallow it? Did he accept to refrain from eating and drinking - and he is not eating in this case? Or did he accept not to have any benefit from food, and there is a little benefit here? Rabbi Ami answered, "He can taste, and there is no problem in that." From his reply we also understand that "in that" there is no problem, but he should not swallow, and no blessing is required. Some understand it differently and say that swallowing a small morsel is allowed.
If one greets a fellow before morning prayer, he is like praying to an idol. How could that be? We know that while reading the Shema prayer, one is allowed to interrupt, and initiate a greeting to another person, to show honor? - That is because this person chanced to come by, but the prohibition to greet others applies only when one goes out of his way to greet somebody before morning prayer, and he intends to derive some material benefit thereby.
Art: Daniele Crespi - Fasting