Wa alaykum salam wa rahmatuLlahi wa barakatuHu,

The fast is valid if it [nukhamah] was swallowed while it had not reached the oral cavity [i.e. zahir]. If one brought it up and was then unable to spit it out, or if it came by itself and one was able to remove it but did not, then the fast is invalidated; reason being, its being swallowed is on the part of the one fasting. If it came by itself and one was unable to spit it out, then the fast is valid. (al-Minhaj al-Qawim v. 2, p. 178; Rawdat al-Talibin v. 2, p. 224)

According to the Malikis and Hanafis, swallowing such does not invalidate the fast. This is also one of Imam Ahmad’s views. He considered that swallowing such does not invalidate the fast, as it is a substance generally present in the mouth; thus being analogous to saliva. (al-Mawsu’at al-Fiqhiyyah v. 28, p. 65)

And Allah knows best.

Shafiifiqh.com Fatwa Dept.