השרוי בתענית יכול לטעום כדי רביעית ובלבד שיפלוט וביום הכפורים ובתשעה באב אסור: הגה ויש מחמירין בכל תענית צבור והכי נוהגין: (תוס' והגהות מיימוני פ"א ות"ה סימן קנ"ח):

One who is fasting can taste up to a revi'it, provided he spits it out. This is forbidden on Yom Kippur and Tisha B'Av. Rem"a: There are those who are stringent on all public fasts, and this is the practice (Tosafot, Hagahot Maimoni chapter 1, Trumat Hadeshen siman 158).

There are those who say that a revi'it is only permitted bit by bit and not all at once. And there are those who say that one can taste a revi'it even all at once if he knows that he will be able to prevent himself from swallowing anything.

Somebody who is accustomed to washing his mouth in the morning of a public fast is not acting properly. However, on a private fast, this is permitted provided he spits it out, even if the water he is rinsing with is more than a revi'it. Rem"a: It is permitted to chew and spit out cinnamon, other spices, or sweet wood in order to moisten the throat, except for on Yom Kippur when this is forbidden (Mordechai on Ta'anit and Hagahot Maimoni chapter 1).