Laws of Public Fast Days
10. It is forbidden to rinse one's mouth in the morning on all public fast
days (1). Regarding a build up of saliva: if one can spit it out, one
should. If this is impossible, one may swallow it, even on Yom Kippur,
because one's intent is not to enjoy it (2).
It is forbidden to taste food on a communal fast day, even if one later
spits it out. In contrast, one may taste food and spit it out during a
personal fast that one voluntarily accepts upon oneself (3). Similarly,
rinsing one's mouth is permitted on such a fast.
(1) If one is experiencing discomfort ("tza'ar"), there is room to permit
("yeish lehatir") rinsing out one's mouth, as long as one is careful to
lean forward so that no water is swallowed. This is true even on
Tisha'B'Av, if one is experiencing extreme discomfort ("tza'ar gadol").
However, on Yom Kippur, one should be stringent, and not rinse out one's
mouth even if one is experiencing extreme discomfort (Mishna Berura 567:11).
(2) The Mishna Berura does not even mention that one should try to spit out
the saliva (Ibid. 13).
(3) It is assumed that when an individual accepts a fast upon himself, he
commits himself to refrain only from actual eating and drinking, and not
from simply tasting food. The obligation for a communal fast is not
dependent on an individual's conscious acceptance ("kabalah"), and
therefore, one cannot claim that there was no commitment to refrain from